1 Flowchart notation and loops Implementation of loops in C –while loops –do-while loops –for loops Auxiliary Statements used inside the loops –break –continue –goto Today’s Material

2 Often you would need to repeatedly execute some statements as long as a condition is true Consider computing “a n “, where “a” is a real number and “n” is an integer >= 0 –E.g., Compute 3 5 –3 5 = 3*3*3*3*3 The Need For Loops

3 Keep a running power, initialized to 1 –power = 1 (a 0 =1) Multiply power with “a”, and count the number of times we have multiplied “power” with “a” –Initially count = 0 When “count” reaches “n”, that is, when we have multiplied “power” with “a” “n” times, we are done How to compute a n?

4 1.Prompt the user and get “a” and “n” 2.Set count to 0 3.Set power to 1 /* power = a 0 = 1 */ 4.repeat while (count < n) 4.1. power = power * a; /* power = a count now */ 4.2. count++; 5.Print power Algorithm for Computing a n

5 Flowchart for Computing a n count = 0 Start power = 1 count < n? Prompt the user and get “a” and “n” Print power no End yes power *= a; count++;

6 Clearly, we need to repeatedly execute steps 4.1 and 4.2 until “count” reaches “n” That is, we need to loop around steps 4.1, 4.2 as long as a condition is true C provides 3 looping constructs –while loops –do-while loops –for loops C Looping Constructs

7 while Statement The while loop keeps repeating an action until an associated test returns false Useful when the programmer does not know in advance how many times the loop will be iterated Syntax: while(expression) { statement1; statement2;... } expression Y N statement 1 statement 2...

8 Code for Computing a n int count; int n; double a; double power; printf(“Enter a: “); scanf(“%lf”, &a); printf(“Enter n: “); scanf(“%d”, &n); count = 0; power = 1; /* power = a 0 */ while (count < n){ power *= a; count++; } /* end-while */ printf(“a^n: %lf\n”, power); count = 0 Start power = 1 count < n? Prompt the user and get “a” and “n” Print power no End yes power *= a; count++;

9 Trace of the Code for a = 3, n = 5 int count; int n; double a; double power; printf(“Enter a: “); scanf(“%lf”, &a); printf(“Enter n: “); scanf(“%d”, &n); count = 0; power = 1; while (count < n){ power *= a; count++; } /* end-while */ printf(“a^n: %lf\n”, power); 0 1 count power 3 5 a n Test: 0 < 5?  True 1 3 Test: 1 < 5?  True 2 9 Test: 2 < 5?  True 3 27 Test: 3 < 5?  True 4 81 Test: 5 < 5?  False Test: 4 < 5?  True 5 243

10 Computing a n : Alternative Code int i; int n; double a; double power; printf(“Enter a: “); scanf(“%lf”, &a); printf(“Enter n: “); scanf(“%d”, &n); i = 0; power = 1; while (i < n){ power *= a; i++; } /* end-while */ printf(“a^n is: %lf\n”, power); int i; int n; double a; double power; printf(“Enter a: “); scanf(“%lf”, &a); printf(“Enter n: “); scanf(“%d”, &n); i = 0; power = 1; while (i++ < n) power *= a; printf(“a^n is: %lf\n”, power);

11 1.Prompt the user and get “n” 2.Set i to 1 /* Iteration variable */ 3.Set sum to 0 /* Running sum */ 4.repeat while (i <= n) 4.1. sum += i; 4.2. i++; 5.Print sum Computing N

12 Flowchart and Code for Computing N int i; int n; int sum = 0; printf(“Enter n: “); scanf(“%d”, &n); i = 1; while (i<= n){ sum += i; i++; } /* end-while */ printf(“Sum is: %d\n”, sum); i = 1 Start sum = 0 i <= n? Prompt the user and get “n” Print sum no End yes sum += i; i++;

13 Trace of the Code for n=5 int i; int n; int sum = 0; printf(“Enter n: “); scanf(“%d”, &n); i = 1; while (i<= n){ sum += i; i++; } /* end-while */ printf(“Sum is: %d\n”, sum); 1 0 i sum Test: 1 <= 5?  True 2 1 Test: 2 <= 5?  True 3 3 Test: 3 <= 5?  True 4 6 Test: 4 <= 5?  True 5 10 Test: 6 <= 5?  False Test: 5 <= 5?  True n

14 Computing N: Alternative Code int i; int n; int sum = 0; printf(“Enter n: “); scanf(“%d”, &n); i = 1; while (i<= n){ sum += i; i++; } /* end-while */ printf(“Sum is: %d\n”, sum); int i; int n; int sum = 0; printf(“Enter n: “); scanf(“%d”, &n); i = 1; while (i<= n) sum += i++; printf(“Sum is: %d\n”, sum);

15 Printing a Table of Squares | i | i*i | | 1| 1| | 2| 4| | 3| 9| | 4| 16| | 5| 25| | 6| 36| We want to print a table of squares for numbers 1, 2, 3, 4,.., n for some number “n” Here is how the table should look like for n = 6

