Identifiers

punctuation marks or symbols can be part of an identifier. Only letters, digits and single underscore characters are
valid. In addition, variable identifiers always have to begin with a letter. They can also begin with an underline
character (_ ), but in some cases these may be reserved for compiler specific keywords or external identifiers, as
well as identifiers containing two successive underscore characters anywhere. In no case they can begin with a
digit.
Another rule that you have to consider when inventing your own identifiers is that they cannot match any keyword
of the C++ language nor your compiler's specific ones, which are reserved keywords. The standard reserved
keywords are:
asm, auto, bool, break, case, catch, char, class, const, const_cast, continue, default, delete,
do, double, dynamic_cast, else, enum, explicit, export, extern, false, float, for, friend, goto,
if, inline, int, long, mutable, namespace, new, operator, private, protected, public, register,
reinterpret_cast, return, short, signed, sizeof, static, static_cast, struct, switch, template,
this, throw, true, try, typedef, typeid, typename, union, unsigned, using, virtual, void,
volatile, wchar_t, while
Additionally, alternative representations for some operators cannot be used as identifiers since they are reserved
words under some circumstances:
and, and_eq, bitand, bitor, compl, not, not_eq, or, or_eq, xor, xor_eq
Your compiler may also include some additional specific reserved keywords.

The selective structure: switch

The syntax of the switch statement is a bit peculiar. Its objective is to check several possible constant values for an
expression. Something similar to what we did at the beginning of this section with the concatenation of several if
and else if instructions. Its form is the following:
switch (expression)
{
case constant1:
group of statements 1;
break;
case constant2:
group of statements 2;
break;
.
.
.
default:
default group of statements
}
It works in the following way: switch evaluates expression and checks if it is equivalent to constant1, if it is, it
executes group of statements 1 until it finds the break statement. When it finds this break statement the
program jumps to the end of the switch selective structure.
If expression was not equal to constant1 it will be checked against constant2. If it is equal to this, it will execute
group of statements 2 until a break keyword is found, and then will jump to the end of the switch selective
structure.
Finally, if the value of expression did not match any of the previously specified constants (you can include as
many case labels as values you want to check), the program will execute the statements included after the
default: label, if it exists .
Both of the following code fragments have the same behavior:

The switch statement is a bit peculiar within the C++ language because it uses labels instead of blocks. This
forces us to put break statements after the group of statements that we want to be executed for a specific
condition. Otherwise the remainder statements -including those corresponding to other labels- will also be
executed until the end of the switch selective block or a break statement is reached.
For example, if we did not include a break statement after the first group for case one, the program will not
automatically jump to the end of the switch selective block and it would continue executing the rest of statements
until it reaches either a break instruction or the end of the switch selective block. This makes unnecessary to
include braces { } surrounding the statements for each of the cases, and it can also be useful to execute the same
block of instructions for different possible values for the expression being evaluated. For example:
switch (x) {
case 1:
case 2:
case 3:
cout << "x is 1, 2 or 3";
break;
default:
cout << "x is not 1, 2 nor 3";
}
Notice that switch can only be used to compare an expression against constants. Therefore we cannot put variables
as labels (for example case n: where n is a variable) or ranges (case (1..3):) because they are not valid C++
constants.
If you need to check ranges or values that are not constants, use a concatenation of if and else if statements.

goto statement

goto allows to make an absolute jump to another point in the program. You should use this feature with caution
since its execution causes an unconditional jump ignoring any type of nesting limitations.
The destination point is identified by a label, which is then used as an argument for the goto statement. A label is
made of a valid identifier followed by a colon (:).
Generally speaking, this instruction has no concrete use in structured or object oriented programming aside from
those that low-level programming fans may find for it. For example, here is our countdown loop using goto:

// goto loop example
#include <iostream>
using namespace std;
int main ()
{
int n=10;
loop:
cout << n << ", ";
n--;
if (n>0) goto loop;
cout << "FIRE!\n";
return 0;
}
10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!


The exit function
exit is a function defined in the cstdlib library.
The purpose of exit is to terminate the current program with a specific exit code. Its prototype is:
void exit (int exitcode);
The exitcode is used by some operating systems and may be used by calling programs. By convention, an exit
code of 0 means that the program finished normally and any other value means that some error or unexpected
results happened.

