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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab Repetition in programs  Loop  A control structure that repeats a group of steps in a program  Loop body  The statements that are repeated in the loop  Three 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?

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-3 Figure 5.1 Flow Diagram of Loop Choice Process

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-4

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-5 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.

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 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.

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-7 Figure 5.2 Program Fragment with a Loop

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-8 Figure 5.3 Flowchart for a while Loop

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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 repetition  The loop control variable must be  Initialization  Testing  Updating

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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-11 Figure 5.4 Program to Compute Company Payroll

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-12 Figure 5.4 Program to Compute Company Payroll (cont’d) 跑個 GDB?

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-13

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Compound Assignment Operators  Assignment statements of the form  variable = variable op expression  variable op = expression  如 += -= /= %= /=

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 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)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-16 Figure 5.5 Using a for Statement in a Counting Loop

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-17 The for statement  The for (loop) statement for (init expr; loop condition; step expr) { stmts; } Initial expression Loop condition Step expression statements Yes No

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-18 An Example #include 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 …

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-19 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 effect a change in the value of a variable as a result of carrying out an operation

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-20 Figure 5.6 Comparison of Prefix and Postfix Increments

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-21 Conditional loops  In many programming situations, you will not be able to determine the exact number of loop repetitions before loop execution begins.

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-23 Figure 5.9 Program to Monitor Gasoline Storage Tank

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-24 Figure 5.9 Program to Monitor Gasoline Storage Tank (cont’d)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-25 Figure 5.9 Program to Monitor Gasoline Storage Tank (cont’d)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Example 5.5 (Figure 5.9)  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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-27 Loop design… An example on Ex 5.5 (Fig 5.9) Fig 5.1

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-28 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 value An end marker that follows the last item in a list of data

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Example 5.6 (Figure 5.10)  A program that calculates the sum of a collection of exam scores is a candidate for using a sentinel value.  Sentinel loop 1. Initialize sum to zero 2. Get first score 3. while score is not the sentinel 4. Add score to sum 5. Get next score 有兩次 Get 會很怪嗎 ?

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-30 Figure 5.10 Sentinel-Controlled while Loop

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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); }

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-32 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 value  get next data value and save input status

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-33 Result value of scanf and fscanf input_status = scanf(“%d%d%lf”,&a, &b, &c);  number of data items it actually obtained  number of data before encountering error  EOF: when detecting the endfile before get (End of File)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-34 Figure 5.11 Batch Version of Sum of Exam Scores Program

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-35 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 outer and an inner for loop in the same nest.

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-36 An Example #include main( ) { int I,j; for (i=1;i<=5 ;i++){ for(j=1;j<=5;j++){ printf (“(%d, %d)”, I, j); } printf( “\n”); }

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-37 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)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 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 ? No Yes

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-39

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Example 5.10 (Figure 5.14)  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.

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-42 Figure 5.14 Validating Input Using do-while Statement

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab Problem Solving Illustrated Case 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 factors  Average number of heating degree days for the coldest month of a year  The heating requirements per square foot of floor space  Floor space  Efficiency of the collection method

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Case Study ： Collecting Area For Solar-Heated House  Problem  The program will have to access two data files  File hdd.txt  Contains numbers representing the average heating degree days in the construction location for each of 12 months  File solar.txt  Contains the average solar insolation for each month

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-45 Solar-heated house Heating degree * days Heating requirement BTU / degree-day / ft^2 Floor space Solar insolation 日射能量 BTU / ft^2 / day Efficiency Area = heat loss / energy resource

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-46 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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Case Study ： Collecting Area For Solar-Heated House  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 2  1.3 Scan a value from the file into next_hdd, saving status  1.4 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 ct  1.9 Scan a value from the file into next_hdd, saving status

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Case Study ： Collecting Area For Solar-Heated House  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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (cont) Case Study ： Collecting Area For Solar-Heated House  Design  Functions  nth_item  days_in_month  Input file hdd.txt   Input file solar.txt 

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-50 Figure 5.15 Structure Chart for Computing Solar Collecting Area Size

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-51 Figure 5.16 Program to Approximate Solar Collecting Area Size 好長哦 … 直接看程式

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-52 Figure 5.16 Program to Approximate Solar Collecting Area Size (cont’d)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-53 Figure 5.16 Program to Approximate Solar Collecting Area Size (cont’d)

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-54 Figure 5.16 Program to Approximate Solar Collecting Area Size (cont’d)

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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-56 How to debug and test programs  Debugging without a debugger  Off-by-one loop errors

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 3-58 補充資料  break  強制離開這層迴圈  continue  強制進行下次迴圈

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab 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 =

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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 (condition 1 ) do { … } while(condition1);

中正大學通訊工程系 潘仁義老師 Advanced Network Technology Lab (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