16 Code for Printing a Table of Squares int i; int n; printf(“Enter n: “); scanf(“%d”, &n); printf(“ \n”); printf(“| i | i*i |\n”); printf(“ \n”); i = 1; while (i <= n){ printf(“|%5d|%5d|\n”, i, i*i); i++; } /* end-while */ printf(“ \n”); i = 1 Start i <= n? Prompt the user and get and “n” Print bottom line of the table no End yes print (i, i*i) i++; Print the heading

17 Trace of the code for n=4 printf(“ \n”); printf(“| i | i*i |\n”); printf(“ \n”); i = 1; while (i <= n){ printf(“|%5d|%5d|\n”, i, i*i); i++; } /* end-while */ printf(“ \n”); 1 i 4 n Test: 1 <= 4?  True | i | i*i | | 1| 1| | 2| 4| | 3| 9| | 4| 16| Test: 2 <= 4?  True 3 Test: 3 <= 4?  True 4 Test: 4 <= 4?  True 5 Test: 5 <= 4?  False

18 Another while Example int i = 0; printf(“How do you like C programming?\n”); while(i < 10){ printf(“Programming is fun!\n”); i++; } /* end-while */ Repeats 10 times (for i from 0 to 9) Prints the same message 10 times

19 Yet Another while Example int i = 20; printf(“How do you like C programming?\n”); while(i < 10){ printf(“Programming is fun!\n”); i++; } /* end-while */ Repeats 0 times (i is 20, not less than 10) Does not print any "... is fun" messages.

20 do while Statement do { statement1; statement2;... } while(expression); expression Y N statement 1 statement 2... While loop tests the loop condition at the beginning of the loop, before the first iteration begins Sometimes you want to test the loop condition at the end of the loop. In such cases do-while loops are used –This ensures that the loop body is run at least once Syntax:

21 Asking for a Password(1) Assume you want to repeatedly ask the user for the password until the user enters the password correctly /* Implementation using while */ #define PASSWORD int passwd; printf(“Enter the password: “); scanf(“%d”, &passwd); while (passwd != PASSWD){ printf(“Enter the password: “); scanf(“%d”, &passwd); } /* end-while */ printf(“Password is OK\n”); Clearly you want to ask for the password at least once! –Using a while loop, we have to repeat printf/scanf statements

22 Asking for a Password(2) Here is the same loop with do-while #define PASSWORD int passwd; do { printf(“Enter the password: “); scanf(“%d”, &passwd); } while(passwd != PASSWD); printf(“Password is OK\n”); This is cleaner compared to while loop

23 do while Example Asking for a positive integer int no; do { printf(“Enter a positive integer: “); scanf(“%d”, &no); } while(no <= 0);

24 Another do while Example char option = 'x'; do{ printf("Select an option: \n"); printf("(a) Calculate grades \n"); printf("(b) Calculate class average \n"); printf("(c) Print grades \n"); printf("(q) Quit \n"); option = getchar(); getchar(); /* Skip ‘\n’ */ if (option == 'a')... else if (option == 'b')... else if (option == 'c')... } while(option != ‘q’);

25 Another do while Example int i = 0; printf(“How do you like C programming?\n”); do { printf(“Programming is fun!\n”); i++; } while(i < 10); Repeats 10 times (for i from 0 to 9) Prints the same message 10 times

26 Yet Another do while Example int i = 20; printf(“How do you like C programming?\n”); do { printf(“Programming is fun!\n”); i++; } while(i < 10); Repeats once (for i == 20) Prints the same message once

27 What are the differences between the while and the do while statements? whiledo while Entry control structureExit control structure Loop may or may not be executedLoop is executed at least once

28 for Statement More frequently used Ideal for loops that have a “counting” variable –i.e., We need to loop a fixed number of times Is general enough to be used by other kinds of loops as well Syntax: for (initializing list; expression; altering list) { statement1; statement2;... }

29 for Statement Flowchart, and Equivalent while Statement for(initialize; check; modify) { statement1; statement2;... } modify check Y N initialize statement 1 statement 2... initialize; while(check) { statement1; statement2;... modify; }

30 Another for Example int count; for(count = 1; count <= 10; count++){ printf("%d ",count); } /* end-for */ printf("\n"); Printed output: Start count++ End Displaycount count <= 10 Y N count = 1 Problem: Print numbers from 1 to 10

31 Using for Statement for (i=0; i<N; i++) … For statement is usually the best choice for loops that “count up” (increment a variable) or “count down” (decrement a variable) Counting up from 0 to N-1 for (i=1; i<=N; i++) … Counting up from 1 to N for (i=N-1; i>=0; i--) … Counting down from N-1 to 0 for (i=N; i>=1; i--) … Counting down from N to 1

32 for Example: Compute a n power = 1; for(i=1; i<=N; i++){ power *= a; } /* end-for */ Start i++ End Displaypower i <= N Y N i = 1 power *=a; 1 1 i power 3 5 a n Test: 1 <= 5?  True 2 3 Test: 2 <= 5?  True 3 9 Test: 3 <= 5?  True 4 27 Test: 4 <= 5?  True 5 81 Test: 6 <= 5?  False Test: 5 <= 5?  True 6 243

