Control Structures in c++

A program is usually not limited to a linear sequence of instructions. During its process it may need to 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 have a concept of the compound-statement 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.

Conditional structure:

if and else:

Simple If:
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 10 only if the value stored in the x variable is indeed 10:

if (x == 10) cout << "x is 10";

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; }

If else:
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 statement2For example:

if (x == 10) cout << "x is 10"; else cout << "x is not 10";

prints on the screen x is 10 if indeed x has a value of 10, but if it has any other vaue, it prints out x is not 10.

Multiple else if:

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 { }.

another substitute of multiple else  if is Nested if. i.e if into the block of another if and if else can also b nested either in the block of if or in the block of if else.