1. 240-222 Computer Programming Techniques Semester 1, 1998
2. Control Flow Section 3.10
Objectives of these slides:
to introduce the main kinds of C control flow

2. Overview 1. Three/Four Kinds of Control Flow 2. The if Statement
3. The while Statement
4. Grades Average Example
5. Incrementing and Decrementing
continued

3. 6. Counter-controlled Repetition
7. The for Statement
8. The switch Statement
9. The break Statement
10. More Complex Input

4. 1. Three/Four Kinds of Control Flow
sequence
choice (branches)
e.g. if and switch statements
loops (iteration)
e.g. while and for statements
recursion

5. 2. The if statement
: scanf("%d", &grade); if (grade >= 60) printf("Passed\n"); else { printf("Failed\n"); printf("Repeat Course\n"); } scanf("%d", &grade); :

6. : if (temp < 0) setting = setting + 20; else if (temp < 10) setting = setting + 10; else setting = setting + 1; :

7. 2. The while statement (sum.c)
/* sum between 1 and 10 */ #include <stdio.h> int main() { int i = 1, sum = 0; while (i <= 10) { sum = sum + i; i = i + 1; } printf("Sum is %d\n", sum); return 0; }

8. Compile and run
$ gcc -Wall -o sum sum.c
$ sum Sum is 55

9. The while statement has the form:
while ( condition ) statement
The while keeps executing its (compound) statement until its condition becomes false
Deciding on a loop condition can be very difficult

10. Flow Chart for while condition true statement false
code after while-loop

11. Square Root Example
/* find square root of n using Newton Raphson method */ #include <stdio.h> int main() { float n, x0, eps = ; printf("Enter number\n"); scanf("%f", &n); x0 = n; while ( abs(x0*x0 - n)/n > eps) x0 = (x0 + n/x0)/2; printf("Square Root is %f\n", x0); return 0; }

12. 4. Grades Average Example
4.1. The Problem Statement
4.2. How to Start Writing a Program
4.3. Dissect the Problem Statement
4.4. Flesh out the Skeleton
4.5. Consider the Loop
4.6. What is an Average?
4.7. The Final Code: average.c

13. 4.1. The Problem Statement Sec. 3.9
Develop a class averaging program that will process an arbitrary number of grades each time the program is run.

14. 4.2. How to Start Writing a Program
Program = Algorithm + Data Structures
Algorithm:
a series of functions, each using a mix of sequencing, branches, loops, recursion and other functions
Data Structures:
they reflect the structure of the data in the Problem Statement
e.g int, float, arrays, etc

15. Consider the input/output requirements
Dissect (‘Pull apart’) the Problem Statement
Use top-down design (also called stepwise refinement)

16. 4.3. Dissect the Problem Statement
The inputs are the grades
The output is the average
Use a loop to read in ‘an arbitrary number of grades’

17. 4.4. Flesh Out the Skeleton
/* grades.c */ #include <stdio.h> int main() { /* declare the variables */ float average; /* calculate class average using a loop to read in the grades */ printf("Class average is %f\n", average); return 0; }

18. 4.5. Consider the Loop The big problem:
while ( condition?? ) { printf("Enter grade:"); scanf("%d", &grade); /* calculate the on-going average using grade */ }
The big problem:
what is the loop condition?

19. The Loop with a Condition
printf("Enter grade, -1 to end: "); scanf("%d", &grade); while (grade != -1) { /* calculate the on-going average using grade */ printf("Enter grade, -1 to end: "); scanf("%d", &grade); }

20. The Skeleton with the Loop
/* grades.c */ #include <stdio.h> int main() { /* declare the variables */ float average; int grade; printf("Enter grade, -1 to end: "); scanf("%d", &grade); :
continued

21. while (grade. = -1) { /. calculate the on-going average using grade
while (grade != -1) { /* calculate the on-going average using grade */ printf("Enter grade, -1 to end: "); scanf("%d", &grade); } printf("Class average is %f\n",average); return 0; }

22. 4.6. What is an Average? average = total of grades / number of grades
Use two new variables:
total counter

23. 4.7. The Final Code: average.c Sec. 3.9
/* Class Average Program */ #include <stdio.h> int main() { float average; int counter, grade, total; /* initialization phase */ total = 0; counter = 0; :
continued

24. /* looping phase */ printf("Enter grade, -1 to end: "); scanf("%d", &grade); while(grade != -1) { total = total + grade; counter = counter + 1; printf("Enter grade, -1 to end: "); scanf("%d", &grade); } : :
continued

25. /. termination phase. / average = (float) total / counter; /
/* termination phase */ average = (float) total / counter; /* can you see a bug ? */ printf("Class average is %.2f", average); return 0; }

26. Compile and run $ gcc -Wall -o average average.c
$ average Enter grade, -1 to end: 75 Enter grade, -1 to end: 94 Enter grade, -1 to end: 6 Enter grade, -1 to end: -1 Class average is 58.33

27. 5. Incrementing and Decrementing
Add 1 to c by writing:
c = c + 1;
Also: c += 1;
Also: c++;
Also: ++c;

28. Fig. 3.13
/* Preincrementing and postincrementing */ #include <stdio.h> int main() { int c; c = 5; printf("%d\n", c); printf("%d\n",c++); /*post-increment*/ printf("%d\n\n", c); :
continued

29. c = 5; printf("%d\n", c); printf("%d\n",++c); /. pre-increment
c = 5; printf("%d\n", c); printf("%d\n",++c); /*pre-increment*/ printf("%d\n", c); return 0; }

