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CS1010: Programming Methodology comp.nus.sg/~cs1010/

CS1010: Programming Methodology http://www.comp.nus.edu.sg/~cs1010/. Week 5: Repetition Statements. Objectives: Understand the program control structure called loops Compare the different types of repetition structure. References: Chapter 4 Lessons 4.7 – 4.11. Week 5: Outline (1/2).

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CS1010: Programming Methodology comp.nus.sg/~cs1010/

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  1. CS1010: Programming Methodologyhttp://www.comp.nus.edu.sg/~cs1010/

  2. Week 5: Repetition Statements Objectives: • Understand the program control structure called loops • Compare the different types of repetition structure • References: • Chapter 4 Lessons 4.7 – 4.11 CS1010 (AY2013/4 Semester 1)

  3. Week 5: Outline (1/2) • Week 4 Exercise #3: NRIC Check Code • Loops! • The while Loop 3.1 Demo 3.2 Loop condition 3.3 Tracing • The do-while Loop • The for Loop 5.1 Odd Integers • Exercise #1: Sum of Multiples of 3 • Exercise #2: Asterisks CS1010 (AY2013/4 Semester 1)

  4. Week 5: Outline (2/2) • Common Errors • Some Notes of Caution • Exercise #3: Tracing Nested Loops • Using break in Loop • Using continue in Loop • Exercise #4: Prime Number (take-home) CS1010 (AY2013/4 Semester 1)

  5. 1. Week 4 Ex3: NRIC Check Code (1/3) Algorithm for NRIC check code NRIC consists of 7 digits. Eg: 8730215 Step 1: Multiply the digits with corresponding weights 2,7,6,5,4,3,2 and add them up. Eg: 82 + 77 + 36 + 05 + 24 + 13 + 52 = 16+49+18+0+8+3+10 = 104 Step 2: Divide step 1 result by 11 to obtain the remainder. Eg: 104 % 11 = 5 CS1010 (AY2013/4 Semester 1) Week5 - 5

  6. 1. Week 4 Ex3: NRIC Check Code (2/3) Algorithm for NRIC check code (cont…) Step 3: Subtract step 2 result from 11 Eg: 11 – 5 = 6 Step 4: Match step 3 result in this table for the check code Eg: The check code corresponding to 6 is ‘F’. Therefore, the check code for 8730215 is ‘F’. Sample run: Enter 7-digit NRIC number: 8730215 Check code is F CS1010 (AY2013/4 Semester 1) Week5 - 6

  7. 1. Week 4 Ex3: NRIC Check Code (3/3) Write a program Week4_NRIC.c to generate the check code given a 7-digit NRIC number. Your program should include a function char generateCode(int) that takes in a single integer (the NRIC number) and returns a character (which is the check code). You need to use the char type. (Explore this on your own.) A character constant is enclosed in single quotes (eg: 'A', 'Z'). The format specifier for char type is %c (to be used in a printf() statement). Do not use techniques that are not covered in class, such as array. Your program may be long now. You can write an improved version later. This is your take-home exercise. This exercise is mounted on CodeCrunch. CS1010 (AY2013/4 Semester 1) Week5 - 7

  8. Recall: Control Structures Sequence Selection if-else, switch Repetition CS1010 (AY2013/4 Semester 1) Week5 - 8

  9. 2. LOOPS! (1/2) “A program without a loop and a structure variable isn’t worth writing.” Alan J.Perlis Yale University The first recipient of ACM Turing Award • A loop is a statement whose job is to repeatedly execute some other statement(s). CS1010 (AY2013/4 Semester 1) Week5 - 9

  10. 2. LOOPS! (2/2) Each round of the loop is called an iteration. Loop condition loop body false cond? true Some statement(s) CS1010 (AY2013/4 Semester 1) Week5 - 10

  11. 2. Loop: Demo (1/3) • Keep prompting the user to input a non-negative integer, and output that integer. • Halt the loop when the input is negative. Enter a number: 12 You entered: 12 Enter a number: 0 You entered: 0 Enter a number: 26 You entered: 26 Enter a number: 5 You entered: 5 Enter a number: -1 • Key observations: • You keep repeating a task while certain condition is met, or alternatively, you repeat until the condition is not met. • You do not know beforehand how many iterations there will be. CS1010 (AY2013/4 Semester 1) Week5 - 11

  12. 2. Loop: Demo (2/3) Algorithm: read num if (num >= 0) { print the value entered read num } else end input request if (num >= 0) { print the value entered read num } else end input request .... Enter a number: 12 You entered: 12 Enter a number: 0 You entered: 0 Enter a number: 26 You entered: 26 Enter a number: 5 You entered: 5 Enter a number: -1 Same code repeated condition body CS1010 (AY2013/4 Semester 1) Week5 - 12

  13. 2. Loop: Demo (3/3) Week5_Read_print.c intmain(void) { int num; printf("Enter a number: "); scanf("%d", &num); while (num >= 0) { printf("You entered: %d\n", num); printf("Enter a number: "); scanf("%d", &num); } return 0; } false num >= 0? true printf … printf … scanf… CS1010 (AY2013/4 Semester 1) Week5 - 13

