a first book of ansi c fourth edition chapter 4 selection

A First Book of ANSI CFourth Edition

Chapter 4

Selection

A First Book of ANSI C, Fourth Edition 2

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


Introduction

• Flow of control refers 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– Selection is used 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


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)


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


• 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.


• 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.

•


• 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


• 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.

• .


• 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


• Comparing letters is essential in alphabetizing names or using characters to select a particular choice in decision-making situations.


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.


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.


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.


• 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.


• 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


• Expression Value Interpretation

• a > b 1 True

• i==j ︱︱ a < b ︱︱ complete 0 False

• a/b > 5 && i<=20 1 True


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.


• 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.


Table 4.6 Precedence of operators in C


• 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:


• (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

4.2 The if and if-else Statements

if (expression)

statement1;



4.2 The if and if-else Statements

No semicolon here One-way if statement


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;• }


Compound Statements

• Although only a single statement is permitted in an if statement, this statement can be a single compound statement


Compound Statements (continued)


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;


The if-else Statement

• The most commonly used if-else statement isif (expression)

statement1;

else

statement2;

– If the value of expression is 0 statement2, the statement after the reserved word else, is executed


The if-else Statement (continued)


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.


• The general form of the if-else statement is

• if (expression) statement1;

• else statement2;


• 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.


• 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.


• 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;


• 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.


• 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).


• 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.


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*/


• 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 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.


• 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


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.


• 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.


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);


• 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;• }


• Program 4.3 checks whether the value in tempType is f.

• If the value is f, the compound statement corresponding to the if part of the if-else statement is executed.

• Any other letter results in execution of the compound statement corresponding to the else part.


• Following is a sample run of Program 4.2.

• Enter the temperature to be converted: 212

• Enter an f if the temperature is in Fahrenheit

• or a c if the temperature is in Celsius: f

• The equivalent Celsius temperature is 100.00


The if-else Chain

• Nested if statement:if (expression1) statement1;else if (expression2) statement2; else statement3;

• Whether the indentation exists or not, the compiler will, by default, associate an else with the closest previous unpaired if, unless braces are used to alter this default pairing


Nested if Statements

• An if-else statement can contain simple or compound statements .

• Any valid C statement can be used , including another if-else statement .

• Thus , one or more if-else statements can be included within either part of an if-else statement .

• This construction is called an if-else chain .


• It is used extensively in applications programs . • Each condition is evaluated in order , and if any condition

is true the corresponding statement is executed and the remainder of the chain is terminated .

• The final else statement is only executed if none of the previous conditions are satisfied .

• This serves as a default or catchall case that is useful for detecting an impossible or error condition


The if-else Chain (continued)



• if-else chain:

if (expression1) statement1;else if (expression2) statement2;else statement3;


• As with all C statements , each individual statement can be a compound statement bounded by the braces { and } .

• To illustrate an if-else chain, Program 4.4 displays a person’s marital status corresponding to a letter input .

• The following letter codes are used :


Marital Status Input Code

Married M

Single S

Divorced D

Widowed W


Program 4.4• #include <stdio.h>• int main ( )• {• char marcode ;• printf (“Enter a marital code : ”) ;• scanf (“%c” , &marcode ) ;• if (marcode == ‘M’) • printf (“\nIndividual is married .”) ;• else if (marcode == ‘S’)• printf (“\nIndividual is single .”);


• else if (marcode == ‘D’)

• printf (“\nIndividual is divorced .\ );

• else if (marcode == ‘W’)

• printf (“\nIndividual is widowed .”);

• else

• printf (“\nAn invalid code was entered .”);

• return 0 ;

• }


Monthly Sales Income

greater than or equal to $50,000 $375 plus 16% of sales

less than $50,000 but greater than or equal to $40,000

$350 plus 14% of sales







less than $10,000 $200 plus 3% of sales



Program 4.5

• #include <stdio.h>

• int main ( )

• {

• float monthlySales ,income ;

• printf (“Enter the value of monthly sales :”);

• scanf (“%f”, &monthlySales );

• if (monthlySales >= 50000.00)

• income = 375.00 + .16 * monthlySales ;

• else if (monthlySales >= 40000.00)









• else income = 200.00 + .03 * monthlySales ;

• printf (“The income is $%7.2f”, income );

• return 0;

• }

• A sample run using Program 4.5 is illustrated below .

• Enter the value of monthly sales : 36243.89

• The income is $4674.27


4.4 The switch Statement

• The if-else chain is used in programming applications where one set of instructions must be selected from many possible alternatives .

• The switch statement provides an alternative to the if-else chain for cases that compare the value of an integer expression to a specific value .


The switch Statement

Terminated with a colon

default is optional

If the break statement was omitted, the following case would be executed


The switch Statement (continued)


• The switch statement uses four new keywords : switch , case , default , and break .

• Let’s see what each of these words does .

• The keyword switch identifies the start of the switch statement .

•


• The expression in parentheses following this word is evaluated , and the result of the expression is compared to various alternative values contained within the compound statement .

• The expression in the switch statement must evaluate to an integer result or a compilation error occurs .


• Internal to the switch statement , the keyword case is used to identify or label individual values that are compared to the value of the switch expression .

• The switch expression’s value is compared to each of these case values in the order that these values are listed until a match is found .

• When a match occurs , execution begins with the statement immediately following the match .


• Any number of case labels may be contained within a switch statement , in any order .

• If the value of the expression does not match any of the case values , however , no statement is executed unless the keyword default is encountered .


