true (any other value but zero) false (zero) expression Statement 2 Selection Selection allows you to choose between two or more alternatives. In C this means that the course of your executing program will depend on the result of an expression. true (any other value but zero) false (zero) expression Statement 2 Statement 1 Logical Flow

Selection Logical data in C - C recognizes zero as a false value and any other nonzero value is considered true. Logical Operators - logical operators form conditions or logical expressions. The not operator ( ! ) changes a true value (nonzero ) to false ( zero ) and a false value ( zero ) to true (one ).

Selection The and operator ( && ) is a binary operator with four distinct possible combinations of values in its operands. The or operator ( || ) is a binary operator with four distinct combinations of values in its operands.

Selection Short-circuit evaluation - C will stop evaluation when it knows for sure what the final result will be. false && ( anything ) true || ( anything ) after the first operand is evaluated and found to be false and the operator is the and operator ( && ) the second operand will not be evaluated ( this could cause unexpected results if the second operand has side effects ) Relational Operators - Relational operators support logical relations. They are all binary operators that accept two operands and compare them. The result is logical data, that is, it is always a zero or one.

Syntactical rules for if…else statements: Selection The if …. else Statement - An if…else statement is a composite statement used to make a decision between two alternatives. Syntax: if ( expression ) statement 1 else statement 2 The expression can be any C expression. After it has been evaluated, if its value is true (not zero ), statement 1 is executed: otherwise, statement2 is executed. It is impossible for both statements to be executed in the same evaluation. Syntactical rules for if…else statements: The expression must be enclosed in parentheses. No semicolon ( ; ) is needed for an if..else statement. Statement 1 and statement 2 may have a semicolon as required by their types. The expression can have a side effect. Both the true and false statements can be any statement (even another if…else statement) or can be a null statement.

expression statements Selection We can swap the position of statement 1 and statement2 if we use the complement of the original expressions. if ( x > y) printf( “ x is greater than y\n”) ; else printf(“ y is greater than x\n”) ; An if…else with a compound statement if ( x != y) { printf( “ x is not equal to y\n”) ; x = y; } printf(“ x is equal to y\n”) ; The semicolons belong to the expression statements not to the if…else statement Curly brackets

printf( “ x is either 8 or 9\n”) ; } Selection A null else statement: if ( x > 7 && x < 10) { printf( “ x is either 8 or 9\n”) ; } else is null

The compiler pairs this if and else! Selection Nested if statements - when an if…else is included within an if…else, it is known as a nested if. if ( x <= y) if ( x < y) printf( “ %d < %d\n”, x ,y); else printf( “ %d == %d\n”, x ,y); printf(“ %d > %d\n”, x ,y); Dangling else problem - This problem is created when there is no matching else for every if. Simple rule: else is always paired with the most recent unpaired if The compiler pairs this if and else!

Selection Multiway selection - multiway selection chooses among several alternatives. There are two different ways to implement multiway selection in C. The first is by using the switch statement. The other is a programming technique known as the else-if that provides a convenient style to nest if statements. The else-if -There is no such C construct as the else-if. Rather, it is a style of coding that is used when you need a multiway selection based on a value that is not integral. if ( score >= 90 ) grade = ‘A’ ; else if (score >= 80 ) grade = ‘B’ ; else if (score >= 70 ) grade = ‘C’ ; else if (score >= 60 ) grade = ‘D’ ; else grade = ‘F’ ; The else-if is used when: The selection variable is not an integral and The same variable is being tested in the expression

Selection The switch Statement - Switch is a composite statement used to make a decision between many alternatives. The selection condition must be one of the C integral types. Syntax: switch ( expression ) { case constant-1 : statement; statement; case constant-2 : statement; …… case constant-3 : statement; ……. case constant-n : statement; default : statement; } /* end switch */

Selection Syntactical rules for the switch statement: There must be at least one case statement. Each case expression is associated with a constant. The case expression is followed by a colon ( : ) and then the statement with which it is associated. There may be one or more statements for each case. The case label simply provides an entry point to start executing the code. Default is executed whenever none of the previous case values matched the value in the switch expression. The default is optional. When the statements associated with one case have been executed, the program flow continues with the statements for the next case unless a break statement is used. The break statement causes the program to jump out of the switch statement (goes to the closing brackets and continues with code following the switch