  14. 3. The whileLoop while ( condition ) { // loop body } false If condition is true, execute loop body; otherwise, terminate loop. cond? true Loop body CS1010 (AY2013/4 Semester 1)

  15. 3.1 The whileLoop: Demo (1/3) • Keep prompting the user to enter a non-negative integer. • Halt the loop when the input is negative, and output the maximum integer entered so far. Enter a number: 12 Enter a number:0 Enter a number: 26 Enter a number: 5 Enter a number: -1 The maximum number is 26 CS1010 (AY2013/4 Semester 1) Week5 - 15

  16. 3.1 The whileLoop: Demo (2/3) maxi = 0; read num; while (num >= 0) { if (maxi < num) maxi = num; read num; } print maxi; maxi = 0; read num; if (num >= 0) { if (maxi < num) maxi = num; read num; } else stop; if (num >= 0) { if (maxi < num) maxi = num; read num; } else stop; ... print maxi; CS1010 (AY2013/4 Semester 1) Week5 - 16

  17. 3.1 The whileLoop: Demo (3/3) Week5_Find_max.c intmain(void) { int num, maxi = 0; printf("Enter a number: "); scanf("%d", &num); while (num >= 0) { if (maxi < num) { maxi = num; } printf("Enter a number: "); scanf("%d", &num); } prinf("The maximum number is %d\n", maxi); return 0; } CS1010 (AY2013/4 Semester 1) Week5 - 17

  18. 3.2 whileLoop Condition (1/2) When the loop condition is always false, the loop body is not executed. Output: ? a = 2; b = 7; while (a == b) { print a; a = a + 2; } CS1010 (AY2013/4 Semester 1) Week5 - 18

  19. 3.2 whileLoop Condition (2/2) Output: ? a = 2; b = 7; while (a != b) { print a; a = a + 2; } CS1010 (AY2013/4 Semester 1) Week5 - 19 

  20. 3.3 Tracing whileLoop (1/4) • Trace the following codes manually and write out their outputs (assume all variables are of type int) (a) a = 1; while (a*a < 100) { printf("%d ", a); a *= 2; } printf("\n"); (b) 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); CS1010 (AY2013/4 Semester 1) 

  21. 3.3 Tracing whileLoop (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 CS1010 (AY2013/4 Semester 1)

  22. 3.3 Tracing whileLoop (3/4) • Example: Given a positive integer n, print out its digits from least significant to most significant. Week5_Print_digits.c // Precond: n > 0 void print_digits(intn) { int digit; while (n > 0) { digit = n%10; printf("%d\n", digit); n /= 10; } } CS1010 (AY2013/4 Semester 1)

  23. 3.3 Tracing whileLoop (4/4) Week5_Print_digits.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? CS1010 (AY2013/4 Semester 1) 

  24. 4. The do-whileLoop (1/2) do { // loop body } while ( condition ); Execute loop body at least once. Loop body cond? true false CS1010 (AY2013/4 Semester 1)

  25. 4. The do-whileLoop (2/2) do { // loop body } while ( condition ); Week5_Count_digits.c // Precond: n > 0 intcount_digits(intn) { int counter = 0; do { counter++; n /= 10; } while (n > 0); return counter; } • Example: Count the number of digits in an integer. CS1010 (AY2013/4 Semester 1)

  26. 5. The for Loop (1/2) for ( initialization; condition; update ) { // loop body } Initialization: initialize the loop variable Condition: repeat loop while the condition on loop variableis true Update: change value ofloop variable CS1010 (AY2013/4 Semester 1)

  27. 5. The for Loop (2/2) • Example: Print numbers 1 to 10 int n; for (n=1; n<=10; n++) { printf("%3d", n); } • Steps: • n=1; • if (n<=10) {printf(…);n++;Go to step 2} • Exit the loop CS1010 (AY2013/4 Semester 1)

  28. Week5_OddIntegers_v1.c #include <stdio.h> void print_odd_integers(int); intmain(void) { int num; printf("Enter a positive integer: "); scanf("%d", &num); print_odd_integers(num); return0; } // Precond: n > 0 void print_odd_integers(intn) { inti; for (i=1; i<=n; i+=2) printf("%d ", i); printf("\n"); } 5.1 The for Loop: Odd Integers (1/2) CS1010 (AY2013/4 Semester 1)

  29. Week5_OddIntegers_v2.c Week5_OddIntegers_v3.c // Precond: n > 0 void print_odd_integers(intn) { inti; for (i=1; i<=n; i++) if (i%2 != 0) printf("%d ", i); printf("\n"); } // Precond: n > 0 void print_odd_integers(intn) { for ( ; n > 0; n--) if (n%2 != 0) printf("%d ", n); printf("\n"); } 5.1 The for Loop: Odd Integers (2/2) Values printed from largest to smallest. Empty statement CS1010 (AY2013/4 Semester 1)

