A First Book of ANSI C Fourth Edition

A First Book of ANSI CFourth Edition Chapter 4 Selection

Objectives • Relational Expressions • The if and if-else Statements • The if-else Chain • The switch Statement • Case Study: Data Validation • Common Programming and Compiler Errors A First Book of ANSI C, Fourth Edition

Introduction • Flow of controlrefers to the order in which a program’s statements are executed • Any algorithm can be built using combinations of four standardized flow of control structures: • Normal flow of control for all programs is sequential • Selectionisused to select which statements are performed next based on a condition • Repetition is used to repeat a set of statements • Invocation is used to invoke a sequence of instructions using a single statement, as in calling a function A First Book of ANSI C, Fourth Edition

4.1 Relational Expressions • Simplest decision structure: if (condition) statement executed if condition is true • The condition is evaluated to determine its numerical value, which is interpreted as either true (non-zero) or false (0) • If condition is “true” the statement following the if is executed; otherwise, this statement is not executed • The condition used in all of C’s if statements can be any valid C expression • Most commonly, a relational expression (can yield only 0 or 1) A First Book of ANSI C, Fourth Edition

Relational Expressions (continued) A First Book of ANSI C, Fourth Edition

Relational Expressions (continued) • Relational expressions are also known as conditions • A relational expression evaluates to 1 (true) or 0 (false) • The expression 3 < 4 has a value of 1 • The expression 2.0 > 3.3 has a value of 0 • The value of hours > 0 depends on the value of hours • Character data can also be compared using relational operators A First Book of ANSI C, Fourth Edition

Relational expressions are sometimes called conditions, and we will use both terms to refer to these expressions. • Like all C expressions, relational expressions are evaluated to yield a numerical result. • In the case of relational expressions, the value of the expression can only be an integer value of 1 or 0. • A condition that we would interpret as true evaluates to an integer value of 1, and a false condition results in an integer value of 0. A First Book of ANSI C, Fourth Edition

For example, because the relationship 3<4 in always true, the expression has a value of 1, and because the relationship 2.0>3.3 is always false, the expression has a value of 0. A First Book of ANSI C, Fourth Edition

This can be verified using the statements • printf(“The value of 3<4 is %d”, 3<4); • printf(“\nThe value of 2.0>3.3 is %d”, 2.0>3.3); • which results in the display • The value of 3<4 is 1 • The value of 2.0>3.3 is 0 A First Book of ANSI C, Fourth Edition

The value of a relational expression such as hours >0 depends on the value stored in the variable hours. • In addition to numerical operands, character data can also be compared using relational operators. • For example, in the ASCII code the letter A is stored using a code having a lower numerical value than the letter B, the code for a B is lower in value than the code for a C, and so on. • . A First Book of ANSI C, Fourth Edition

For character sets coded in this manner, the following expressions are evaluated as listed • Expression Value Interpretation • ‘A’ > ‘C’ 0 False • ‘D’ <= ’Z’ 1 True • ‘E’ == ’F’ 0 False • ‘G’ >= ‘M’ 0 False • ‘B’ != ‘C’ 1 True A First Book of ANSI C, Fourth Edition

Comparing letters is essential in alphabetizing names or using characters to select a particular choice in decision-making situations. A First Book of ANSI C, Fourth Edition

Relational Expressions (continued) A First Book of ANSI C, Fourth Edition

Logical Operators • In addition to using simple relational expressions as conditions, more complex conditions can be created using the logical operations AND,OR, and NOT. • These operations are represented by the symbols &&, ︱︱, and !, respectively. • When the AND operator, &&, is used with two simple expressions, the condition is true only if both expressions are true by themselves. A First Book of ANSI C, Fourth Edition

AND Operator • Thus, the compound condition • (age > 40) && (term < 10) • is true (has a value of 1) only if age is greater than 40 and term is less than 10. • Because relational operations have a higher precedence than logical operators, the parentheses in this logical expression could have been omitted. A First Book of ANSI C, Fourth Edition

