control struc

7/30/2019 Control Struc

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Copyright 2004 Pearson Addison-Wesley. All rights reserved. 5-1

Repetition in Programs Loops

Three types of program control structure:

sequence, selection, repetition

Loop: a control structure that repeats agroup of steps in a program

C loop control statements

while, for, and do-while

loop body: the statements that are

repeated in the loop


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Repetition in Programs (Cont)

After you solve the sample case, askyourself some of the following questions todetermine whether loops will be required in

the general algorithm: Were there any steps I repeated as I solved the

problem? If so, which ones?

If the answer to question 1 is yes, did I know in

advance how many times to repeat the steps?

If the answer to question 2 is no, how did I knowhow long to keep repeating the steps?


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Loop Kinds


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Counting Loops and the while Statement

counter-controlled loop (counting loop)

a loop whose required number of iterations can

be determined before loop execution begins

Set loop control variable to an initial value of 0

while loop control variable < final value

. . .Increase loop control variable by 1


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The while Statement


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The while Statement (Cont)

After executing the last step in the loop body,

control returns to the line beginning with

while and the condition is reevaluated.

loop repetition condition

the condition that controls loop repetition


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Figure 5.3 Flowchart for a while Loop


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The loop control variable count_emp must be (1)initialized, (2) tested, and (3) updated for the loopto execute properly. Initialization. count_emp is set to an initial value of 0

(initialized to 0) before the while statement is reached.

Testing. count_emp is tested before the start of eachloop repetition (called an iteration or apass).

Updating. count_emp is updated (its value increased

by 1) during each iteration. infinite loop

a loop that executes forever


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Computing a Sum or a Product in a Loop

accumulator

a variable used to store a value being computed

in increments during the execution of a loop


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Figure 5.4 Program to Compute Company

Payroll


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Multiplying a List of Numbers


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Compound Assignment Operators

variable =variable op(expression);

variable op=expression;


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The for Statement

Review of the three loop control

components

initialization of the loop control variable,

test of the loop repetition condition, and

change (update) of the loop control variable.

/* Display nonnegative numbers < max */

for (i = 0; i < max; i += 1)

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


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Figure 5.5 Using a for Statement in a

Counting Loop


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Increment and Decrement Operators

The increment operator++ takes a single variableas its operand.

for (counter = 0; counter < limit; ++counter)

prefix increment ++ is placed immediately in front of its operand

value of the expression is the variables value afterincrementing

postfix increment

++ comes immediately after the operand expressions value is the value of the variable before it

is incremented


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Figure 5.6 Comparison of Prefix and Postfix

Increments


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Increment and Decrement Operators (Cont)

avoid using the increment and decrementoperators in complex expressions in which thevariables to which they are applied appear morethan once.

Example:x = 5;

i = 2;

y = i * x + ++i; Implementation dependent

13 (2 * 5 + 3) or (3 * 5 + 3)


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Figure 5.7 Function to Compute Factorial


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Figure 5.8

Displaying

a Celsius-to-Fahrenheit

Conversion

Table


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Conditional Loops

1. Print an initial prompting message.

2. Get the number of observed values.

3. while the number of values is negative1) Print a warning and another prompting

message.

2) Get the number of observed values.


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Conditional Loops(Cont)

Example

printf("Enter number of observed values> ");

scanf("%d", &num_obs); /* initialization */

while (num_obs < 0) {

printf("Negative number invalid; try again> ");

scanf("%d", &num_obs); /* update */

}


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Figure 5.9

Program to

MonitorGasoline

Storage Tank


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Calculates the sum of scores

Sentinel Loop Initialize sum to zero.

Get first score.

while score is not the sentinel Add score to sum.

Get next score.

Incorrect Sentinel Loop Initialize sum to zero.

while score is not the sentinel Get score

Add score to sum


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Using a for Statement to Implement a

Sentinel Loop

/* Accumulate sum of all scores. */

printf("Enter first score (or %d to quit)> ", SENTINEL);

for (scanf("%d", &score);

score != SENTINEL;scanf("%d", &score))

{

sum += score;

printf("Enter next score (%d to quit)> ", SENTINEL);

}


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Endfile-Controlled Loops

The return value ofscanfis the number of data

items it actually obtained.

input_status = scanf("%d%d%lf", &part_id,

&num_avail, &cost); returns a result of 3 on success

if scanf runs into difficulty with invalid or

insufficient data, the function returns as its value

the number of data items scanned before

encountering the error or running out of data.


