CS201 - Repetition

1. CS201 - Repetition loops

2. Types Of Loops • Counting Loop (while, for) • Sentinel-Controlled Loop (while, for) • Endfile-Controlled Loop (while, for) • Input Validation Loop (do-while) • General Conditional Loop (while, for)

3. While Loop (same as Java) • while (condition) { Do Something } • When condition is true, Do Something and return to while. • When condition is false, skip and exit loop. • i = 0; • while ( i<10 ) • { printf(“%d “, i); i=i+1;}

4. 0 1 2 3 4 5 6 7 8 9

5. For Loop (almost like Java) • No internal type statement allowed! • for (initialization; test; update) { Do Something } • for(i=0; i<10; i=i+1) { printf(“%d\n”,i); } • (same output) • for (int j=0; j<10; j=j+1) { … } NOT ALLOWED IN C !

6. Interpretation • First the initialization expression(s) is(are) executed. • Then the test expression(s) is(are) evaluated. • If true, statements are executed, update expression(s) is(are) executed, and test is evaluated again. • If false, loop exits. • Last expression controls multiple expressions.

7. What Prints? int i=0, j=0; for (i=0, j=-2; i<8, j<2; i=i+1, j=j+2) printf(“%d %d\n”, i,j);

8. What Prints? int i=0, j=0; for (i=0, j=-2; i<8, j<2; i=i+1, j=j+2) printf(“%d %d\n”, i,j); 0 -2 1 0

9. Do While Loop (just like Java) • Do {something} while ( expression); • Always does “something” at least once! • Test expression. • If true, return and to something again. • If false, exit loop at that point.

10. What prints? int i=0; do { i=i+3; printf(“%d\n”, i); } while (i<12);

11. What prints? int i=1; do { i=i+3; printf(“%d\n”, i); } while (i<12); 4 7 10 13

12. Compound Assignment • Like Java. • x += 3; is the same as x = x + 3; • In general: • var op = expression; • Is the same as • var = var op expression; • += -= *= /= %=

13. Increment and Decrement Operators • Like Java • Prefix and postfix versions • ++i i++ • i++; is the same as i = i + 1; • j--; is the same as j = j – 1; • However k=++i; is different than k=i++;

14. Prefix increment • What prints? int i=0, j=0; printf(“%d\n”, i++); printf(%d\n”,++i); j = i++; printf(“%d\n,j);

15. Prefix increment • What prints? 0 2 2 int i=0, j=0; printf(“%d\n”, i++); printf(%d\n”,++i); j = i++; printf(“%d\n,j); What’s the value of i here?

16. Prefix increment • What prints? 0 2 2 int i=0, j=0; printf(“%d\n”, i++); printf(%d\n”,++i); j = i++; printf(“%d\n,j); What’s the value of i here? 3

17. Sentinel-Controlled Loops • Loops that run as long as needed. • Signal the end of the loop with a special value (negative for ages, etc.) • Get a line of data • While the sentinel value has not been encountered • Process the data line. • Get another line of data.

18. What prints? int i, sum=0; scanf(“%d”,&i); while(i!=-9) { sum+=i; scanf(“%d”,&i); } printf(“%d\n”, sum); /* input = 1 3 5 7 -9 */

19. What prints? int i, sum=0; scanf(“%d”,&i); while(i!=-9) { sum+=i; scanf(“%d”,&i); } printf(“%d\n”, sum); /* input = 1 3 5 7 -9 */ 16

20. What prints? int i, sum=0; scanf(“%d”,&i); while(i!=-9) { sum+=i; scanf(“%d”,&i); } printf(“%d\n”, sum); /* input = -9 */

21. What prints? int i, sum=0; scanf(“%d”,&i); while(i!=-9) { sum+=i; scanf(“%d”,&i); } printf(“%d\n”, sum); /* input = -9 */ 0

22. EOF • Many library functions return helpful data as the value of the function? • For example, scanf returns the number of successful conversions. • File functions can return a value equal to EOF (a special defined variable). • You can send this to scanf with ctrl-D (linux)

23. What prints? int i, sum=0,status=0; status = scanf(“%d”,&i); while(status!=EOF) { sum+=i; status = scanf(“%d”,&i); } printf(“%d\n”, sum); /* input = */ 2 4 6 ctl-d

24. What prints? int i, sum=0,status=0; status = scanf(“%d”,&i); while(status!=EOF) { sum+=i; status = scanf(“%d”,&i); } printf(“%d\n”, sum); /* input = */ 2 4 6 ctl-d 12

25. Nested Loops • Usually used to work with two dimensional arrays (later). • Simply put the definition for one loop in the body of another loop.

26. What Prints? 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);

27. What Prints? 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);

28. Debugging • Use a source level debugger as part of your IDE. • Use a command line debugger like gdb. • Add printf statements to trace execution. • (Be careful when removing the printf statements that you don’t introduce errors.)

29. ∞ Loops • It’s hard to distinguish between a complex calculation loop and an infinite loop without debugging statements.