About the If Statement

The if statement, which is used to conditionally execute a statement or block of statements, has three basic forms. The first form (if) is used to execute a statement if an expression is true, otherwise, do nothing. The second form (if/else) is used to execute one of two separate statements depending on whether an expression is true or not. The final form (if/else if/else) essentially chains together a sequence of conditionals to determine which statement from a collection of statements should be executed.

Conditional	Statement
if	The first type of if statement has the format ``` if( expression )
statement For example, if(x>0)
y=sqrt(x); This statement first evaluates the expression x>0. If this expression is **true**, the statement y=sqrt(x); is executed. If the expression, x>0 is **false**, the **if** statement does nothing. Note that the statement y=sqrt(x); ends with a semicolon. All statements must be terminated with a semicolon unless the statement is a statement block as in the following example: if(x>0) {
y=sqrt(x);
z=x/(1+x);
} ``` Each statement in a statement block ends with a semicolon. However, the block itself does not include any terminating punctuation other than the closing brace. Wherever you can insert a statement, you can instead insert a statement block.
if/else	The second type of if statement has the format ``` if( expression )
statement1
else
statement2 For example, if(x>0)
y=sqrt(x);
else
y=0; The first part of this statement behaves just like the previous example. That is, the statement first evaluates the expression x>0. If this expression is **true**, the statement y=sqrt(x); is executed. However, if the expression is **false**, the statement y=0; is executed. Just as with the previous form of the **if** statement, each conditional statement can be a statement block. For example, if(x>0) {
y=sqrt(x);
z=x/(1+x);
}
else {
y=0;
z=0;
} ```
if/else if/else	The final type of if statement has the format ``` if( expression1 )
statement1
else if( expression2 )
statement2
. . .
else
statement3 There can be any number of else if parts in the full statement. Also, the final else is optional. For example, if(x>0)
y=sqrt(x);
else if(x==0)
y=-1;
else
y=0; ``` This statement first evaluates the expression x>0. If it is true, the statement y=sqrt(x); is executed. If the expression is false, the expression x==0 is evaluated. If this expression is true, then the statement y=-1; is executed. Finally, if none of the conditional expressions is true, the final statement, y=0;, is executed.
IF Primitive	You can create a conditional using the IF primitive instead of the if statement. The statement ``` if( expression )
statement is the same as IF( expression, statement ) Also, the statement if( expression )
statement1
else
statement2 is the same as IF( expression, statement1, statement2 ) Note that the word *IF* is in capital letters when using the **IF** primitive and it is in lower-case characters when using the **if** statement. For example, the statement if(x>0)
y=sqrt(x);
else
y=0; is the same as IF(x>0,y=sqrt(x),y=0); Using the **IF** primitive, you can further simplify this expression by bringing the y= out in front of **IF**: y=IF(x>0,sqrt(x),0); ```