Repetition

1. Repetition • Loops are common in programming (computers can do boring things over and over again) Type of loop When used C-structures (our tools) Counting We know how many loops while and for Sentinel- Input of data with a special while and for Controlled ending symbol Endfile-controlled Input of data until the file is ended while and for Input validation Repeted input if data is out of range do-while General Process data until desired condition while and for TDBA66, VT-03, Lecture - Ch. 5

2. while-statement while (expr) statement; next statement; • Conditional repetition: the condition (expr) is evaluated before the statement (often a compound statement) is either executed or not • If the value of (expr) is true then the statement is executed, otherwise control is tranfered to ”next statement • I.e. the statement inside the while-loop might not be executed • The value of (expr) should be false at some point; otherwise we have created an infinit loop TDBA66, VT-03, Lecture - Ch. 5

3. Ex. 1 and Ex. 2 factorial = 1.0; i=1; while (i++ < n) factorial *= i; ------------------------------------ lowercase-letter.cnt = 0; total_cnt = 0; while ((c = getchar()) != EOF) { if (c >= ’a’ && c <=’z’) ++lowercase_letter_cnt; ++total_cnt; } TDBA66, VT-03, Lecture - Ch. 5

4. cnt_char.c TDBA66, VT-03, Lecture - Ch. 5

5. do-statement do statement; while (expr); next statement; • The condition (expr) is tested after the statement is executed -> the statement is executed at least once TDBA66, VT-03, Lecture - Ch. 5

6. Compute average Write a program that reads real numbers from the key-board until the value zero is read and computes the average of the read numbers. #include <stdio.h> int main(void) { double sum=0.0, number; int n=0; /* counts the number of values read */ printf(”Type a number of real numbers. End by zero(0.0)\n”); do { scanf(”%lf”, &number); sum = sum +number; n++; } while (number != 0.0); if (n != 0) printf(”\nThe average is %.3f\n”, sum/n); else printf(”\nNo values are typed\n”); return (0); } TDBA66, VT-03, Lecture - Ch. 5

7. for-satsen for (expr1; expr2; expr3) statement; next statement; • Fixed number of loops Semantically the same as expr1; while (expr2) { statement expr3; } next statement i++ for (i=1; i<=5; i=i+1) printf(”2*%d=%d ”,2*i,i); TDBA66, VT-03, Lecture - Ch. 5

8. Examples i = 1; sum = 0; for (; i <= 10; ++i) sum += i; Does the same as sum=0; for (i=1; i <= 10; i++) sum = sum +i; As will this code do i = 1; sum = 0; for (; i <= 10; ) sum += i++; TDBA66, VT-03, Lecture - Ch. 5

9. The comma-operator • Lowest priority of all operators • It is left-right associative • x = (expr1 , expr2); • Means that expr1 is evaluated and than expr2. The value of whole expresion is the value of expr2 • Comma-operator is usually used in for-loops for (sum = 0, i = 1; i <= n; ++i) sum += i; for (sum = 0, i = 1; i <= n; sum += i, ++i); /* Note the empty statement */ for (sum = 0, i = 1; i <= n; ++i, sum += i); /* not same result */ TDBA66, VT-03, Lecture - Ch. 5

10. break & continue • Changes flow of control • break ends ”the current loop” or switch-statement • continue ends the current iteration and continues with the next iteration while (1) { /* 1 is always true */ scanf(”%lf”, &x); if (x < 0.0) break; printf(”%f\n”, sqrt(x)); } /* break jumps to here */ TDBA66, VT-03, Lecture - Ch. 5

11. Figure 5.11 Program Showing a Sentinel-Controlled Loop /* Compute the sum of a list of exam scores. */ #include <stdio.h> #define SENTINEL -99 int main(void) { int sum = 0, /* sum of scores input so far */ score; /* current score */ 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); } printf("\nSum of exam scores is %d\n", sum); return (0); } TDBA66, VT-03, Lecture - Ch. 5

12. Figure 5.12 Batch Version of Sum of Exam Scores Program /* * Compute the sum of the list of exam scores stored in the * file scores.dat */ #include <stdio.h> /* defines fopen, fclose, fscanf, fprintf, and EOF */ int main(void) { FILE *inp; /* input file pointer */ int sum = 0, /* sum of scores input so far */ score, /* current score */ input_status; /* status value returned by fscanf */ inp = fopen("scores.dat", "r"); printf("Scores\n"); for (input_status = fscanf(inp, "%d", &score); input_status != EOF; input_status = fscanf(inp, "%d", &score)) { printf("%5d\n", score); sum += score; } printf("\nSum of exam scores is %d\n", sum); fclose(inp); return (0); } TDBA66, VT-03, Lecture - Ch. 5

13. Nested loops • Inside a loop there might be another loop and an other….. Ex. 1 Write a program the computes and displays a multiplication table for numbers 1 to 12. The output should look like | 1 2 3 4 5 6 7 8 9 10 11 12 -|------------------------------------ 1| 1 2 3 4 5 6 7 8 9 10 11 12 2| 2 4 6 8 10 12 14 16 18 20 22 24 3| 3 6 9 12 15 18 21 24 27 30 33 36 And so on TDBA66, VT-03, Lecture - Ch. 5

14. peppar:~/c/Ckod> cat multplic.c #include <stdio.h> int main(void){ /* A program that prints the multiplication table for the numbers 1 to 12 */ int i, j; /* Print the heading */ printf("\n |"); for (i=1; i < 13; i++) printf("%3d", i); putchar('\n'); /* underline */ for (i=1; i<40; i++) putchar('-'); putchar('\n'); /* Print the table */ for (i=1; i<=12; i++) { printf("%3d|", i); for (j=1; j<=12; j++) printf("%3d", i*j); putchar('\n'); } /* end of for i-loop */ return 0; } TDBA66, VT-03, Lecture - Ch. 5