repetition and loop statementscmliu/pl/ntut-c06su/notes/ch5...2 mobile computing & software...

1Mobile Computing & Software Engineering LabMobile Computing & Software Engineering Lab

Repetition and Loop Statements

Chuan-Ming LiuComputer Science & Information Engineering

National Taipei University of TechnologyTaiwan


Outline • Repetition in Programs• Counting Loops and the while Statement• Computing a Sum or a Product in a Loop• The for Statement• Conditional Loops• Loop Design• Nested Loops• The do-while Statement and Flag• Illustration• How to Debug and Test Programs• Common Programming Errors


Repetition in Programs

• Three types of program control structure:– sequence, selection, repetition

• loop– a control structure that repeats a group of steps in a

program• C loop control statements

– while, for, and do-while• loop body

– the statements that are repeated in the loop


Determining the Need for Loops

• After you solve the sample case, ask yourself some of the following questions to determine 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 know

how long to keep repeating the steps?


Loop Choice Process


Loop Kinds


Counting Loops

• 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.


The while Statement – Example


The while Statement

• 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


Flowchart for a while Loop


Loop Control Variable • The loop control variable count_emp must be

(1) initialized, (2) tested, and (3) updated for the loop to 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 each loop

repetition (called an iteration or a pass).– Updating. count_emp is updated (its value increased by 1)

during each iteration.

• infinite loop– a loop that executes forever


Syntax of while Statement


A Sum or Product Using a Loop

• accumulator – a variable used to store a value being computed in

increments during the execution of a loop


Computing Company Payroll


Multiplying a List of Numbers


Compound Assignment Operators

• variable = variable op (expression);• variable op= expression;


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.

• Example– /* Display nonnegative numbers < max */

for (i = 0; i < max; i += 1)printf("%d\n", i);


Loop Using for Statement


Syntax – for Statement


Increment and Decrement Operators

• The increment operator ++ takes a single variable as its operand.

• for (counter = 0; counter < limit; ++counter)• prefix increment

– ++ is placed immediately in front of its operand– value of the expression is the variable’s value after

incrementing• postfix increment

– ++ comes immediately after the operand– expression’s value is the value of the variable before it is

incremented


Prefix and Postfix Increments


Using Increment and Decrement Operators

• avoid using the increment and decrement operators in complex expressions in which the variables to which they are applied appear more than once.

• Example:x = 5;i = 2;y = i * x + ++i;– Implementation dependent– 13 (2 * 5 + 3) or (3 * 5 + 3)


Function for Factorial


Celsius-to-Fahrenheit Conversion Table


Conditional Loops (1)

• Using a condition to control the loop─ The exact number of loop repetitions is

undetermined• Example

1. Print an initial prompting message.2. Get the number of observed values.3. while the number of values is negative

1) Print a warning and another prompting message.2) Get the number of observed values.


Conditional Loops (2)

• Exampleprintf("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 */

}


Program – Monitoring Gasoline Storage Tank (1)


Program – Monitoring Gasoline Storage Tank (2)


Execution of the Program


Sentinel-Controlled Loops

• One way to signal the program to stop reading and processing new data.

• sentinel value– an end marker that follows the last item in a list of data– The loop exit when the sentinel value is read

• Structure1. Get a line of data.2. while the sentinel value has not been encountered

3. Process the data line.4. Get another line of data.


Example – 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


Sentinel-Controlled while Loop


for Statement for 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);

}


Endfile-Controlled Loops (1)

• The return value of scanf is 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.


Endfile-Controlled Loops (2)

• To detect the end-of-file condition, scanfreturns as its result the value of the standard constant EOF

• Endfile-controlled loop:1. Get the first data value and save input status2. While input status does not indicate that end of

file has been reached3. Process data value4. Get next data value and save input status


Sum of Scores – Batch Version


Infinite Loops on Faulty Data• Assume the user responds to the prompt

Enter 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 inputif (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);

}


Nested Loops• Nested Loops consists of an outer loop with one

or more inner 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.

• Invalid Examplefor (i=0; i<5; i++){

sum =0;for(i=0; i<4; i++)

sum = sum + i;}


5.12Program to Process

Bald Eagle

Sightings for a Year


The do-while Statement

• A loop must execute at least one time.• Example

– Get a data value.– If data value isn’t in the acceptable range, go back to step 1.

• Implementationdo {

printf("Enter a letter from A through E> ");scanf("%c", &letter_choice);

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


do-while Statement Syntax


Flag-Controlled Loops

• Sometimes a loop repetition condition becomes so complex that placing the full expression in its usual spot is awkward

• In many case, the condition may be simplified by using a flag

• flag– a type int variable used to represent whether or not a

certain event has occurred• A flag has one of two values: 1 (true) and 0

(false)


Program Execution

• Execution of the get_int() function call– next_int = get_int(10, 20)

• 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

• The do-while is often the structure to choose when checking for valid input


Case Study – Collecting Area for Solar-Heated House

• Problem: estimate the appropriate size for the collecting area of a solar-heated house– Factors considered

• Average number of heating degree days for the coldest month of a year

• Heating requirement per square foot of floor space• Floor space• Efficiency of the collection method

– Accessing two files• hdd.txt: average heating degree days in the area for each

month• solar.txt: average solar insolation for each month


Analysis


Data Requirements• Inputs

– Average heating degree days file– Average solar insolation file– heat_deg_days– coldest_mon– solar_insol– heating_req– efficiency– floor_space

• Variables– energy_resrc

• Output– heat_loss– collect_area


Design – Initial Algorithm

1. Determine the coldest month and the average heating degree days for that month

2. Find the average daily solar insolation per ft2

for the coldest month3. Get from the user the other inputs

1. heating_req2. efficiency3. floor_space

4. Estimate the collecting area5. Display results


Structure Chart for Computing Solar Collecting Area Size


Program to Approximate Solar Collecting Area Size (1)


How to Debug and Test Programs

• There are three general categories of errors:– Syntax error– Run-time error– Logic error

• 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.


Using Debugger Programs• Single-step execution

– execute your program one statement at a time using debugger

• Trace your program’s execution and observe the effect of each C statement on variables you select

• A breakpoint is like a fence between two segments of a program.

• When the program stops at a breakpoint, you can examine the values of selected variables to determine whether the program segment has executed correctly.


Debugging without a Debugger

• Insert extra diagnostic calls to printf that 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.


Debugging without a Debugger (2)

• Examplewhile (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


Off-by-One Loop Errors• Common logic error of loops is a loop that

executes one more time or one less time than required

• Make sure that the initial and final values of the loop control variable are correct and that the loop repetition condition is right.– loop boundaries: initial and final values of the loop

control variable• Example

for (count = 0; count <= n; ++count)sum += count;


Common Programming Error (1)

• Remember to end the initialization expression and the loop repetition condition of for loop 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 brace encountered with the innermost structure.


Common Programming Errors (2)



• Be sure to verify that a loop’s repetition condition will eventually become false (0); otherwise, an infinite loop may result.

• An equality test mistyped as an assignment operationdo {

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

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



• Use a do-while only when there is no possibility of zero loop iterations.

• Do not use increment, decrement, or compound assignment operations as subexpressions in complex expressions– Difficult to read or even produce varying results in

different implementation of C• a *= b + c; is equivalent to a = a * (b + c);

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