Jump statements

The break statement
Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to end an infinite
loop, or to force it to end before its natural end. For example, we are going to stop the count down before its
natural end (maybe because of an engine check failure?):
// break loop example
#include <iostream>
using namespace std;
int main ()
{
int n;
for (n=10; n>0; n--)
{
cout << n << ", ";
if (n==3)
{
cout << "countdown aborted!";
break;
}
}
return 0;
}
10, 9, 8, 7, 6, 5, 4, 3, countdown aborted!
The continue statement
The continue statement causes the program to skip the rest of the loop in the current iteration as if the end of the
statement block had been reached, causing it to jump to the start of the following iteration. For example, we are
going to skip the number 5 in our countdown:
// continue loop example
#include <iostream>
using namespace std;
int main ()
{
for (int n=10; n>0; n--) {
if (n==5) continue;
cout << n << ", ";
}
cout << "FIRE!\n";
return 0;
}

for loop using c

Its format is:
for (initialization; condition; increase) statement;

its main function is to repeat statement while condition remains true, like the while loop. But in addition, the
for loop provides specific locations to contain an initialization statement and an increase statement. So this
loop is specially designed to perform a repetitive action with a counter which is initialized and increased on each
iteration.
It works in the following way:
1. initialization is executed. Generally it is an initial value setting for a counter variable. This is executed
only once.
2. condition is checked. If it is true the loop continues, otherwise the loop ends and statement is skipped
(not executed).
3. statement is executed. As usual, it can be either a single statement or a block enclosed in braces { }.
4. finally, whatever is specified in the increase field is executed and the loop gets back to step 2.
Here is an example of countdown using a for loop:
// countdown using a for loop
#include <iostream>
using namespace std;
int main ()
{
for (int n=10; n>0; n--) {
cout << n << ", ";
}
cout << "FIRE!\n";
return 0;
}
10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!
The initialization and increase fields are optional. They can remain empty, but in all cases the semicolon signs
between them must be written. For example we could write: for (;n<10;) if we wanted to specify no initialization
and no increase; or for (;n<10;n++) if we wanted to include an increase field but no initialization (maybe because
the variable was already initialized before).
Optionally, using the comma operator (,) we can specify more than one expression in any of the fields included in
a for loop, like in initialization, for example. The comma operator (,) is an expression separator, it serves to
separate more than one expression where only one is generally expected. For example, suppose that we wanted to
initialize more than one variable in our loop:
for ( n=0, i=100 ; n!=i ; n++, i-- )
{
// whatever here...
}
This loop will execute for 50 times if neither n or i are modified within the loop:

n starts with a value of 0, and i with 100, the condition is n!=i (that n is not equal to i). Because n is increased by
one and i decreased by one, the loop's condition will become false after the 50th loop, when both n and i will be
equal to 50.

Iteration structures (loops)

Loops have as purpose to repeat a statement a certain number of times or while a condition is fulfilled.
The while loop
Its format is:
while (expression) statement
and its functionality is simply to repeat statement while the condition set in expression is true.
For example, we are going to make a program to countdown using a while-loop:

// custom countdown using while    Enter the starting number > 8
//8, 7, 6, 5, 4, 3, 2, 1, FIRE!
#include <iostream>
using namespace std;
int main ()
{
int n;
cout << "Enter the starting number > ";
cin >> n;
while (n>0) {
cout << n << ", ";
--n;
}
cout << "FIRE!\n";
return 0;
}

When the program starts the user is prompted to insert a starting number for the countdown. Then the while loop
begins, if the value entered by the user fulfills the condition n>0 (that n is greater than zero) the block that follows
the condition will be executed and repeated while the condition (n>0) remains being true.
The whole process of the previous program can be interpreted according to the following script:

