Chapter 5

Boolean expressions and Branching

Boolean Algebra

Boolean algebra is a simple but important concept used in computer science. In boolean algebra variable can only contain one of two possible values: TRUE or FALSE. Boolean algebra has several operators called LOGICAL OPERATORS They are: ==, <, >, <=, >=, !=, &&, ||, !

Unlike Pascal, ANSI C does not have a built in boolean data type. Instead, it uses integer variables to perform boolean operations. FALSE is defined as the number 0. TRUE is defined as the number -1. C provides the following boolean operators: ==, <, >, <=, >=, !=, &&, ||, !

"==" Equals

The "equals" operator is used for comparing two variables. Notice that this operator has two equals signs. It is very important that you include two equals signs when using this operator. If only one is included the compiler will think you are trying to make an assignment statement instead of a logical statement.

First	Operation	Second	Result
1	==	1	TRUE
5	==	1	FALSE

"<", ">", "<=" and ">=" Less then, Greater than, Less than or equal to, Greater than or equal to

The greater than and less than symbols are also used for comparing variables.

First	Operation	Second	Result
1	<	2	TRUE
1	<	0	FALSE
1	<	1	FALSE
2	>	1	TRUE
2	>	3	FALSE
2	>	2	FALSE
1	<=	2	TRUE
1	<=	0	FALSE
1	<=	1	TRUE
2	>=	1	TRUE
2	>=	3	FALSE
3	>=	3	TRUE

"!=" Not Equal to

This returns True if and only if both variables are different.

First	Operation	Second	Result
1	!=	0	TRUE
1	!=	1	FALSE

&& "And operator"

The && operator performs the same function as the AND operator in Pascal. It returns TRUE if and only if both the second and the first variables are true. If one of the two other variables happens to be FALSE, the result will be FALSE.

First	Operation	Second	Result
TRUE	&&	TRUE	TRUE
TRUE	&&	FALSE	FALSE
FALSE	&&	FALSE	FALSE
FALSE	&&	TRUE	FALSE

|| "OR operator"

The || performs the same funciton as the OR operator in Pascal. It returns TRUE if either one of the two variables is TRUE. OR only returns FALSE if both variables are FALSE.

First	Operator	Second	Result
TRUE	\|\|	TRUE	TRUE
TRUE	\|\|	FALSE	TRUE
FALSE	\|\|	FALSE	FALSE
FALSE	\|\|	TRUE	TRUE

! NOT

The NOT operator only takes one variable. It returns the complete opposite of the variable that follows it.

Operator	First	Result
!	TRUE	FALSE
!	FALSE	TRUE

THE IF STATEMENT

The format for the IF statement is: if( <Boolean Expression> ) { <body> }

Example

  if(10 > 1) {  
    puts("true"):
  }


Example (with ELSE clause):
  if(10 > 12) {
    puts("true");
  } else {
    puts("false");
  }

The SWITCH statement

The SWITCH statement can be used to compare a value with several other values.

  switch(x) {

    case 1 : printf("its one");
             break;

    case 2 : printf("its two");
             break;

    case 3 : printf("its three");
             break;

    case default : printf("not a choice"):
  }