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## 4.E. Conditional Statements 

A conditional statement selects a single statement of its component statements for execution. Pascal offers two kinds of conditional statements, the if and case statements. 

## 4.E.l The if statement 

The if statement specifies that a statement be executed only if a certain condition (Boolean expression) is true. If it is false, then either no statement or the statement following the symbol else is executed. 

The form of an if statement is: 

<img src='content_image/694718.jpg'>

The expression between the symbols if and then must be oftype Boolean. Note that the first form may be regarded as an abbreviation of the second when the alternative statement is the empty statement. Caution: there is never a semicolon before an else! Hence, the text: 

is incorrect. More deceptive is the text: 

if P then begin Sl; S2; S3 end; else S4 

if P then; begin Sl; S2; S3 end 

Here, the statement controlled by the if is the empty statement between the then and the semicolon; hence, the compound statement following the if statement will always be executed. 

The syntactic ambiguity arising from the construction: 

if expression1 then if expression2 then statement1 else statement2 

is resolved by interpreting this construction as equivalent to 