1. User assigns a value to n
2. The while condition is checked (n>0). At this point there are two posibilities:
* condition is true: statement is executed (to step 3)
* condition is false: ignore statement and continue after it (to step 5)
3. Execute statement:
cout << n << ", ";
--n;
(prints the value of n on the screen and decreases n by 1)
4. End of block. Return automatically to step 2
5. Continue the program right after the block: print FIRE! and end program.
When creating a while-loop, we must always consider that it has to end at some point, therefore we must provide
within the block some method to force the condition to become false at some point, otherwise the loop will
continue looping forever. In this case we have included --n; that decreases the value of the variable that is being
evaluated in the condition (n) by one - this will eventually make the condition (n>0) to become false after a certain
number of loop iterations: to be more specific, when n becomes 0, that is where our while-loop and our countdown
end.
Of course this is such a simple action for our computer that the whole countdown is performed instantly without
any practical delay between numbers.

any practical delay between numbers.
The do-while loop
Its format is:
do statement while (condition);
Its functionality is exactly the same as the while loop, except that condition in the do-while loop is evaluated after
the execution of statement instead of before, granting at least one execution of statement even if condition is
never fulfilled. For example, the following example program echoes any number you enter until you enter 0.
// number echoer
#include <iostream>
using namespace std;
int main ()
{
unsigned long n;
do {
cout << "Enter number (0 to end): ";
cin >> n;
cout << "You entered: " << n << "\n";
} while (n != 0);
return 0;
}
Enter number (0 to end): 12345
You entered: 12345
Enter number (0 to end): 160277
You entered: 160277
Enter number (0 to end): 0
You entered: 0
The do-while loop is usually used when the condition that has to determine the end of the loop is determined within
the loop statement itself, like in the previous case, where the user input within the block is what is used to
determine if the loop has to end. In fact if you never enter the value 0 in the previous example you can be
prompted for more numbers forever.

Operators

Once we know of the existence of variables and constants, we can begin to operate with them. For that purpose,
C++ integrates operators. Unlike other languages whose operators are mainly keywords, operators in C++ are
mostly made of signs that are not part of the alphabet but are available in all keyboards. This makes C++ code
shorter and more international, since it relies less on English words, but requires a little of learning effort in the
beginning.
You do not have to memorize all the content of this page. Most details are only provided to serve as a later
reference in case you need it.
Assignment (=)
The assignment operator assigns a value to a variable.
a = 5;
This statement assigns the integer value 5 to the variable a. The part at the left of the assignment operator (=) is
known as the lvalue (left value) and the right one as the rvalue (right value). The lvalue has to be a variable
whereas the rvalue can be either a constant, a variable, the result of an operation or any combination of these.
The most important rule when assigning is the right-to-left rule: The assignment operation always takes place from
right to left, and never the other way:
a = b;
This statement assigns to variable a (the lvalue) the value contained in variable b (the rvalue). The value that was
stored until this moment in a is not considered at all in this operation, and in fact that value is lost.
Consider also that we are only assigning the value of b to a at the moment of the assignment operation. Therefore
a later change of b will not affect the new value of a.
For example, let us have a look at the following code - I have included the evolution of the content stored in the
variables as comments:
// assignment operator                                      a:4 b:7
#include <iostream>
using namespace std;
int main ()
{
int a, b; // a:?, b:?
a = 10; // a:10, b:?
b = 4; // a:10, b:4
a = b; // a:4, b:4
b = 7; // a:4, b:7
cout << "a:";
cout << a;
cout << " b:";
cout << b;
return 0;
}

This code will give us as result that the value contained in a is 4 and the one contained in b is 7. Notice how a was
not affected by the final modification of b, even though we declared a = b earlier (that is because of the right-toleft
rule).
A property that C++ has over other programming languages is that the assignment operation can be used as the
rvalue (or part of an rvalue) for another assignment operation. For example:
a = 2 + (b = 5);
is equivalent to:
b = 5;
a = 2 + b;
that means: first assign 5 to variable b and then assign to a the value 2 plus the result of the previous assignment
of b (i.e. 5), leaving a with a final value of 7.
The following expression is also valid in C++:
a = b = c = 5;
It assigns 5 to the all the three variables: a, b and c.
Arithmetic operators ( +, -, *, /, % )
The five arithmetical operations supported by the C++ language are:
+ addition
- subtraction
* multiplication
/ division
% modulo
Operations of addition, subtraction, multiplication and division literally correspond with their respective
mathematical operators. The only one that you might not be so used to see is modulo; whose operator is the
percentage sign (%). Modulo is the operation that gives the remainder of a division of two values. For example, if
we write:
a = 11 % 3;
the variable a will contain the value 2, since 2 is the remainder from dividing 11 between 3.
Compound assignment (+=, -=, *=, /=, %=, >>=, <<=, &=,
^=, |=)
When we want to modify the value of a variable by performing an operation on the value currently stored in that
variable we can use compound assignment operators:
expression is equivalent to
value += increase; value = value + increase;
a -= 5; a = a - 5;
a /= b; a = a / b;
price *= units + 1; price = price * (units + 1);
and the same for all other operators. For example:
// compound assignment operators                                     5
#include <iostream>
using namespace std;
int main ()
{
int a, b=3;
a = b;
a+=2; // equivalent to a=a+2
cout << a;
return 0;
}

