The If Statement

Conditional statements help the computer or program to make a decision among alternative paths. It gives the possibility of different outcomes based on conditions.

If blocks

The simplest conditional statement is an if block, as shown:

When run, this will compare the value of the variable x to 100. If it is larger, "What a big number!" will be printed. Otherwise, nothing happens.

If-Else blocks

It is also possible to specify that something should happen if the condition is not true, as shown in this example:

As with the previous block, "What a big number!" will be printed if x > 100; otherwise, "That's not very big." will be printed.
An if block may have zero or one else sections but not more than one.

If-Else-If blocks

It is also possible to test multiple conditions with a single if block by adding else if clauses:

The block first checks if x > 100, printing "What a big number!" if it is. If it is not, it goes on to check if x = 42. If so, it prints "That's my lucky number." Otherwise, nothing happens.
An if block may have any number of else if sections. Conditions are evaluated top to bottom until one is satisfied, or until no more conditions are left.

If-Else-If-Else blocks

As shown here, if blocks may have both else if and else sections:

The else section guarantees that some action is performed, even if none of the prior conditions are true.
An else section may occur after any number of else if sections, including zero.