30. Output:

31. 5.2. Decrementing Take 1 from c by writing: Also: c -= 1; Also: c--;
c = c - 1;
Also: c -= 1;
Also: c--;
Also: --c;

32. 6. Counter-controlled Repetition Fig. 4.1.
/* Counter-controlled repetition */ #include <stdio.h> int main() { int counter = 1; while (counter <= 10) { printf ("%d\n", counter); counter; } return 0; }

33. Output
1 2 3 : 10

34. /. Counter-controlled repetition. / #include <stdio
/* Counter-controlled repetition */ #include <stdio.h> int main() { int counter = 0; while (++counter <= 10) printf ("%d\n", counter); return 0; }

35. 7. The for Statement 7.1. The Parts of a for
7.2. The for is a sort of while
7.3. Some Examples
7.4. Summation
7.5. Compound Interest

36. 7.1. The Parts of a for Sec. 4.4
#include <stdio.h> int main() { int cnt; for (cnt = 1; cnt <= 10; cnt++) printf("%d\n", cnt); return 0; }
the increment occurs after the loop body has been executed
test for
continuing
initial
value
final value
increment

37. Flow Chart for for-loop
initial value
test for continuing
true
body (e.g. printf)
increment
false
code after for-loop

38. 7.2. The for is a sort of while
for (expr1; expr2; expr3) statement;
is equivalent to:
expr1; while (expr2) { statement; expr3; }

39. 7.3. Some Examples for(i = 7; i <=77; i += 7) statement;

40. x =2; y = 10; for(j = x; j <= 4*x*y; j += y/x) statement;
equivalent to:
for(j = 2; j <= 80; j += 5) statement;

41. for(j = 10; j > 20; j++) statement;
equivalent to:
j = 10;

42. 7.4. Summation fig. 4.5
/* Summation using a for-loop */ #include <stdio.h> int main() { int sum = 0, num; for (num = 2; num <= 100; num += 2) sum += num; printf("Sum is %d", sum); return 0; }

43. 7.5. Compound Interest Fig. 4.6
Amount = principal * (1 + rate)year

44. /. Calculating compound interest for 10 years. / #include <stdio
/* Calculating compound interest for 10 years */ #include <stdio.h> #include <math.h> int main() { int year; double amount, principal = , rate = 0.05; printf("%4s%21s\n", "Year", "Amount on deposit"); :
continued

45. for (year = 1; year <= 10; year++) { amount = principal
for (year = 1; year <= 10; year++) { amount = principal * pow(1 + rate, year); printf("%4d%10.2f\n", year, amount); } return 0; }

46. Some Points On p.864 of D&D: Also look at math.h in /usr/include
pow()'s arguments should be doubles,: if they are integer or float then they are cast into doubles (e.g. year).
On p.864 of D&D:
double pow(double x, double y);
Also look at math.h in /usr/include

47. Compile and run % gcc -Wall -o compound compound.c
% compound Year Amount on deposit : :

48. Explicit Libraries But on some machines: Must type:
% gcc -Wall -o compound compound.c ld: undefined: pow
Must type:
% gcc -Wall -o compound compound.c -lm

49. 8. The switch Statement 8.1. The Parts of a switch
8.2. Counting Letter Grades

50. 8.1. The Parts of a switch For multiple choices: condition c1 c2 c3
task-A
task-B
task-C

51. switch (condtion) { case c1 : task-A; break; case c2 : task-B; break; case c3 : task-C; break; : }

52. 8.2. Counting Letter Grades Fig. 4.7
/* Counting letter grades */ #include <stdio.h> int main() { int grade; int acount = 0, bcount = 0, ccount = 0, dcount = 0, fcount = 0; printf("Enter the letter grades.\n"); printf("Enter EOF to end.\n"); :
continued

53. while ((grade = getchar())
while ((grade = getchar()) != EOF) { 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; : :
continued

54. case 'F': case 'f': fcount; break; case '\n': case' ': break; default: printf("Incorrect letter grade entered."); printf(" Enter a new grade.\n"); break; } } : :
important
continued

55. printf("\nThe totals for each
printf("\nThe totals for each letter grade are:\n"); printf("A: %d\n", acount); printf("B: %d\n", bcount); printf("C: %d\n", ccount); printf("D: %d\n", dcount); printf("F: %d\n", fcount); return 0; }

56. Some Points Very common coding style:
while((grade = getchar()) != EOF) { : }
getchar() reads the next character as an integer and assigns it to grade
EOF is an integer constant representing the end-of-file value. Defined in stdio.h

57. The line includes the newline character (represented by '\n')
getchar() will only start reading a line of characters when it has been terminated with a return or EOF.
The line includes the newline character (represented by '\n')

58. Output from letter grades program
Enter the letter grades.
Enter EOF to end input. A B C C A X Incorrect letter grade entered. Enter a new grade. D :
I typed return and <ctrl>D
continued

59. Totals for each letter grade were: A: 2 B: 1 C: 2 D: 1

60. 9. The break Statement Sec. 4.9
Causes execution to jump to the next statement after the loop (or switch) containing the break.
while(1) { scanf("%lf", &x); if (x < 0.0) break; printf("%f\n", sqrt(x)); } /* break jumps to here */ :

61. 10. More Complex Input
If you want to read several values from a line, then use scanf(), and check its return value.
scanf() returns the number of values it has read in.
Example: assume each line contains two integers and a float:

62. Code Fragment
int d1, d2; float f; : while (scanf(“%d %d %f”, &d1, &d2, &f) == 3) /* do something with input values */ :

63. Note
The spaces between the numbers (including tabs and newlines) are ignored by scanf() when it is looking for things to read in.
e.g. could write: