Chapter 5 Repetition and Loop Statements Dr. J.-Y. Pan Dept. Comm. Eng. Nat. Chung Cheng Univ. [email protected]

Chapter 5Repetition and Loop Statements

Dr. J.-Y. Pan Dept. Comm. Eng.Nat. Chung Cheng Univ.http://[email protected]

3-2中正大學通訊工程系潘仁義老師 Advanced Network Technology Lab

5.1 Repetition in programs

Loop A control structure that repeats a group of steps in a

programLoop body

The statements that are repeated in the loopThree questions to determine whether loops will

be required in the general algorithm: (Figure5.1) 1. Were there any steps I repeated as I solved the

problem? If so, which ones? 2. If the answer to question 1 is yes, did I know in

advance how many times to repeat the steps? 3. If the answer to question 2 is no, how did I know how

long to keep repeating the steps?


Figure 5.1 Flow Diagram of Loop Choice Process



The while statement:

while (bool-expr) statement ;while (bool-expr) { stmts;}(1)First evaluates bool-expr;(2)If bool-expr’s value is TRUE, it executes

stmts,and back to step 1.(3)If bool-expr’s value is FALSE, it terminates.


5.2 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

Figure 5.2 shows a program fragment that computes and displays the gross pay for seven employees.


Figure 5.2 Program Fragment with a Loop


Figure 5.3 Flowchart for a while Loop


5.2 (cont) Counting Loops and the while Statement

Loop repetition condition The condition that controls loop repetition

Loop control variable The variable whose value controls loop

repetitionThe loop control variable must be

Initialization Testing Updating


5.3 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

Fig.5.4 Program to Compute Company Payroll


Figure 5.4 Program to Compute Company Payroll


Figure 5.4 Program to Compute Company Payroll (cont’d)

跑個GDB?



5.3 (cont) Compound Assignment Operators

Assignment statements of the form variable = variable op expression variable op = expression

如 += -= /= %= /=


5.4 The for Statement

Three loop control components Initialization of the loop variable Test of the loop repetition condition Change (update) of the loop control variable

Using a for statement in a counting loop.

(Figure 5.5)


Figure 5.5 Using a for Statement in a Counting Loop


The for statementThe for (loop) statement

for (init expr; loop condition; step expr) {

stmts;

}

Initial expression

Loop condition

Step expression

statements

Yes

No


An Example

#include <stdio.h>

int main(void) {

int i, N;

printf(“How many = ?”);

scanf (“%d”,&N);

/* Display N asterisks */

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

printf(“*”);

} // 跑個 GDB 吧…


Increment and decrement operators

The counting loops that you have seen have all included assignment expressions of form

counter = counter +1

or

counter +=1

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

…….. Side effecta change in the value of a variable as a result of carrying out an operation


Figure 5.6 Comparison of Prefix and Postfix Increments


Conditional loops

In many programming situations, you will not be able to determine the exact number of loop repetitions before loop execution begins.


5.5 (cont) Example 5.5(Figure 5.9)

The program is designed to assist in monitoring the gasoline supply in a storage tank at the Super Oil Company refinery.

The program is to alert the supervisor when the supply of gasoline in the tank falls below 10% of the tank’s 80,000 barrel storage capacity.

The barrel used in the petroleum industry equals 42 U.S. gallons


Figure 5.9 Program to Monitor Gasoline Storage Tank


Figure 5.9 Program to Monitor Gasoline Storage Tank (cont’d)


Figure 5.9 Program to Monitor Gasoline Storage Tank (cont’d)



There are three critical steps in Fig.5.9 that involve the loop control variable current: current is initialized to the starting supply in the

for statement initialization expression current is tested before each execution of the

loop body current is updated (by subtraction of the amount

removed) during each iteration


Loop design… An example on Ex 5.5 (Fig 5.9)

Fig5.1


Loop design Sentinel-Controlled Loops

A loop that process data until the sentinel value is entered has the form get a line of data (initialization) while the sentinel value has not been

encountered (a loop repetition condition) process the data line. get another line of data (update)

Sentinel valueAn end marker that follows the last item in a list of data



A program that calculates the sum of a collection of exam scores is a candidate for using a sentinel value.

Sentinel loop1. Initialize sum to zero2. Get first score3. while score is not the sentinel

4. Add score to sum5. Get next score

有兩次 Get

會很怪嗎 ?


Figure 5.10 Sentinel-Controlled while Loop


5.6 (cont) Using a for Statement to Implement a Sentinel Loop

The for statement form of the while loop in Fig.5.10

/* 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

Here is the pseudo code for an endfile-controlled loop: get the first data value and save input status while input status does not indicate that end of

file has been reached process data valueget next data value and save input status


Result value of scanf and fscanf

input_status = scanf(“%d%d%lf”,&a, &b, &c);number of data items it actually obtainednumber of data before encountering errorEOF: when detecting the endfile before get

(End of File)


Figure 5.11 Batch Version of Sum of Exam Scores Program


Nested loop

If a for loop is nested inside another for loop, the inner for loop is executed once for each iteration of the outer for loop

Each for loop must have its own (distinct) index variables so that the variables do not interfere with each other

You CANNOT use the same variable as the loop control variable of both an outerand an inner for loop in the same nest.


