Repetition Statements and Tracing Loops in CS1010

1. Lecturer’s slides http://www.comp.nus.edu.sg/~cs1010/ WEEK 4 Class Activities

2. CS1010 (AY2014/5 Semester 1) Week 4: Repetition Statements • Tracing whileLoop • Tracing for Loop • Warm-up: List a Range of Integers • Exercise #1: Sum of Multiples of 3 • Exercise #2: Asterisks • Tracing Nested Loop • Exercise #3: Prime Number • Testing and Debugging (running theme)

3. CS1010 (AY2014/5 Semester 1) Tracing while Loop (1/4) • Trace the following codes manually and write out their outputs (a) int a = 1; while (a*a < 100) { printf("%d ", a); a *= 2; } printf("\n"); 1 2 4 8 b=0, c=9 b=1, c=8 b=2, c=7 b=3, c=6 b=4, c=5 outside:b=5, c=4 (b) int b = 0,c = 9; while (b < c) { printf("b=%d,c=%d\n", b, c); b++; c--; } printf("outside: b=%d,c=%d\n", b, c);

4. CS1010 (AY2014/5 Semester 1) Tracing while Loop (2/4) • Example: Given a positive integer n, print out its digits from least significant to most significant. • Sample run: Enter a positive integer: 28943 3 4 9 8 2

5. CS1010 (AY2014/5 Semester 1) Tracing while Loop (3/4) • Example: Given a positive integer n, print out its digits from least significant to most significant. Week4_PrintDigits.c // Precond: n > 0 void print_digits(intn) { int digit; while (n > 0) { digit = n%10; printf("%d\n", digit); n /= 10; } }

6. CS1010 (AY2014/5 Semester 1) Tracing while Loop (4/4) Week4_PrintDigits.c // Precond: n > 0 void print_digits(intn) { int digit; while (n > 0) { digit = n%10; printf("%d\n", digit); n /= 10; } } What are the values of n and digit after exiting the loop?

7. CS1010 (AY2014/5 Semester 1) Tracing for Loop • Trace the following codes manually and write out their outputs (a) int i, sum = 0; for (i=0; i <= 10; i+=2) { sum += i; } printf("sum = %d\n", sum); sum = 30 i=1, sum=1 i=2, sum=3 i=4, sum=7 i=8, sum=15 i=16, sum=31 Final i=32 Final sum=31 (b) int i, sum = 0; for (i=1; sum < 20; i*=2) { sum += i; printf("i=%d,sum=%d\n", i, sum); } printf("Final i=%d\n", i); printf("Final sum=%d\n", sum);

8. CS1010 (AY2014/5 Semester 1) Warm-up: List a Range of Integers (1/3) • Ask the user for 2 integers: a (the lower limit), and b (the upper limit), and print the list of integers from a to b. • Write a function list_integers(int lower, int upper) • Main function given: #include <stdio.h> void list_integers(int, int); intmain(void) { inta, b; printf("Enter 2 integers a and b (a<=b): "); scanf("%d %d", &a, &b); list_integers(a, b); return 0; }

9. CS1010 (AY2014/5 Semester 1) Warm-up: List a Range of Integers (2/3) • What should be the pre-condition of list_integer()? // List integers in the range [lower, upper] // Precond: void list_integers(int lower, int upper) { ... } lower <= upper int num; for (num=lower; num<=upper; num++) { printf("%d", num); } printf("\n"); • Use a for loop to implement the function

10. CS1010 (AY2014/5 Semester 1) Warm-up: List a Range of Integers (3/3) • Now, use a while loop to implement the function, without using any local variables // List integers in the range [lower, upper] // Precond: void list_integers(int lower, int upper) { ... } lower <= upper while (lower <= upper) { printf("%d", lower); lower++; } printf("\n");

11. CS1010 (AY2014/5 Semester 1) Exercise #1: Sum of Multiples of 3 (1/2) • Modify the program Unit6_OddIntegers_v1.c to read a positive integer n and then compute the sum of all integers which are multiples of 3 between 1 and n inclusive using a for loop. Write a function called sum_multiples_of_3(int). • This problem can be solved with a formula, but we will use the for loop just for exercise. • Call this program SumMultiples3.c • Sample run: Enter a positive integer: 50 Sum = 408

12. CS1010 (AY2014/5 Semester 1) Exercise #1: Sum of Multiples of 3 (2/2) • How about using a while loop instead? • Pseudo-code using a while loop: precondition: n > 0 sum  0 i n while (i > 0) if iis a multiple of 3 sum  sum + i i  i - 1 return sum

13. CS1010 (AY2014/5 Semester 1) Exercise #2: Asterisks (1/2) • Write a program Asterisks.cto read an integer n and print a certain number of asterisks on a single line. Write a function print_asterisks(int). • If n is non-positive, then no asterisk should be printed. • Sample runs: Think! What is the relationship between n and the number of *? Enter n: 3 ***** Done! Enter n: 6 *********** Done! Enter n: -2 Done! Enter n: 10 ******************* Done!

