1. Beginning C For Engineers Fall 2005 Lecture 3: While loops, For loops, Nested loops, and Multiple Selection Section 2 – 9/14/05 Section 4 – 9/15/05 Bettina Schimanski

2. Homework  Read Chapter 4  HW 3  Don’t forget the classroom change next week: Section 2 (Wednesday): Sage 5101 Section 4 (Thursday): Walker 5113

3. More C Operators 8 int age; age = 8; age = age + 1; age 9

4. PREFIX FORM Increment Operator 8 int age; age = 8; ++age; age 9

5. POSTFIX FORM Increment Operator 8 int age; age = 8; age++; age 9

6. Decrement Operator 100 int dogs; dogs = 100; dogs--; --dogs; dogs 98 dogs

7.  When the increment (or decrement) operator is used in a “stand alone” statement solely to add one (or subtract one) from a variable’s value, it can be used in either prefix or postfix form  But… when the increment (or decrement) operator is used in a statement with other operators, the prefix and postfix forms can yield different results Which Form to Use dogs-- ; --dogs ; USE EITHER

8. The += operator  If you wish to add a value to a variable and put the result back in the variable, use the += operator  ex: int stuff = 4; stuff += 3; /* same as stuff = stuff + 3; stuff is now 7 */ Adds the value on the right to the variable on the left.

9. Other similar operators  As you might expect, there are also operators for -=, *=, /=, and %=. x += y; /* same as x = x + y; */ z *= 3; /* same as z = z * 3; */ y /= 2; /* same as y = y / 2; */ i %= 5; /* same as i = i % 5; */

10.  A loop is a repetition control structure.  It causes a single statement or block to be executed repeatedly What is a loop?

11. Loop constructs in C  while execute block of statements repeatedly as long as some condition is true condition is tested before block of statements are executed  for an abbreviation for a collection of statements that use a while loop for creating counting loops ideal when the number of iterations of the loop is “known” beforehand  do – while similar to while condition is tested after block of statements are executed

12. While Statement SYNTAX while ( Expression ) {.. // loop body. } NOTE: Loop body can be a single statement, a null statement, or a block.

13. Event-controlled Loops Keep processing data until a certain event happens, for example  a special value is entered (e.g. -1) to indicate that processing should stop  there is more data in the file  until the value of a flag changes 13

14. #include int main( ) /* Example from Lecture 2 */ { int total = 0, num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); /* Read until the user enters -1 */ while (num != -1) { total = total + num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); } printf( “Total = %d”, total); return 0; }

15. #include int main( ) { int total = 0, num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); /* Read until the user enters -1 */ while (num != -1) { total += num; printf( “Enter a number (-1 to stop ) ”); scanf(“%d”, &num); } printf( “Total = %d”, total); return 0; }

16. Flag-controlled Loops  Initialize a flag to 1(true) or 0 (false)  Use a meaningful name for the flag  A condition in the loop body changes the value of the flag  Test for the flag in the loop test expression

17. int numpos = 0; int isSafe = 1 /* initialize Boolean flag */ /*Loop while flag is true, i.e. while only positive numbers */ while (isSafe) { scanf(“%d”, &num); if ( num < 0 ) isSafe = 0; /* change flag value */ else numpos++; } printf(“%d positive numbers were entered\n”, numpos);

18. Count-controlled Loops contain:  An initialization of the loop control variable  An expression to test for continuing the loop  An update of the loop control variable to be executed with each iteration of the body  * Although count-controlled loops can be implemented using while loops, it is more common to use for loops for counting

19. Using For loops for Counting SYNTAX for ( initialization ; test expression ; update ) { 0 or more statements to repeat } initialization update test statement... done true false

20. Example of Repetition int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } initializationtestupdate

21. 21 Example of Repetition num int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } OUTPUT ?