Increase and decrease (++, --)
Shortening even more some expressions, the increase operator (++) and the decrease operator (--) increase or
reduce by one the value stored in a variable. They are equivalent to +=1 and to -=1, respectively. Thus:
c++;
c+=1;
c=c+1;
are all equivalent in its functionality: the three of them increase by one the value of c.
In the early C compilers, the three previous expressions probably produced different executable code depending on
which one was used. Nowadays, this type of code optimization is generally done automatically by the compiler,
thus the three expressions should produce exactly the same executable code.
A characteristic of this operator is that it can be used both as a prefix and as a suffix. That means that it can be
written either before the variable identifier (++a) or after it (a++). Although in simple expressions like a++ or ++a
both have exactly the same meaning, in other expressions in which the result of the increase or decrease operation
is evaluated as a value in an outer expression they may have an important difference in their meaning: In the case
that the increase operator is used as a prefix (++a) the value is increased before the result of the expression is
evaluated and therefore the increased value is considered in the outer expression; in case that it is used as a suffix
(a++) the value stored in a is increased after being evaluated and therefore the value stored before the increase
operation is evaluated in the outer expression. Notice the difference:
Example 1 Example 2
B=3;
A=++B;
// A contains 4, B contains 4
B=3;
A=B++;
// A contains 3, B contains 4
In Example 1, B is increased before its value is copied to A. While in Example 2, the value of B is copied to A and
then B is increased.
Relational and equality operators ( ==, !=, >, <, >=, <= )
In order to evaluate a comparison between two expressions we can use the relational and equality operators. The
result of a relational operation is a Boolean value that can only be true or false, according to its Boolean result.
We may want to compare two expressions, for example, to know if they are equal or if one is greater than the
other is. Here is a list of the relational and equality operators that can be used in C++:
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to
Here there are some examples:
(7 == 5) // evaluates to false.
(5 > 4) // evaluates to true.
(3 != 2) // evaluates to true.
(6 >= 6) // evaluates to true.
(5 < 5) // evaluates to false.
Of course, instead of using only numeric constants, we can use any valid expression, including variables. Suppose
that a=2, b=3 and c=6,
(a == 5) // evaluates to false since a is not equal to 5.
(a*b >= c) // evaluates to true since (2*3 >= 6) is true.
(b+4 > a*c) // evaluates to false since (3+4 > 2*6) is false.
((b=2) == a) // evaluates to true.
Be careful! The operator = (one equal sign) is not the same as the operator == (two equal signs), the first one is an
assignment operator (assigns the value at its right to the variable at its left) and the other one (==) is the equality
operator that compares whether both expressions in the two sides of it are equal to each other. Thus, in the last
expression ((b=2) == a), we first assigned the value 2 to b and then we compared it to a, that also stores the
value 2, so the result of the operation is true.
Logical operators ( !, &&, || )
The Operator ! is the C++ operator to perform the Boolean operation NOT, it has only one operand, located at its
right, and the only thing that it does is to inverse the value of it, producing false if its operand is true and true if its
operand is false. Basically, it returns the opposite Boolean value of evaluating its operand. For example:
!(5 == 5) // evaluates to false because the expression at its right (5 == 5) is true.
!(6 <= 4) // evaluates to true because (6 <= 4) would be false.
!true // evaluates to false
!false // evaluates to true.
The logical operators && and || are used when evaluating two expressions to obtain a single relational result. The operator && corresponds with Boolean logical operation AND. This operation results true if both its two operands are true, and false otherwise. The following panel shows the result of operator && evaluating the expression a &&b:
&& OPERATOR
a b a && b
true true true
true false false
false true false
false false false
The operator || corresponds with Boolean logical operation OR. This operation results true if either one of its two
operands is true, thus being false only when both operands are false themselves. Here are the possible results of a|| b:
|| OPERATOR
a b a || b
true true true
true false true
false true true
false false false
For example:
( (5 == 5) && (3 > 6) ) // evaluates to false ( true && false ).
( (5 == 5) || (3 > 6) ) // evaluates to true ( true || false ).
Conditional operator ( ? )
The conditional operator evaluates an expression returning a value if that expression is true and a different one if
the expression is evaluated as false. Its format is:
condition ? result1 : result2
If condition is true the expression will return result1, if it is not it will return result2.
7==5 ? 4 : 3 // returns 3, since 7 is not equal to 5.
7==5+2 ? 4 : 3 // returns 4, since 7 is equal to 5+2.
5>3 ? a : b // returns the value of a, since 5 is greater than 3.
a>b ? a : b // returns whichever is greater, a or b.
// conditional operator                           7
#include <iostream>
using namespace std;
int main ()
{
int a,b,c;
a=2;
b=7;
c = (a>b) ? a : b;
cout << c;
return 0;
}

In this example a was 2 and b was 7, so the expression being evaluated (a>b) was not true, thus the first value
specified after the question mark was discarded in favor of the second value (the one after the colon) which was b,
with a value of 7.
Comma operator ( , )
The comma operator (,) is used to separate two or more expressions that are included where only one expression
is expected. When the set of expressions has to be evaluated for a value, only the rightmost expression is
considered.
For example, the following code:
a = (b=3, b+2);
Would first assign the value 3 to b, and then assign b+2 to variable a. So, at the end, variable a would contain the
value 5 while variable b would contain value 3.
Bitwise Operators ( &, |, ^, ~, <<, >> )
Bitwise operators modify variables considering the bit patterns that represent the values they store.
operator asm equivalent description
& AND Bitwise AND
| OR Bitwise Inclusive OR
^ XOR Bitwise Exclusive OR
~ NOT Unary complement (bit inversion)
<< SHL Shift Left
>> SHR Shift Right
Explicit type casting operator
Type casting operators allow you to convert a datum of a given type to another. There are several ways to do this
in C++. The simplest one, which has been inherited from the C language, is to precede the expression to be
converted by the new type enclosed between parentheses (()):
int i;
float f = 3.14;
i = (int) f;
The previous code converts the float number 3.14 to an integer value (3), the remainder is lost. Here, the
typecasting operator was (int). Another way to do the same thing in C++ is using the functional notation:
preceding the expression to be converted by the type and enclosing the expression between parentheses:
i = int ( f );
Both ways of type casting are valid in C++.
sizeof()
This operator accepts one parameter, which can be either a type or a variable itself and returns the size in bytes of
that type or object:
a = sizeof (char);
This will assign the value 1 to a because char is a one-byte long type.
The value returned by sizeof is a constant, so it is always determined before program execution.
Other operators
Later in these tutorials, we will see a few more operators, like the ones referring to pointers or the specifics for
object-oriented programming. Each one is treated in its respective section.
Precedence of operators
When writing complex expressions with several operands, we may have some doubts about which operand is
evaluated first and which later. For example, in this expression:
a = 5 + 7 % 2
we may doubt if it really means:
a = 5 + (7 % 2) // with a result of 6, or
a = (5 + 7) % 2 // with a result of 0
The correct answer is the first of the two expressions, with a result of 6. There is an established order with the
priority of each operator, and not only the arithmetic ones (those whose preference come from mathematics) but
for all the operators which can appear in C++. From greatest to lowest priority, the priority order is as follows:

Grouping defines the precedence order in which operators are evaluated in the case that there are several
operators of the same level in an expression.
All these precedence levels for operators can be manipulated or become more legible by removing possible
ambiguities using parentheses signs ( and ), as in this example:
a = 5 + 7 % 2;
might be written either as:
a = 5 + (7 % 2);
or
a = (5 + 7) % 2;
depending on the operation that we want to perform.
So if you want to write complicated expressions and you are not completely sure of the precedence levels, always
include parentheses. It will also become a code easier to read.

