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4

C Program�Control

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Not everything that can be counted counts, and�not every thing that counts can be counted.

Albert Einstein

Who can control his fate?

William Shakespeare

The used key is always bright.

Benjamin Franklin

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Intelligence… is the faculty of making artificial objects, especially tools to make tools.

Henri Bergson

Every advantage in the past is judged in the light�of the final issue.

Demosthenes

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OBJECTIVES

In this chapter you will learn:

The essentials of counter-controlled repetition.
To use the for and do...while repetition statements to execute statements in a program repeatedly.
To understand multiple selection using the switch selection statement.
To use the break and continue program control statements to alter the flow of control.
To use the logical operators to form complex conditional expressions in control statements.
To avoid the consequences of confusing the equality and assignment operators.

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4.1 Introduction

4.2 Repetition Essentials

4.3 Counter-Controlled Repetition

4.4 for Repetition Statement

4.5 for Statement: Notes and Observations

4.6 Examples Using the for Statement

4.7 switch Multiple-Selection Statement

4.8 do...while Repetition Statement

4.9 break and continue Statements

4.10 Logical Operators

4.11 Confusing Equality (==) and Assignment (=) Operators

4.12 Structured Programming Summary

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4.1 Introduction

This chapter introduces

Additional repetition control structures

for
do…while

switch multiple selection statement
break statement

Used for exiting immediately and rapidly from certain control structures

continue statement

Used for skipping the remainder of the body of a repetition structure and proceeding with the next iteration of the loop

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4.2 Repetition Essentials

Loop

Group of instructions computer executes repeatedly while some condition remains true

Counter-controlled repetition

Definite repetition: know how many times loop will execute
Control variable used to count repetitions

Sentinel-controlled repetition

Indefinite repetition
Used when number of repetitions not known
Sentinel value indicates "end of data"

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4.3 Counter-Controlled Repetition

Counter-controlled repetition requires

The name of a control variable (or loop counter)
The initial value of the control variable
An increment (or decrement) by which the control variable is modified each time through the loop
A condition that tests for the final value of the control variable (i.e., whether looping should continue)

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4.3 Counter-Controlled Repetition

Example:

int counter = 1; // initialization

while ( counter <= 10 ) { // repetition condition

printf( "%d\n", counter );

++counter; // increment

}

The statement

int counter = 1;

Names counter
Defines it to be an integer
Reserves space for it in memory
Sets it to an initial value of 1

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Outline

fig04_01.c

Definition and assignment are performed simultaneously

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4.3 Counter-Controlled Repetition

Condensed code

C Programmers would make the program more concise
Initialize counter to 0

while ( ++counter <= 10 )� printf( “%d\n, counter );

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Common Programming Error 4.1

Because floating-point values may be approximate, controlling counting loops with floating-point variables may result in imprecise counter values and inaccurate tests for termination.

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Error-Prevention Tip 4.1

Control counting loops with integer values.

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Good Programming Practice 4.1

Indent the statements in the body of each control statement.

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Good Programming Practice 4.2

Put a blank line before and after each control statement to make it stand out in a program.

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Good Programming Practice 4.3

Too many levels of nesting can make a program difficult to understand. As a general rule, try to avoid using more than three levels of nesting.

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Good Programming Practice 4.4

The combination of vertical spacing before and after control statements and indentation of the bodies of control statements within the control-statement headers gives programs a two-dimensional appearance that greatly improves program readability.

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Outline

fig04_02.c

for loop begins by setting counter to 1 and repeats while counter <= 10. Each time the end of the loop is reached, counter is incremented by 1.

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Fig. 4.3 | for statement header components.

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Common Programming Error 4.2

Using an incorrect relational operator or using an incorrect initial or final value of a loop counter in the condition of a while or for statement can cause off-by-one errors.

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Error-Prevention Tip 4.2

Using the final value in the condition of a while or for statement and using the <= relational operator will help avoid off-by-one errors. For a loop used to print the values 1 to 10, for example, the loop-continuation condition should be counter <= 10 rather than counter < 11 or counter < 10.

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4.4 for Repetition Statement

Format when using for loops

for ( initialization; loopContinuationTest; increment ) � statement

Example:

for( int counter = 1; counter <= 10; counter++ )

printf( "%d\n", counter );

Prints the integers from one to ten

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4.4 for Repetition Statement

For loops can usually be rewritten as while loops:

initialization;�while ( loopContinuationTest ) {� statement;� increment;�}

Initialization and increment

Can be comma-separated lists
Example:

for (int i = 0, j = 0; j + i <= 10; j++, i++)

printf( "%d\n", j + i );

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Software Engineering Observation 4.1

Place only expressions involving the control variables in the initialization and increment sections of a for statement. Manipulations of other variables should appear either before the loop (if they execute only once, like initialization statements) or in the loop body (if they execute once per repetition, like incrementing or decrementing statements).