22. Example of Repetition num OUTPUT 1 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); }

23. Example of Repetition num OUTPUT 1 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } true

24. Example of Repetition num OUTPUT 1 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato

25. Example of Repetition num OUTPUT 2 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato

26. Example of Repetition num OUTPUT 2 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato true

27. Example of Repetition num OUTPUT 2 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato 2 Potato

28. Example of Repetition num OUTPUT 3 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato 2 Potato

29. Example of Repetition num OUTPUT 3 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } true 1 Potato 2 Potato

30. Example of Repetition num OUTPUT 3 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato 2 Potato 3 Potato

31. Example of Repetition num OUTPUT 4 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato 2 Potato 3 Potato

32. Example of Repetition num OUTPUT 4 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } 1 Potato 2 Potato 3 Potato false

33. Example of Repetition num 4 int num; for ( num = 1 ; num <= 3 ; num++ ) { printf(“%d Potato\n”, num); } false When the loop control condition is evaluated and has value false, control passes to the statement following the for statement.

34. int count ; for ( count = 0 ; count < 4 ; count ++ ) { printf(“%d \n”, count); } printf( “Done\n”); Count-controlled Loop OUTPUT: 0 1 2 3 Done

35. int count ; for ( count = 0 ; count <= 4 ; count ++ ) { printf(“%d \n”, count); } printf( “Done\n”); Count-controlled Loop OUTPUT: 0 1 2 3 4 Done

36. Equivalence with while loop  The following while loop is equivalent to the previous for loop (on the previous slide) int count ; count = 0; /* initialization */ while (count <= 4) /* test expression */ { printf(“%d \n”, count); count++;/* update loop variable*/ } printf( “Done\n”);

37. Summing Values in Loops int num, count; int total = 0; for ( count = 0; count < 10; count++ ) { printf(“nter a numer:”; scanf (“%d”, &num); total += num; }

38. For Loop Variations  The counter variable can be initialized to anything.  The test expression can be any logical expression involving the counter variable.  The increment can be any value, positive or negative.  Examples: for (i = 0; i < 10; i += 2) for (k = 20; k > 0; k -= 1) for (j = 1; j != 6; j += 2) Warning: What happens in the last example?

39. What Is Displayed By Each Program Segment? for (i = 0; i < 10; i +=2) { printf(“%d\n”, i ); } for (j = 20; j >= 0; j -= 3) { printf(“%d\n”, j ); } 0246802468 20 17 14 11 8 5 2

40. Nested For Loops for (j = 0; j <= 3; j ++) { for (k = j; k < 5; k ++) { printf(“%d %d\n”, j, k); } printf (“\n”); } What is printed to the screen? 0 0 1 0 2 0 3 0 4 1 1 2 1 3 1 4 2 2 3 2 4 3 3 4

41. Multiple Selection  An algorithm may contain a series of decisions in which a variable or expression is tested Many if…else if… else statements Switch statement

42. Switch Statement case a case b case z default true false case a action(s) case b action(s) case z action(s) break;

43. /* count the letter grades */ switch(grade) { case ‘A’: aCount++; break; case ‘B’: bCount++; break; case ‘C’: cCount++; break; case ‘D’: dCount++; break; default: fCount++; } Place within a while loop to capture many grades:

44. Switch Statement  Each case can have one or more actions  Each break statement causes control to immediately exit the switch statement What happens if you leave out a break?

45. Error: What is printed out for bCount and cCount? /* count the letter grades */ switch(grade) { case ‘A’: aCount++; break; case ‘B’: bCount++; /* No break! */ case ‘C’: cCount++; break; case ‘D’: dCount++; break; default: fCount++; }

46. More than one case can have the same action(s). /* count the letter grades */ switch(grade) { case ‘A’: case ‘a’: aCount++; break; case ‘B’: case ‘b’: bCount++; break; case ‘C’: case ‘c’: cCount++; break; case ‘D’: case ‘d’: dCount++; break; default: fCount++; }

47. The Lab  Tracing code by hand  Summing, Average  Max/Min In there is INT_MAX and INT_MIN These defined constants are useful for initializing your min and max variables