14. CS1010 (AY2014/5 Semester 1) Exercise #2: Asterisks (2/2) • Write a program Asterisks.cto read an integer n and print a certain number of asterisks on a single line. Write a function print_asterisks(int). Pseudo-code: read input n; if n is non-positive print “Done!” and end program; m compute the number of asterisks given n print_asterisks(m) end program;

15. CS1010 (AY2014/5 Semester 1) Tracing Nested Loops (1/5) • Given the following 3 programs, hand trace each of them and write out the output without running the program.

16. CS1010 (AY2014/5 Semester 1) Tracing Nested Loops (2/5) Week4_NestedLoop1.c #include <stdio.h> int main(void) { int a, b; a = 1; while (a <= 4) { b = a + 3; while (b <= 10) { printf("a =%d, b = %d\n", a, b); b += 3; } a++; } return0; } a = 1, b = 4 a = 1, b = 7 a = 1, b = 10 a = 2, b = 5 a = 2, b = 8 a = 3, b = 6 a = 3, b = 9 a = 4, b = 7 a = 4, b = 10

17. CS1010 (AY2014/5 Semester 1) Tracing Nested Loops (3/5) Week4_NestedLoop2.c #include <stdio.h> int main(void) { int x, y; for (x=10; x<30; x+=5) for (y=x; y>4; y/=2) printf("x = %d, y = %d\n", x, y); return 0; } x = 10, y = 10 x = 10, y = 5 x = 15, y = 15 x = 15, y = 7 x = 20, y = 20 x = 20, y = 10 x = 20, y = 5 x = 25, y = 25 x = 25, y = 12 x = 25, y = 6

18. CS1010 (AY2014/5 Semester 1) Tracing Nested Loops (4/5) Week4_NestedLoop3.c #include <stdio.h> int main(void) { intp, q; for (p=0; p<10; p++) { if (p%2 == 0) { for (q=4; q>0; q--) printf("p = %d, q = %d\n", p, q); } else { for (q=p; q<20; q+=5) printf("p = %d, q = %d\n", p, q); } } return 0; }

19. CS1010 (AY2014/5 Semester 1) Tracing Nested Loops (5/5) Week4_NestedLoop3.c for (p=0; p<6; p++) { if (p%2 == 0) { for (q=4; q>0; q--) printf("p = %d, q = %d\n", p, q); } else { for(q=p; q<20; q+=5) printf("p = %d, q = %d\n", p, q); } } p = 0, q = 4 p = 0, q = 3 p = 0, q = 2 p = 0, q = 1 p = 1, q = 1 p = 1, q = 6 p = 1, q = 11 p = 1, q = 16 p = 2, q = 4 p = 2, q = 3 p = 2, q = 2 p = 2, q = 1 p = 3, q = 3 p = 3, q = 8 p = 3, q = 13 p = 3, q = 18 p = 4, q = 4 p = 4, q = 3 p = 4, q = 2 p = 4, q = 1 p = 5, q = 5 p = 5, q = 10 p = 5, q = 15

20. CS1010 (AY2014/5 Semester 1) Exercise #3: Prime Number • Primality test is a classic programming problem • Given a positive integer, determine whether it is a prime • A primenumber has two distinct factors (divisors): 1 and itself. Examples: 2, 3, 5, 7, 11, ... (Note: 1 is not a prime!) • Write a program PrimeTest.c. You should include a function is_prime(int). (What value should the function return?) • This exercise is mounted on CodeCrunch. Sample runs: Enter a positive integer: 131 131 is a prime. Enter a positive integer: 713 713 is not a prime.

21. CS1010 (AY2014/5 Semester 1) Things-To-Do • Revise • Chapter 4 Lessons 4.1 – 4.6, Beginning Decision Making • Deadline for Lab #1 • Deadline: 6 September 2014, Saturday, 9am • Lab #2 released • Deadline: 13 September 2014, Saturday, 9am • Preparation for next week • Chapter 6: Numeric Arrays • Continue to do practice exercises on CodeCrunch

22. CS1010 (AY2014/5 Semester 1) End of File