Chapter 5 Repetition and Loop Statements Instructor: Alkar & Demirer

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-2 Repetition in Programs In most software, the statements in the program may need to repeat for many times. –e.g., calculate the value of n!. –If n = 10000, it’s not elegant to write the code as 1*2*3*…* LoopLoop is a control structure that repeats a group of steps in a program. –Loop body –Loop body stands for the repeated statements. There are three C loop control statements: –whilefordo-while –while, for, and do-while.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-3 Flow Diagram of Loop Choice Process e.g., calculate the value of n! e.g., read the content in a file

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-4 Comparison of Loop Choices (1/2) KindWhen to UseC Structure Counting loopWe know how many loop repetitions will be needed in advance. while, for Sentinel- controlled loop Input of a list of data ended by a special value while, for Endfile- controlled loop Input of a list of data from a data file while, for

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-5 Comparison of Loop Choices (2/2) KindWhen to UseC Structure Input validation loop Repeated interactive input of a value until a desired value is entered. do-while General conditional loop Repeated processing of data until a desired condition is met. while, for

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-6 The while Statement in C The syntax of while statement in C: loop repetition condition while (loop repetition condition) statement Loop repetition conditionLoop repetition condition is the condition which controls the loop. trueThe statement is repeated as long as the loop repetition condition is true. infinite loopA loop is called an infinite loop if the loop repetition condition is always true.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-7 An Example of a while Loop Statement Loop repetition condition Loop control variable Loop control variable is the variable whose value controls loop repetition. In this example, count_emp is the loop control variable.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-8 Flowchart for a while Loop Loop repetition condition Statement

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-9 Compound Assignment Operators (1/2) The loop body usually consists of statements of the form: variable = variable op expression. –e.g., count_emp = count_emp + 1; compound assignment operatorsC provides compound assignment operators which enable a more concise notation for this kind of statements. –“variable op = expression” –“variable op = expression” is the same to “variable = variable op expression.”

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-10 Compound Assignment Operators (2/2) Simple Assignment Operators Compound Assignment Operators count_emp = count_emp + 1; count_emp += 1; time = time -1;time -= 1; product = product * item; product *= item; total = total / number; total /= number; n = n % (x+1);n %= x+1;

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-11 The for Statement in C The syntax of for statement in C: initialization expression for (initialization expression; loop repetition condition loop repetition condition; update expression update expression) statement initialization expressionThe initialization expression set the initial value of the loop control variable. loop repetition conditionThe loop repetition condition test the value of the loop control variable. update expressionThe update expression update the loop control variable.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-12 An Example of the for Loop Loop repetition condition Initialization Expression Update Expression count_emp is set to 0 initially. count_emp should not exceed the value of number_emp. count_emp is increased by one after each iteration.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-13 Increment and Decrement Operators The statements of increment and decrement are commonly used in the for loop. ++--The increment (i.e., ++) or decrement (i.e., --) operators are the frequently used operators which take only one operand. The increment/decrement operators increase or decrease the value of the single operand. i++ –e.g., for (int i = 0; i < 100; i++){ … } –The variable i increase one after each iteration.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-14 Comparison of Prefix and Postfix Increments The value of the expression (that uses the ++/-- operators) depends on the position of the operator. The value of j is increased The value of j is not increased

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-15 Sentinel-Controlled Loops Sometimes we may not know how many times the loop will repeat. sentinel valueOne way to do this is to choose a sentinel value as an end marker. sentinel value –The loop exits when the sentinel value is read. sentinel valueIf the user wish to exit the loop, he or she has to input the sentinel value. –It is similar to the “logout” function in many applications.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-16 An Example of Sentinel-Controlled while Loops If the user wish to exit the loop, he or she has to input -99.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-17 An Example of Endfile-Controlled Loops fscanffscanf is a function used to read file. EOF fscanfEOF stands for the special value of end-file returned by fscanf. This loop repeats until reading the end of the file.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-18 Nested Loops Usually used to work with two dimensional arrays (later). Nested loops consist of an outer loop with or more inner loops. Each time the outer loop is repeated, the inner loops are reentered –Their loop control expressions are reevaluated –All required iterations are performed again.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-19 Nested Loops outer loop inner loopsNested loops consist of an outer loop with one or more inner loops. e.g., for (i=1;i<=100;i++){ for(j=1;j<=50;j++){ … } The above loop will run for 100*50 iterations. Inner loop Outer loop

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-20 What is the Output? lcv1 = 0; sum = 0; while (lcv1 < 100) { lcv2 = 0; while (lcv2 < 100) { sum = sum + 1; lcv2++; } lcv1++; } printf(“Sum is %d\n”, sum); /* Output = */ Sum is 10000

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-21 What is the Output? int a=0, b=0, sum=0; for(a=0; a<6; a+=2) for(b=0; b>4; b--) sum=sum+1; printf(“%d”,sum); /* Output = */ Sum is 0

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-22 Do While statement Both the for statement and the while statement evaluate the loop condition before the first execution of the loop body. In most cases, this pretest is desirable and prevents the loop from executing when there may be no data items to process There are some situations, generally involving interactive input, when we know that a loop must execute at least one time.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-23 The do-while Statement in C The syntax of do-while statement in C: loop repetition condition do statement while (loop repetition condition); The statement is first executed. loop repetition conditionIf the loop repetition condition is true, the statement is repeated. Otherwise, the loop is exited.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-24 An Example of the do-while Loop /* Find even number input */ do{ printf(“Enter a value: ”); scanf(“%d”, &num); }while (num % 2 !=0) This loop will repeat if the user inputs odd number.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-25 Do-While Example do { printf("Enter a letter from A through E> "); scanf("%c", &letter_choice); }while (letter_choice ’E’);

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-26 Flag Controlled Loops Sometimes a loop repetition condition becomes so complex that placing the full expression in its usual spot is awkward. In many cases, the condition may be simplified by using a flag. while (flag) { …. } A flag is 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).

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-27 Endfile-controlled loops Many library functions return helpful data as the value of the function. For example, scanf returns the number of data items it actually obtained. num = scanf(“%d%d%d”, &n1, &n2, &n3); A data file is always terminated by an endfile character (EOF) that can be detected by the scanf and fscanf functions. You can write a program that processes a list of data of any length from a file without requiring a special sentinel value at the end of the data file.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-28 #include int main(void) { FILE *inp; int sum = 0, score, input_status; inp = fopen("scores.dat", "r"); input_status = fscanf(inp, "%d", &score); while (input_status != EOF) { printf("%d ", score); sum += score; input_status = fscanf(inp, "%d", &score); } printf("\nSum of exam scores is %d\n", sum); fclose(inp); return 0; } /* Output */ Sum of exam scores is 165

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-29 What is the Output? int i, sum=0,status=0; status = scanf(“%d”,&i); while(status!=EOF) { sum+=i; status = scanf(“%d”,&i); } printf(“%d\n”, sum); /* Input = */ ctl-d /* Output = */ 12

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-30 In class work (1/2) Write a program that prompts the user to input an integer n. Draw a triangle with n levels by star symbols. For example, n = 3, * ** *** After drawing the triangle, repeat the above process until the user input a negative integer.

Copyright ©2004 Pearson Addison-Wesley. All rights reserved.5-31 In class work #4 (2/2) An usage scenario: Please input: 2 * ** Please input: 3 * ** *** Please input: -9 Thank you for using this program.