Conditional structure: if and else

The if keyword is used to execute a statement or block only if a condition is fulfilled. Its form is:
if (condition) statement
Where condition is the expression that is being evaluated. If this condition is true, statement is executed. If it is
false, statement is ignored (not executed) and the program continues right after this conditional structure.
For example, the following code fragment prints x is 100 only if the value stored in the x variable is indeed 100:
if (x == 100)
cout << "x is 100";
If we want more than a single statement to be executed in case that the condition is true we can specify a block
using braces { }:
if (x == 100)
{
cout << "x is ";
cout << x;
}
We can additionally specify what we want to happen if the condition is not fulfilled by using the keyword else. Its
form used in conjunction with if is:
if (condition) statement1 else statement2
For example:
if (x == 100)
cout << "x is 100";
else
cout << "x is not 100";
prints on the screen x is 100 if indeed x has a value of 100, but if it has not -and only if not- it prints out x is
not 100.
The if + else structures can be concatenated with the intention of verifying a range of values. The following
example shows its use telling if the value currently stored in x is positive, negative or none of them (i.e. zero):
if (x > 0)
cout << "x is positive";
else if (x < 0)
cout << "x is negative";
else
cout << "x is 0";
Remember that in case that we want more than a single statement to be executed, we must group them in a block
by enclosing them in braces { }.

Control Structures

A program is usually not limited to a linear sequence of instructions. During its process it may bifurcate, repeat
code or take decisions. For that purpose, C++ provides control structures that serve to specify what has to be done
by our program, when and under which circumstances.
With the introduction of control structures we are going to have to introduce a new concept: the compoundstatement
or block. A block is a group of statements which are separated by semicolons (;) like all C++
statements, but grouped together in a block enclosed in braces: { }:
{ statement1; statement2; statement3; }
Most of the control structures that we will see in this section require a generic statement as part of its syntax. A
statement can be either a simple statement (a simple instruction ending with a semicolon) or a compound
statement (several instructions grouped in a block), like the one just described. In the case that we want the
statement to be a simple statement, we do not need to enclose it in braces ({}). But in the case that we want the
statement to be a compound statement it must be enclosed between braces ({}), forming a block.

Defined constants

Defined constants (#define)
You can define your own names for constants that you use very often without having to resort to memoryconsuming
variables, simply by using the #define preprocessor directive. Its format is:
#define identifier value
For example:
#define PI 3.14159
#define NEWLINE '\n'
This defines two new constants: PI and NEWLINE. Once they are defined, you can use them in the rest of the code
as if they were any other regular constant, for example:
// defined constants: calculate circumference            31.4159
#include <iostream>
using namespace std;
#define PI 3.14159
#define NEWLINE '\n'
int main ()
{
double r=5.0; // radius
double circle;
circle = 2 * PI * r;
cout << circle;
cout << NEWLINE;
return 0;
}

In fact the only thing that the compiler preprocessor does when it encounters #define directives is to literally
replace any occurrence of their identifier (in the previous example, these were PI and NEWLINE) by the code to
which they have been defined (3.14159 and '\n' respectively).
The #define directive is not a C++ statement but a directive for the preprocessor; therefore it assumes the entire
line as the directive and does not require a semicolon (;) at its end. If you append a semicolon character (;) at the
end, it will also be appended in all occurrences within the body of the program that the preprocessor replaces.
Declared constants (const)
With the const prefix you can declare constants with a specific type in the same way as you would do with a
variable:
const int pathwidth = 100;
const char tabulator = '\t';
Here, pathwidth and tabulator are two typed constants. They are treated just like regular variables except that
their values cannot be modified after their definition.