OR Operator • The logical OR operator, ︱︱, is also applied between two expressions. • When using the OR operator, the condition is satisfied if either one or both of the two expressions are true. • Thus, the compound condition (age > 40) ︱︱ (term < 10) is true if either age is greater than 40, term is less than 10, or both conditions are true. A First Book of ANSI C, Fourth Edition

For the declarations • int i, j; • float a, b, complete; • the following represent valid conditions; • a > b • i ==j ︱︱a < b︱︱complete • a/b > 5 && i<=20 • Before these conditions can be evaluated, the values of a,b,i,j, and complete must be known. A First Book of ANSI C, Fourth Edition

Assuming • a = 12.0, b=2.0,i=15，j=30, and complete = 0.0 • the expressions yield the folowing results: • a > b • i==j︱︱a < b︱︱complete • a/b > 5 && i<=20 A First Book of ANSI C, Fourth Edition

Expression Value Interpretation • a > b 1 True • i==j︱︱a < b︱︱complete 0 False • a/b > 5 && i<=20 1 True A First Book of ANSI C, Fourth Edition

NOT Operator • The NOT operator is used to change an expression to its opposite state; that is, if the expression has any nonzero value (true), !expression produces a zero value (false). • For example, assuming the number 26 is stored in the variable age, the expression age> 40 has a value of zero (it is false), while the expression !(age>40) has a value of 1. • Since the NOT operator is used with only one expression, it is a unary operator. A First Book of ANSI C, Fourth Edition

The relational and logical operators have a hierarchy of execution similar to that of the arithmetic operators. • Table 4.6 lists the precedence of these operators in relation to the other operators we have used. A First Book of ANSI C, Fourth Edition

Table 4.6 Precedence of operators in C A First Book of ANSI C, Fourth Edition

As with all expressions, parentheses can be used to alter the assigned operator priority and improve the readability of relational expressions. • By evaluating the expressions within parentheses first, the following compound condition is evalusted as: A First Book of ANSI C, Fourth Edition

(6 * 3 == 36 / 2) ︱︱(13 < 3* 3 + 4) && ! (6-2<5) • (18 == 18) ︱︱(13 <9 + 4) && !(4 < 5) • 1︱︱(13 < 13) && !1 • 1︱︱0 && 0 • 1︱︱0 • 1 A First Book of ANSI C, Fourth Edition

4.2 The if and if-else Statements if (expression) statement1; A First Book of ANSI C, Fourth Edition

4.2 The if and if-else Statements No semicolon here One-way if statement A First Book of ANSI C, Fourth Edition

Program 4.1 • #define LIMIT 3000.0 • #include <stdio.h> • int main() • { • int idNum; • float miles; • printf(“Please type in car number and mileage:”); • scanf(“%d %f”, &idNum, &miles); • if(miles > LIMIT) • printf(“ Car %d is over the limit.\n”,idNum); • printf(“End of program output.\n”); • return 0; • } A First Book of ANSI C, Fourth Edition

Compound Statements • Although only a single statement is permitted in an if statement, this statement can be a single compound statement A First Book of ANSI C, Fourth Edition

Compound Statements (continued) A First Book of ANSI C, Fourth Edition

Compound Statements (continued) • For example, if(expression) { statement1; /*as many statements as necessary*/ statement2; /*can be placed within the braces*/ • /*each statement must end with ; */ • • statementn; } • For very short statements, you can code a complete if statement placed on a single line • if (grade > 69) ++passTotal; A First Book of ANSI C, Fourth Edition

The if-else Statement • The most commonly used if-else statement is if (expression) statement1; else statement2; • If the value of expression is 0 statement2, the statement after the reserved word else, is executed A First Book of ANSI C, Fourth Edition

The if-else Statement (continued) A First Book of ANSI C, Fourth Edition

4.2 The if-else Statement • The if-else statement directs the computer to select a sequence of one or more instructions based on the result of a comparison. A First Book of ANSI C, Fourth Edition

The general form of the if-else statement is • if (expression)statement1; • else statement2; A First Book of ANSI C, Fourth Edition