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Endfile-Controlled Loops (Cont)

To detect the end-of-file condition, scanfreturns as its result the value of the standardconstant EOF

Endfile-controlled loop: Get the first data value and save input status

While input status does not indicate that end offile has been reached Process data value

Get next data value and save input status


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Figure 5.11

Batch

Version ofSum of

Exam

ScoresProgram


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Infinite Loops on Faulty Data

assume the user responds to the promptEnter next score (-99 to quit)>

in Fig. 5.10 with the faulty data 7o Infinite loops!!

Changing the loop repetition condition to input_status == 1 To warn of bad input

if (input_status == EOF) {printf("Sum of exam scores is %d\n", sum);

} else {fscanf(inp, "%c", &bad_char);

printf("*** Error in input: %c ***\n", bad_char);

}


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Nested Loops

Do not use the same variable as the loop

control variable of both an outer and an

inner for loop in the same nest.


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The do-while Statement and Flag-

Controlled Loops

a loop must execute at least one time.

Example

Get a data value.

Ifdata value isnt in the acceptable range, go back tostep 1.

Implementation

do {

printf("Enter a letter from A through E> ");

scanf("%c", &letter_choice);

} while (letter_choice < 'A' || letter_choice > 'E');


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Flag-Controlled Loops for Input Validation

(Cont)

Execution results

Enter an integer in the range from 10 to 20 inclusive> @20

Invalid character >>@>>. Skipping rest of line.

Enter an integer in the range from 10 to 20 inclusive> 2o

Number 2 is not in range.

Enter an integer in the range from 10 to 20 inclusive> 20


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Figure 5.16

Program to

ApproximateSolar Collecting

Area Size (contd)


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Figure 5.16

Program to


Area Size(contd)


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Figure 5.16

Program to


Area Size (contd)


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How to Debug and Test Programs

The first step in locating a hidden error is to

examine the program output to determine

which part of the program is generating

incorrect results.


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Using Debugger Programs

single-step execution execute your program one statement at a time

Trace your programs execution and observe theeffect of each C statement on variables you select

A breakpoint is like a fence between twosegments of a program.

When the program stops at a breakpoint, you can

examine the values of selected variables todetermine whether the program segment hasexecuted correctly.


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Debugging without a Debugger

Insert extra diagnostic calls toprintfthat

display intermediate results at critical points

in your program.

By comparing these results at the end of a

run, you may be able to determine which

segment of your program contains bugs.


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Debugging without a Debugger (Cont)

Example

while (score != SENTINEL) {

sum += score;

if (DEBUG)

printf("***** score is %d, sum is %d\n", score, sum);

printf("Enter next score (%d to quit)> ", SENTINEL);

scanf("%d", &score); /* Get next score. */

}

turn your diagnostics on by inserting

#define DEBUG 1


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Off-by-One Loop Errors

loop boundaries

initial and final values of the loop controlvariable

Make sure that the initial and final values ofthe loop control variable are correct and thatthe loop repetition condition is right.

Example

for (count = 0; count


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Common Programming Errors

Remember to end the initialization expression and

the loop repetition condition with semicolons.

Remember to use braces around a loop body

consisting of multiple statements.while (x > xbig)

x -= 2;

++xbig;

The compiler will associate the first closing braceencountered with the innermost structure.


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Common Programming Errors (Cont)


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Be sure to verify that a loops repetition condition

will eventually become false (0); otherwise, an

infinite loop may result.

An equality test mistyped as an assignmentoperation

do {

. . .

printf("One more time? (1 to continue/0 to quit)> ");scanf("%d", &again);

} while (again = 1); /* should be: again == 1 */


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Use a do-while only when there is no

possibility of zero loop iterations.

a *= b + c; is equivalent to a = a * (b + c);

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