An Example

#include <stdio.h> main( ) { int I,j; for (i=1;i<=5 ;i++){ for(j=1;j<=5;j++){ printf (“(%d, %d)”, I, j); } printf( “\n”); } }


An Example

(1,1)(1,2)(1,3)(1,4)(1,5)

(2,1)(2,2)(2,3)(2,4)(2,5)

(3,1)(3,2)(3,3)(3,4)(3,5)

(4,1)(4,2)(4,3)(4,4)(4,5)

(5,1)(5,2)(5,3)(5,4)(5,5)


5.8 The do-while statement and flag-controlled loops

The do-while (loop) statement do statement; while ( bool-expr );

do { stmts; }while ( bool-expr );

statement

Bool-expr ?

NoYes



5.8 (cont) Flag-Controlled Loops for Input Validation

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) ( 或非 0 值 )0 (false)



Function get_int returns an integer value that is in the range specified by its two arguments.

The outer do-while structure implements the stated purpose of the function.

The type int variable error acts as a program flag.

error is initialized to 0 and is changed to 1 when an error is detected.


Figure 5.14 Validating Input Using do-while Statement


5.9 Problem Solving IllustratedCase Study ：

Collecting Area For Solar-Heated House

Problem An architect needs a program that can estimate

the appropriate size for the collecting area of a solar-heated house.

Considerate factorsAverage number of heating degree days for the

coldest month of a yearThe heating requirements per square foot of floor

spaceFloor spaceEfficiency of the collection method


5.9 (cont) Case Study ：Collecting Area For Solar-Heated House

Problem The program will have to access two data files

File hdd.txtContains numbers representing the average

heating degree days in the construction location for each of 12 months

File solar.txtContains the average solar insolation for each

month


Solar-heated house Heating degree * days

Heating requirementBTU / degree-day / ft^2

Floor space

Solar insolation 日射能量BTU / ft^2 / day

Efficiency

Area = heat loss / energy resource


Problem solving illustrated BTU, 英制熱量單位

使 1磅 (b) 水升高華氏 1度所需能量

日射能量



Design Refinement

1.1 Scan first value from heating degree days file into heat_deg_days, and initialize coldest_mon to 1.

1.2 Initialize ct to 21.3 Scan a value from the file into next_hdd, saving

status1.4 as long as no faulty data/end of file, repeat

1.5 if next_hdd is greater than heat_deg_days» 1.6 copy next_hdd into heat_deg_days» 1.7 copy ct into coldest_days

1.8 Increment ct1.9 Scan a value from the file into next_hdd, saving status



Design Refinement

4.1 Calculate heat_loss as the product of heating_req, floor_space, and heat_deg_days

4.2 Calculate energy_resrc as the product of efficiency, solar_insol, and the number of days in the coldest month

4.3 Calculate collect_area as heat_loss divided by energy_resrc. Round result to nearest whole square foot



Design Functions

nth_itemdays_in_month

Input file hdd.txt995 900 750 400 180 20 10 10 60 290 610 1051

Input file solar.txt500 750 1100 1490 1900 2100 2050 1550 1200 900

500 500


Figure 5.15 Structure Chart for Computing Solar Collecting Area Size


Figure 5.16 Program to Approximate Solar Collecting Area Size

好長哦…直接看程式


Figure 5.16 Program to Approximate Solar Collecting Area Size (cont’d)






5.10 How to Debug and Test Programs

Using debugger programs Execute programs one statement at a time

(single-step execution) Set breakpoints at selected statements when a

program is very long

Debugging without a debugger Insert extra diagnostic calls to printf that display

intermediate results at critical points


How to debug and test programs

Debugging without a debugger

Off-by-one loop errors


5.10 (cont) Off-by-One Loop Errors

A common logic error A loop executes one more time or one less time

Loop boundaries Initial and final values of the loop control

variable


補充資料

break 強制離開這層迴圈

continue 強制進行下次迴圈


5.11 Common Programming Errors

Do not confuse if and while statements if statement implement a decision step while/for statement implement a loop

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 (compound statements) 不要括錯了

Remember to provide a prompt for the users, when using a sentinel-controlled loop.

Make sure the sentinel value cannot be confused with a normal data item.

== and =


5.11 (cont) Common Programming Errors

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

Replace the segment with a while or for statement when adding an if statement. if (condition1)

do {

…

} while(condition1);


5.11 (cont) Common Programming Errors

Do not use increment, decrement, or compound assignment operators as subexpressions in complex expressions. (difficult to read)

Be sure that the operand of an increment or decrement operator is a variable.

Do not use a variable twice in an expression in which it is incremented/decremented

Documents

Chapter 5 Repetition and Loop Statements Dr. J.-Y. Pan Dept. Comm. Eng. Nat. Chung Cheng Univ. [email protected]