33 Printing a Table of Squares printf(“ \n”); printf(“| i | i*i |\n”); printf(“ \n”); for (i=1; i <= n; i++){ printf(“|%5d|%5d|\n”, i, i*i); } /* end-while */ printf(“ \n”); | i | i*i | | 1| 1| | 2| 4| | 3| 9| | 4| 16| | 5| 25| | 6| 36| Output for n=6

34 Another for Example sum = 0; for(i=1; i<=N; i++){ sum += i; } Problem: Compute …+N sum=0; i=1; for(; i<=N; i++){ sum += i; } sum=0; for(i=1; i<=N;){ sum += i++; } Initialize (i=1), check(i<=N) and modify(i++) statements are all optional and can be omitted sum=0; i=1; for(; i<=N;){ sum += i++; }

35 It is possible to nest loops inside each other –Many applications require nesting of loops Example: Print the multiplication table Nested Loops | * | 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| | 1| 1| 2| 3| 4| 5| 6| 7| 8| 9| 10| | 2| 2| 4| 6| 8| 10| 12| 14| 16| 18| 20| | 3| 3| 6| 9| 12| 15| 18| 21| 24| 27| 30| | 4| 4| 8| 12| 16| 20| 24| 28| 32| 36| 40| | 5| 5| 10| 15| 20| 25| 30| 35| 40| 45| 50|

36 Printing the Multiplication Table int i, j; /* Print the header */ printf(“ \n”); printf(“| * |”); for (j=1; j<=10; j++) printf(“%3d|”, j); printf(“\n \n”); /* Print the table. i goes over the rows, j over the columns */ for (i=1; i <= 10; i++){ printf(“|%3d:|“, i); /* Print 1 row of the table for i */ for (j=1; j <= 10; j++){ printf(“%3d|”, i*j); } /* end-for-inner */ printf(“\n”); } /* end-for-outer */ /* Print the bottom line of the table */ printf(“ \n”);

37 We have seen that “break” transfers the control out of switch statements Similarly, when used within loops, “break” transfers the control out of the current loop code block break & continue in loops

38 Code for Computing the sum of a series of numbers int sum = 0; int n; while (1){ /* Infinite loop */ printf("Enter an int or -1 to stop: "); scanf("%d", &n"); if (n == -1) break; /* Get out of the loop */ sum += n; } /* end-while */ printf("Sum is %d\n", sum);

39 Code for Checking if a number “n” is prime or not int d; int n; printf(“Enter an integer: “); scanf(“%d”, &n); for (d=2; d<n; d++){ if (n%d == 0) break; } /* end-for */ if (d<n) printf(“n=%d is divisible by %d\n”, n, d); else printf(“n=%d is prime\n”, n);

40 break is particularly useful if the escape point is somewhere in the middle of the loop rather than at the beginning or at the end break (cont) while (1){ printf(“Enter a number or 0 to stop: “); scanf(“%d”, &n); if (n == 0) break; printf(“n=%d, n*n*n*=%d\n”, n, n*n*n); } /* end-while */

41 break transfers the control out of the innermost enclosing code block. –Thus when you have a nested loop, break only escapes one level of nesting break (cont) while (…){ … switch(…){ … break; /* takes us out of switch to (A) */ … } (A) } (B)

42 Transfers the control to the end of the loop Note that we are still inside the loop “continue” simply skips the rest of the statements in the loop body, and moves the control to the end continue int i; int n = 0; int sum = 0; while (n<10){ scanf(“%d”, &i); if (i == 0) continue; /* takes us to (B) */ n++; sum += i; /*(B)*/ } /* end-while */

43 Transfers the control to an arbitrary point in the code designated by a label –goto is strictly discouraged as it leads to spaghetti code, which is hard to understand and maintain –But it may be useful in certain situations goto while (…){ … switch(…){ … goto loop_done; /* break won’t work here, as it takes */ … /* us out of switch only */ } /* end-switch */ } /* end-while */ loop_done:

44 Infinite Loops A loop that iterates forever while (1){ … } do{ … } while (1); How do we get out of these loops then? –Simply have a “break” somewhere within the loop for (;;;){ … } while (1){ printf(“Enter a number or 0 to stop: “); scanf(“%d”, &n); if (n == 0) break; printf(“n=%d, n*n*n*=%d\n”, n, n*n*n); } /* end-while */

45 Occasionally we may want to combine several expressions together into a single statement –This is where we use the comma operator Syntax Comma Operator expr1, expr2, …, exprN; expr1 is evaluated, its value discarded expr2 is evaluated, its value discarded … exprN is evaluated, its value becomes the value of the statement

46 Comma Operator Example (1) i=1, j=2, k=i+j; is equivalent to ((i=1), (j=2), (k=i+j)); Evaluation proceeds left to right The result of the statement is k=i+j; which is 3

47 Comma Operator Example (2) sum=0; for(i=1; i<=N; i++) sum += i; Problem: Compute …+N for(sum=0, i=1; i<=N; i++) sum += i; for(sum=0, i=1; i<=N; sum+=i, i++) ; Empty statement: Loop body is empty