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Common Programming Error 4.3

Using commas instead of semicolons in a for header is a syntax error.

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Common Programming Error 4.4

Placing a semicolon immediately to the right of a for header makes the body of that for statement an empty statement. This is normally a logic error.

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4.5 for Statement : Notes and Observations

Arithmetic expressions

Initialization, loop-continuation, and increment can contain arithmetic expressions. If x equals 2 and y equals 10

for ( j = x; j <= 4 * x * y; j += y / x )

is equivalent to

for ( j = 2; j <= 80; j += 5 )

Notes about the for statement:

"Increment" may be negative (decrement)
If the loop continuation condition is initially false

The body of the for statement is not performed
Control proceeds with the next statement after the for statement

Control variable

Often printed or used inside for body, but not necessary

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Error-Prevention Tip 4.3

Although the value of the control variable can be changed in the body of a for loop, this can lead to subtle errors. It is best not to change it.

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Fig. 4.4 | Flowcharting a typical for repetition statement.

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Outline

fig04_05.c

Note that number has a different value each time this statement is executed

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Good Programming Practice 4.5

Although statements preceding a for and statements in the body of a for can often be merged into the for header, avoid doing so because it makes the program more difficult to read.

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Good Programming Practice 4.6

Limit the size of control-statement headers to a single line if possible.

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Outline

fig04_06.c

(1 of 2 )

additional header

pow function calculates the value of the first argument raised to the power of the second argument

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Outline

fig04_06.c

(2 of 2 )

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Error-Prevention Tip 4.4

Do not use variables of type float or double to perform monetary calculations. The impreciseness of floating-point numbers can cause errors that will result in incorrect monetary values.

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4.7 switch Multiple-Selection Statement

switch

Useful when a variable or expression is tested for all the values it can assume and different actions are taken

Format

Series of case labels and an optional default case

switch ( value ){

case '1':

actions

case '2':

actions

default:

actions

}

break; exits from statement

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Outline

fig04_07.c

(1 of 4 )

Outline

EOF stands for “end of file;” this character varies from system to system

switch statement checks each of its nested cases for a match

break statement makes program skip to end of switch

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Outline

fig04_07.c

(2 of 4 )

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Outline

fig04_07.c

(3 of 4 )

default case occurs if none of the cases are matched

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Outline

fig04_07.c

(4 of 4 )

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Portability Tip 4.1

The keystroke combinations for entering EOF (end of file) are system dependent.

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Portability Tip 4.2

Testing for the symbolic constant EOF rather than –1 makes programs more portable. The C standard states that EOF is a negative integral value (but not necessarily –1). Thus, EOF could have different values on different systems.

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Common Programming Error 4.5

Forgetting a break statement when one is needed in a switch statement is a logic error.

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Fig. 4.8 | switch multiple-selection statement with breaks.

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Good Programming Practice 4.7

Provide a default case in switch statements. Cases not explicitly tested in a switch are ignored. The default case helps prevent this by focusing the programmer on the need to process exceptional conditions. There are situations in which no default processing is needed.

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Good Programming Practice 4.8

Although the case clauses and the default case clause in a switch statement can occur in any order, it is considered good programming practice to place the default clause last.

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Good Programming Practice 4.9

In a switch statement when the default clause is listed last, the break statement is not required. But some programmers include this break for clarity and symmetry with other cases.

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Common Programming Error 4.6

Not processing newline characters in the input when reading characters one at a time can cause logic errors.

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Error-Prevention Tip 4.5

Remember to provide processing capabilities for newline (and possibly other white-space) characters in the input when processing characters one at a time.

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4.8 do…while Repetition Statement

The do…while repetition statement

Similar to the while structure
Condition for repetition only tested after the body of the loop is performed

All actions are performed at least once

Format:

do {

statement;

} while ( condition );

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4.8 do…while Repetition Statement

Example (letting counter = 1):

do {

printf( "%d ", counter );

} while (++counter <= 10);

Prints the integers from 1 to 10

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Good Programming Practice 4.10

Some programmers always include braces in a do...while statement even if the braces are not necessary. This helps eliminate ambiguity between the do...while statement containing one statement and the while statement.

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Common Programming Error 4.7

Infinite loops are caused when the loop-continuation condition in a while, for or do...while statement never becomes false. To prevent this, make sure there is not a semicolon immediately after the header of a while or for statement. In a counter-controlled loop, make sure the control variable is incremented (or decremented) in the loop. In a sentinel-controlled loop, make sure the sentinel value is eventually input.