The expression is evaluated first. • If the value of the expression is nonzero, statement1 is executed. • If the value is zero, statement2, the statement after the reserved word else, is executed. A First Book of ANSI C, Fourth Edition

Thus, one of the two statements (either statement1 or statement2) is always executed depending on the value of the expression. • Notice that the tested expression must be put in parentheses and a semicolon is placed after each statement. A First Book of ANSI C, Fourth Edition

For clarity, the if-else statement may also be written on four lines using the form • if (expression) ← no semicolon here • statement1; • else ← no semicolon here • statement2; A First Book of ANSI C, Fourth Edition

The form of the if-else statement that is selected generally depends on the length of statements 1 and 2. • However, when using the second form, do not put a semicolon after the parentheses or the reserved word else. • The semicolons go only after the ends of the statements. A First Book of ANSI C, Fourth Edition

As an example, let us write an income tax computation program containing an if-else statement. • As preciously described, New Jersey state income tax is assessed at 2 percent of taxable income for incomes less than or equal to $20,000. • For taxable income greater than $20,000, state taxes are 2.5 percent of the income that exceeds $20,000 plus a fixed amount of $400(which is 2 percent of $20,000). A First Book of ANSI C, Fourth Edition

The expression to be tested is whether taxable income is less than or equal to $20,000. • if (taxable <= 20000.0) • taxes = 0.02 * taxable; • else • taxes = 0.025 * (taxable -2000.0) +400.0; • Program 4.2 illustrates the use of this statement in a complete program using named constants for the actual numerical values. A First Book of ANSI C, Fourth Edition

Program 4.2 • #include <stdio.h> • #define LOWRATE 0.02 /*lower tax rate */ • #define HIGHRATE 0.025 /*higher tax rate */ • #define CUTOFF 20000.0 /*cut off for low rate*/ • #define FIXEDAMT 400 /*fixed dollar amount for higher rate amounts*/ A First Book of ANSI C, Fourth Edition

int main () • { • float taxable, taxes; • printf (“Please type in the taxable income: ”); • scanf (“%f ”, &taxable); • if (taxable <= CUTOFF) • taxes = LOWRATE * taxable; • else • taxes = HIGHRATE * (taxable -CUTOFF) + FIXEDAMT; • printf (“Taxes are $%7.2f”, taxes); • return 0; • } A First Book of ANSI C, Fourth Edition

A blank line was inserted before and after the if-else statement to highlight it in the complete program. • We will continue to do this throughout the text to emphasize the statement being presented. • To illustrate this selection in action, Program 4.2 was run twice with different input data. A First Book of ANSI C, Fourth Edition

The results are • Please type in the taxable income: 10000. • Taxes are $ 200.00 • and • Please type in the taxable income: 30000. • Taxes are $ 650.00 A First Book of ANSI C, Fourth Edition

The if-else Statement (continued) A First Book of ANSI C, Fourth Edition

Compound Statements • Although only a single statement is permitted in both the if and else parts of the if-else statement, this statement can be a single compound statement. A First Book of ANSI C, Fourth Edition

The use of braces to enclose a set of individual statements creates a single block of statements, which may be used anywhere in a C program in place of a single statement. • The next example illustrates the use of a compound statement within the general form of an if-else statement. A First Book of ANSI C, Fourth Edition

Program 4.3 • #include <stdio.h> • int main() • { • char tempType; • float temp, fahren, Celsius; • printf(“Enter the temperature to be converted:”); • scanf(“%f”,&temp); • printf(“Enter an f if the temperature is in Fahrenheit”); • printf(“\n or a c if the temperature is in Celsius:”); • scanf(“\n%c”,&tempType); A First Book of ANSI C, Fourth Edition

if (tempType == ‘f’) • { • Celsius = (5.0/9.0) * (temp – 32.0); • printf(“\nThe equivalent Celsius temperature is %6.2f”, celsiur); • } • else • { • fahren = (9.0 / 5.0) * temp + 32.0; • printf(“\nThe equivalent Fahrenheit temperature is %6.2F”, fahren); • } • return 0; • } A First Book of ANSI C, Fourth Edition

A First Book of ANSI C Fourth Edition