Constants

 now we understand Constants in detail :::::::::::
Constants are expressions with a fixed value.
Literals
Literals are used to express particular values within the source code of a program. We have already used these
previously to give concrete values to variables or to express messages we wanted our programs to print out, for
example, when we wrote:
a = 5;
the 5 in this piece of code was a literal constant.
Literal constants can be divided in Integer Numerals, Floating-Point Numerals, Characters, Strings and Boolean
Values.
Integer Numerals
1776
707
-273
They are numerical constants that identify integer decimal values. Notice that to express a numerical constant we
do not have to write quotes (") nor any special character. There is no doubt that it is a constant: whenever we
write 1776 in a program, we will be referring to the value 1776.
In addition to decimal numbers (those that all of us are used to use every day) C++ allows the use as literal
constants of octal numbers (base 8) and hexadecimal numbers (base 16). If we want to express an octal number
we have to precede it with a 0 (zero character). And in order to express a hexadecimal number we have to precede
it with the characters 0x (zero, x). For example, the following literal constants are all equivalent to each other:
75 // decimal
0113 // octal
0x4b // hexadecimal
All of these represent the same number: 75 (seventy-five) expressed as a base-10 numeral, octal numeral and
hexadecimal numeral, respectively.
Literal constants, like variables, are considered to have a specific data type. By default, integer literals are of type
int. However, we can force them to either be unsigned by appending the u character to it, or long by appending l:
75 // int
75u // unsigned int
75l // long
75ul // unsigned long
In both cases, the suffix can be specified using either upper or lowercase letters.
Floating Point Numbers
They express numbers with decimals and/or exponents. They can include either a decimal point, an e character
(that expresses "by ten at the Xth height", where X is an integer value that follows the e character), or both a
decimal point and an e character:
3.14159 // 3.14159
6.02e23 // 6.02 x 10^23
1.6e-19 // 1.6 x 10^-19
3.0 // 3.0
These are four valid numbers with decimals expressed in C++. The first number is PI, the second one is the
number of Avogadro, the third is the electric charge of an electron (an extremely small number) -all of them
approximated- and the last one is the number three expressed as a floating-point numeric literal.
The default type for floating point literals is double. If you explicitly want to express a float or long double
numerical literal, you can use the f or l suffixes respectively:
3.14159L // long double
6.02e23f // float
Any of the letters that can be part of a floating-point numerical constant (e, f, l) can be written using either lower
or uppercase letters without any difference in their meanings.
Character and string literals
There also exist non-numerical constants, like:
'z'
'p'
"Hello world"
"How do you do?"
The first two expressions represent single character constants, and the following two represent string literals
composed of several characters. Notice that to represent a single character we enclose it between single quotes (')
and to express a string (which generally consists of more than one character) we enclose it between double quotes
(").
When writing both single character and string literals, it is necessary to put the quotation marks surrounding them
to distinguish them from possible variable identifiers or reserved keywords. Notice the difference between these
two expressions:
x
'x'
x alone would refer to a variable whose identifier is x, whereas 'x' (enclosed within single quotation marks) would
refer to the character constant 'x'.
Character and string literals have certain peculiarities, like the escape codes. These are special characters that are
difficult or impossible to express otherwise in the source code of a program, like newline (\n) or tab (\t). All of
them are preceded by a backslash (\). Here you have a list of some of such escape codes:
\n newline
\r carriage return
\t tab
\v vertical tab
\b backspace
\f form feed (page feed)
\a alert (beep)
\' single quote (')
\" double quote (")
\? question mark (?)
\\ backslash (\)
For example:
'\n'
'\t'
"Left \t Right"
"one\ntwo\nthree"
Additionally, you can express any character by its numerical ASCII code by writing a backslash character (\)
followed by the ASCII code expressed as an octal (base-8) or hexadecimal (base-16) number. In the first case
(octal) the digits must immediately follow the backslash (for example \23 or \40), in the second case
(hexadecimal), an x character must be written before the digits themselves (for example \x20 or \x4A).
String literals can extend to more than a single line of code by putting a backslash sign (\) at the end of each
unfinished line.
"string expressed in \
two lines"
You can also concatenate several string constants separating them by one or several blank spaces, tabulators,
newline or any other valid blank character:
"this forms" "a single" "string" "of characters"
Finally, if we want the string literal to be explicitly made of wide characters (wchar_t), instead of narrow characters
(char), we can precede the constant with the L prefix:
L"This is a wide character string"
Wide characters are used mainly to represent non-English or exotic character sets.
Boolean literals
There are only two valid Boolean values: true and false. These can be expressed in C++ as values of type bool by
using the Boolean literals true and false.