  30. 6. Exercise #1: Sum of Multiples of 3 (1/2) • Modify the program Week5_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 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 Week5_SumMultiples3.c • Sample run: Enter a positive integer: 50 Sum = 408 CS1010 (AY2013/4 Semester 1)

  31. 6. 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 i is multiple of 3 then sum  sum + i i  i - 1 return sum CS1010 (AY2013/4 Semester 1)

  32. 7. Exercise #2: Asterisks (1/2) • Write a program Week5_Asterisks.c to 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! CS1010 (AY2013/4 Semester 1)

  33. 7. Exercise #2: Asterisks (2/2) • Write a program Week5_Asterisks.c to 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; CS1010 (AY2013/4 Semester 1)

  34. 8. Common Errors (1/2) • What are the outputs for the following programs? (Do not code and run them. Trace the programs manually.) inti; for (i=0; i<10; i++); printf("%d\n", i); Week5_CommonErrors1.c inti = 0; while (i<10); { printf("%d\n", i); i++; } Week5_CommonErrors2.c CS1010 (AY2013/4 Semester 1) 

  35. 8. Common Errors (2/2) intz = 3; while (z = 1) { printf("z = %d\n", z); z = 99; } • Off-by-one error; make sure the loop repeats exactly the correct number of iterations. • Make sure the loop body contains a statement that will eventually cause the loop to terminate. • Using ‘=’ where it should be ‘==’ • Putting ‘;’ where it should not be (just like for the ‘if’ statement) Week5_CommonErrors3.c CS1010 (AY2013/4 Semester 1) 

  36. 9. Some Notes of Caution (1/2) • Involving real numbers • Trace the program manually without running it. double one_seventh = 1.0/7.0; double f = 0.0; while (f != 1.0) { printf("%f\n", f); f += one_seventh; } Week5_Caution1.c CS1010 (AY2013/4 Semester 1) 

  37. 9. Some Notes of Caution (2/2) • Involving ‘wrap-around’ • Trace the program manually without running it. inta = 2147483646; inti; for (i=1; i<=5; i++) { printf("%d\n", a); a++; } Week5_Caution2.c CS1010 (AY2013/4 Semester 1) 

  38. 10. Exercise #3: Tracing Nested Loops • You are given Week5_NestedLoop1.c, Week5_NestedLoop2.c and Week5_NestedLoop3.c • Hand trace the programs and write out the outputs without running the programs • Verify your answers by running the programs CS1010 (AY2013/4 Semester 1)

  39. 11. Using break in Loop (1/2) • You have seen ‘break’ in switch statement • ‘break’ can also be used in a loop • Test out Week5_BreakInLoop.c CS1010 (AY2013/4 Semester 1)

  40. 11. Using break in Loop (2/2) • Use ‘break’ sparingly, because it violates the one-entry-one-exit control flow. • A loop with ‘break’ can be rewritten into one without ‘break’. // without break intn, i = 1, sum = 0; intisValid = 1; while ((i <= 5) && isValid){ scanf("%d", &n); if (n < 0) isValid = 0; else { sum += n; i++; } } // with break intn, i = 1, sum = 0; while (i <= 5) { scanf("%d", &n); if (n < 0) break; sum += n; i++; } CS1010 (AY2013/4 Semester 1)

  41. 12. Using continue in Loop • Test out Week5_ContinueInLoop.c • ‘continue’ is used even less often than ‘break’ CS1010 (AY2013/4 Semester 1)

  42. 13. Exercise #4: Prime Number (1/2) • Primality test is a classic programming problem • Given a positive integer, determine whether it is a prime • A prime number has two distinct factors (divisors): 1 and itself. Examples: 2, 3, 5, 7, 11, ... (Note: 1 is not a prime!) • Write a program Week5_PrimeTest.c. You should include a function is_prime(int). (What does it return?) • Sample runs: Enter a positive integer: 131 131 is a prime. Enter a positive integer: 713 713 is not a prime. CS1010 (AY2013/4 Semester 1)

  43. 13. Exercise #4: Prime Number (2/2) • This is your take-home exercise. • This exercise is mounted on CodeCrunch. • We will discuss this in the next lecture. CS1010 (AY2013/4 Semester 1)

  44. Summary for Today (1/2) • Repetition statements (loops) • for, while, do-while • Nested loops • breakand continue CS1010 (AY2013/4 Semester 1)

  45. Summary for Today (2/2) • You have learned the 3 control structures: • Sequence, Selection, Repetition • With these, you are able to solve just any computing problem! • However, writing good programs is more than just learning the syntax: • Logic should be clear • Variables should be descriptive • Algorithm should be efficient CS1010 (AY2013/4 Semester 1)

  46. Announcements/Things-to-do • Revise Chapter 4 (Lessons 4.7 – 4.11) • Deadline for Lab #2 • Deadline: 14th September 2013, Saturday, 9am • Practical Exam 1 (PE1) • 21st September 2013, Saturday • See web page for details:http://www.comp.nus.edu.sg/~cs1010/3_ca/pe.html • To prepare for next week’s lecture: • Read Chapter 5 Functions • Bring along your Week5_PrimeTest.c program CS1010 (AY2013/4 Semester 1)

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