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C - IF Statements

The ability to control the flow of your program, letting it make decisions on what code to execute, is valuable to the programmer. The if statement allows you to control if a program enters a section of code or not based on whether a given condition is true or false. One of the important functions of the if statement is that it allows the program to select an action based upon the user's input. For example, by using an if statement to check a user-entered password, your program can decide whether a user is allowed access to the program.

Without a conditional statement such as the if statement, programs would run almost the exact same way every time, always following the same sequence of function calls. If statements allow the flow of the program to be changed, which leads to more interesting code.

Here are the relational operators, as they are known, along with examples:

```>     greater than              5 > 4 is TRUE
<     less than                 4 < 5 is TRUE
>=    greater than or equal     4 >= 4 is TRUE
<=    less than or equal        3 <= 4 is TRUE
==    equal to                  5 == 5 is TRUE
!=    not equal to              5 != 4 is TRUE
```

It is highly probable that you have seen these before, probably with slightly different symbols. They should not present any hindrance to understanding. Now that you understand TRUE and FALSE well as the comparison operators, let us look at the actual structure of if statements.

Basic If Syntax

The structure of an if statement is as follows:

```if ( statement is TRUE )
Execute this line of code
```

Else

Sometimes when the condition in an if statement evaluates to false, it would be nice to execute some code instead of the code executed when the statement evaluates to true. The "else" statement effectively says that whatever code after it (whether a single line or code between brackets) is executed if the if statement is FALSE.

It can look like this:

```if ( TRUE ) {
/* Execute these statements if TRUE */
}
else {
/* Execute these statements if FALSE */
}
```

Else if

Another use of else is when there are multiple conditional statements that may all evaluate to true, yet you want only one if statement's body to execute. You can use an "else if" statement following an if statement and its body; that way, if the first statement is true, the "else if" will be ignored, but if the if statement is false, it will then check the condition for the else if statement. If the if statement was true the else statement will not be checked. It is possible to use numerous else if statements to ensure that only one block of code is executed.

Let's look at a simple program for you to try out on your own.

```#include <stdio.h>

int main()                            /* Most important part of the program!  */
{
int age;                          /* Need a variable... */

printf( "Please enter your age" );  /* Asks for age */
scanf( "%d", &age );                 /* The input is put in age */
if ( age < 100 ) {                  /* If the age is less than 100 */
printf ("You are pretty young!\n" ); /* Just to show you it works... */
}
else if ( age == 100 ) {            /* I use else just to show an example */
printf( "You are old\n" );
}
else {
printf( "You are really old\n" );     /* Executed if no other statement is */
}
return 0;
}
```