• The word default is optional and operates the same as the last else in an if-else chain .

• If the value of the expression does not match any of the case values , program execution begins with the statement following the word default .


• Once an entry point has been located by the switch statement , no further case evaluations are done ; all statements that follow within the braces are executed unless a break statement is encountered .

• This is the reason for the break statement , which identifies the end of a particular case and causes an immediate exit from the switch statement .


• Thus , just as the word case identifies possible starting points in the compound statement , the break statement determines terminating points .

• If the break statements are omitted , all cases following the matching case value , including the default case , are executed .


• switch(score)

• { case 5: printf(“Very good!”);

• case 4: printf(“Good!”);

• case 3: printf(“Pass!”);

• case 2: printf(“Fail!”);

• default : printf(“data error!”);

• }


• void main()• { int x=1,y=0,a=0,b=0;• switch(x)• { case 1:• switch(y)• { case 0: a++; break;• case 1: b++; break;• }• case 2: a++;b++; break;• case 3: a++;b++;• }• printf(“\na=%d,b=%d”,a,b);• }


• When you write a switch statement , you can use multiple case values to refer to the same set of statements ; the default label is optional .

• For example , consider the following :

•


• switch (number)

• {

• case 1 :

• printf (“Have a Good Morning\n”) ;

• break ;

• case 2 :

• printf (“Have a Happy Day\n”) ;

• break :

• case 3 : case 4 :case 5 :

• printf (“Have a Nice Evening\n”) ;

• }


• If the value stored in the variable number is 1 , the message Have a Good Morning is displayed .

• Finally , if the value of number is 3 or 4 or 5 , the last message is displayed .

• Since the statement to be executed for these last three cases is the same , the cases for these values can be “stacked together ” as is done in the example .


• Also , since there is no default , no message is printed if the value of number is not one of the listed case values .

• Although it is good programming practice to list case values in increasing order , this is not required by the switch statement .

• A switch statement can have any number of case values , in any order ; only the values being tested for need be listed .


• Program 4.6 uses a switch statement to select the arithmetic operation to be performed on two numbers depending on the value of the variable opselect .


Program 4.6• #include <stdio.h>• int main( )• {• int opselect ;• double fnum , snum ;• printf (“Please type in two numbers :”);• scanf (“%lf %lf”, &fnum , &snum ) ;• printf (“Enter a select code :”) ;• printf (“\n 1 for addition ”) ;• printf (“\n 2 for multiplication ”) ;• printf (“\n 3 for division : ”);• scanf (“%d”, &opselect );•


• switch (opselect)• {• case 1 ;• printf (“The sum of the numbers entered is %6.31f ”, fnum +

snum ) ;• break ;• case 2 ;• printf (“The product of the numbers entered is %6.31f ”, fnum *

snum ) ;• break ;• case 3 ;• printf (“The first number divided by the second is %6.31f ”, fnum

/ snum ) ;• break ; /* this break is optional */• } /* end of switch statement */• return 0 ;• } /* end of main */


• Program 4.6 was run twice . The resulting displays clearly identify the case selected . The results are

• Please type in two numbers : 12 3

• Enter a select code :

• 1 for addition

• 2 for multiplication

• 3 for division : 2


• The product of the numbers entered is 36.000• and • Please type in two numbers : 12 3• Enter a select code :• 1 for addition• 2 for multiplication• 3 for division : 3• The first number divided by the second is 4.000


• In reviewing Program 4.6 , notice the break statement in the last case .

• Although this break is not necessary , it is a good practice to terminate the last case in a switch statement with a break .

• This prevents a possible program error later , if an additional case is subsequently added to the switch statement .


• With the addition of a new case , the break between cases becomes necessary ; having the break in place ensures you will not forget to include it at the time of the modification .


• Since character data are always converted to their integer values in an expression , a switch statement can also be used to “switch” based on the value of a character expression .

• For example , assuming that choice is a character variable , the following switch statement is valid :


• switch (choice)• {• case ‘a’: case ‘e’: case ‘i’: case ‘o’: case ‘u’:• printf (“\nThe character in choice is a vowel”) ;• break ;• default :• printf (“\nThe character in choice is not a vowel”);• } /* end of switch statement */


The switch Statement (continued)


Case Study: Data Validation

• Defensive programming is a technique where the program includes code to check for improper data before an attempt is made to process it further– Checking user input data for erroneous or

unreasonable data is called input data validation

• Requirements:– Write a program to calculate the square root and the

reciprocal of a user-entered number. Validate that the number is not negative before attempting to take its square root, and that the number is not 0 before calculating the number’s reciprocal value.


Case Study: Data Validation (continued)


Common Programming Errors

• Using the assignment operator, =, in place of the relational operator, ==

• Letting the if-else statement appear to select an incorrect choice

• Nesting if statements without including braces to clearly indicate the desired structure

• Using a single & or | in place of the logical && and logical || operators, respectively


Common Compiler Errors


Summary

• Relational expressions, which are also called simple conditions, are used to compare operands

• Conditions can be constructed from relational expressions using C’s logical operators, &&, ||, and !

• A one-way if statement has the general formif (expression) statement;

• A compound statement consists of any number of individual statements enclosed within braces

• An if-else selects between two alternative statements based on the value of an expression


Summary (continued)

• An if-else statement can contain other if-else statements

• The if-else chain is a multiway selection statement

• The switch statement is a multiway selection statement; program execution is transferred to the first matching case and continues through the end of the switch statement unless an optional break statement is encountered

a first book of ansi c fourth edition chapter 4 selection

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