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Outline

fig04_09.c

increments counter then checks if it is less than or equal to 10

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Fig. 4.10 | Flowcharting the do...while repetition statement.

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4.9 break and continue Statements

break

Causes immediate exit from a while, for, do…while or switch statement
Program execution continues with the first statement after the structure
Common uses of the break statement

Escape early from a loop
Skip the remainder of a switch statement

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Outline

fig04_11.c

break immediately ends for loop

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4.9 break and continue Statements

continue

Skips the remaining statements in the body of a while, for or do…while statement

Proceeds with the next iteration of the loop

while and do…while

Loop-continuation test is evaluated immediately after the continue statement is executed

for

Increment expression is executed, then the loop-continuation test is evaluated

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Outline

fig04_12.c

continue skips to end of for loop and performs next iteration

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Software Engineering Observation 4.2

Some programmers feel that break and continue violate the norms of structured programming. Because the effects of these statements can be achieved by structured programming techniques we will soon learn, these programmers do not use break and continue.

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Performance Tip 4.1

The break and continue statements, when used properly, perform faster than the corresponding structured techniques that we will soon learn.

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Software Engineering Observation 4.3

There is a tension between achieving quality software engineering and achieving the best-performing software. Often one of these goals is achieved at the expense of the other.

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4.10 Logical Operators

&& ( logical AND )

Returns true if both conditions are true

|| ( logical OR )

Returns true if either of its conditions are true

! ( logical NOT, logical negation )

Reverses the truth/falsity of its condition
Unary operator, has one operand

Useful as conditions in loops

Expression Result

true && false false�true || false true

!false true

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Fig. 4.13 | Truth table for the && (logical AND) operator.

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Fig. 4.14 | Truth table for the logical OR (||) operator.

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Fig. 4.15 | Truth table for operator ! (logical negation).

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Performance Tip 4.2

In expressions using operator &&, make the condition that is most likely to be false the leftmost condition. In expressions using operator ||, make the condition that is most likely to be true the leftmost condition. This can reduce a program’s execution time.

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Fig. 4.16 | Operator precedence and associativity.

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4.11 Confusing Equality (==) and Assignment (=) Operators

Dangerous error

Does not ordinarily cause syntax errors
Any expression that produces a value can be used in control structures
Nonzero values are true, zero values are false
Example using ==:

if ( payCode == 4 )

printf( "You get a bonus!\n" );

Checks payCode, if it is 4 then a bonus is awarded

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4.11 Confusing Equality (==) and Assignment (=) Operators

Example, replacing == with =:

if ( payCode = 4 )

printf( "You get a bonus!\n" );

This sets payCode to 4

4 is nonzero, so expression is true, and bonus awarded no matter what the payCode was

Logic error, not a syntax error

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Common Programming Error 4.8

Using operator == for assignment or using operator = for equality is a logic error.

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4.11 Confusing Equality (==) and Assignment (=) Operators

lvalues

Expressions that can appear on the left side of an equation
Their values can be changed, such as variable names

x = 4;

rvalues

Expressions that can only appear on the right side of an equation
Constants, such as numbers

Cannot write 4 = x;
Must write x = 4;

lvalues can be used as rvalues, but not vice versa

y = x;

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Good Programming Practice 4.11

When an equality expression has a variable and a constant, as in x == 1, some programmers prefer to write the expression with the constant on the left and the variable name on the right (e.g. 1 == x as protection against the logic error that occurs when you accidentally replace operator == with =.

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Error-Prevention Tip 4.6

After you write a program, text search it�for every = and check that it is being used properly.

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Fig. 4.17 | C’s single-entry/single-exit sequence, selection and repetition statements.

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4.12 Structured Programming Summary

Structured programming

Easier than unstructured programs to understand, test, debug and, modify programs

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Fig. 4.18 | Rules for forming structured programs.

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Fig. 4.19 | Simplest flowchart.

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Fig. 4.20 | Repeatedly applying rule 2 of Fig. 4.18 to the simplest flowchart.

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Fig. 4.21 | Applying rule 3 of Fig. 4.18 to the simplest flowchart.

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Fig. 4.22 | Stacked, nested and overlapped building blocks.

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Fig. 4.23 | An unstructured flowchart.

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4.12 Structured Programming Summary

All programs can be broken down into 3 controls

Sequence – handled automatically by compiler
Selection – if, if…else or switch
Repetition – while, do…while or for

Can only be combined in two ways

Nesting (rule 3)

Stacking (rule 2)

Any selection can be rewritten as an if statement, and any repetition can be rewritten as a while statement

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