1. CS1010: Programming Methodology http://www.comp.nus.edu.sg/~cs1010/ http://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 CS1010 (AY2012/3 Semester 1)Week5 - 2 References:  Chapter 4 Lessons 4.7 – 4.11

3. Week 5: Outline (1/2) 1.Week 4 Exercise #3: NRIC Check Code 2.Loops! 3.The while Loop 3.1 Demo 3.2Loop condition 3.3Tracing 4.The do-while Loop 5.The for Loop 5.1 Odd Integers 6.Exercise #1: Sum of Multiples of 3 7.Exercise #2: Asterisks CS1010 (AY2012/3 Semester 1)Week5 - 3

4. Week 5: Outline (2/2) 8.Common Errors 9.Some Notes of Caution 10.Exercise #3: Tracing Nested Loops 11.Using break in Loop 12.Using continue in Loop 13.Exercise #4: Prime Number (take-home) CS1010 (AY2012/3 Semester 1)Week5 - 4

5. 1. Week 4 Exercise #2: Taxi Fare (1/3) The taxi fare structure in Singapore must be one of the most complex in the world! See http://www.taxisingapore.com/taxi-fare/http://www.taxisingapore.com/taxi-fare/ Write a program Week4_TaxiFare.c that reads the following input data (all are of int type) from the user, and computes the taxi fare:  dayType: 0 represents weekends and public holidays (PH for short); 1 represents weekdays and non-PH  boardHour, boardMin: the hour and minute the passengers board the taxi (eg: 14 27 if the passengers board the taxi at 2:27 PM)  distance: the distance of the journey, in metres Your program should have a function float computeFare(int dayType, int boardTime, int distance)  The parameter boardTime is converted from the input data boardHour and boardMin. It is the number of minutes since 0:00hr. Eg: If boardHour and boardMin are 14 and 27 respectively, then boardTime is 867. Week4 - 5CS1010 (AY2012/3 Semester 1)

6. 1. Week 4 Exercise #2: Taxi Fare (2/3) To implement the actual taxi fare could be a PE question. In this exercise, we use a (grossly) simplified fare structure:  Basic Fare:  Surcharge (applicable at the time of boarding): Week4 - 6CS1010 (AY2012/3 Semester 1) Flag-down (inclusive of 1 st km or less)$3.40 Every 400m thereafter or less up to 10.2km$0.22 Every 350m thereafter or less after 10.2km$0.22 dayTypeMidnight charge (12am – 5:59am) Peak hour charge (6am – 9:29am) Peak hour charge (6pm – 11:59pm) 0: Weekends & PH50% of metered fare None25% of metered fare 1: Weekdays and non-PH 50% of metered fare 25% of metered fare

7. 1. Week 4 Exercise #2: Taxi Fare (3/3) You are given an incomplete program Week4_TaxiFarePartial.c. Complete the program. This exercise is mounted on CodeCrunch. Sample runs below for your checking Week4 - 7CS1010 (AY2012/3 Semester 1) Day type: 0 Boarding hour and minute: 14 27 Distance: 10950 Total taxi fare is $9.12 First 1km: $3.40 Next 9.2km: 23  $0.22 = $5.06 Next 750m: 3  $0.22 = $0.66 Basic fare = $9.12 No surcharge Total fare = $9.12 Day type: 1 Boarding hour and minute: 9 20 Distance: 6123 Total taxi fare is $7.83 First 1km: $3.40 Next 5123m: 13  $0.22 = $2.86 Basic fare = $6.26 Surcharge = 25%  $6.26 = $1.57 Total fare = $7.83 Day type: 1 Boarding hour and minute: 5 59 Distance: 9000 Total taxi fare is $11.70 First 1km: $3.40 Next 8km: 20  $0.22 = $4.40 Basic fare = $7.80 Surcharge = 50%  $7.80 = $3.90 Total fare = $11.70

8. 1. Week 4 Exercise #2: Taxi Fare Week4 - 8CS1010 (AY2012/3 Semester 1) float computeFare(int daytype, int bTime, int dist) { basicFare = calcBasicFare(dist) ; return includeSurcharge(basicFare, daytype, bTime) ; } float calcBasicFare(int dist){ // Pre-cond: 0 <= dist ; if (dist <= 1000) return 3.40; else if (dist 1000) return (3.40 + ceil((dist - 1000) /400.0) * INCREMENT); else if (dist > 10200) return (3.40 + 9200 / 400.0 * INCREMENT + (ceil((dist - 10200) / 350.0)) * INCREMENT); }

9. 1. Week 4 Exercise #2: Taxi Fare Week4 - 9CS1010 (AY2012/3 Semester 1) float computeFare(int daytype, int bTime, int dist) { basicFare = calcBasicFare(dist) ; return includeSurcharge(basicFare, daytype, bTime) ; } float includeSurcharge(float fare, int dType, int bTime){ // Pre-cond: dType = 0 or 1, 0 <= bTime <= 2359 ; if (bTime < 360 && (dType == 0 || dType == 1)) return fare * 1.50; else if (dType == 1 && (bTime =360)) return fare * 1.25; else if (bTime >= 1080) return fare * 1.25; else return fare ; }

10. 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 Week5 - 10 CS1010 (AY2012/3 Semester 1)

11. 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: 1234567891011 ABCDEFGHIZJ Week5 - 11 Enter 7-digit NRIC number: 8730215 Check code is F CS1010 (AY2012/3 Semester 1)

12. 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. Week5 - 12 CS1010 (AY2012/3 Semester 1)

13. 1. Week 4 Ex3: NRIC Check code  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 // Extract the digits digit7 = num%10; num /= 10; digit6 = num%10; num /= 10; digit5 = num%10; num /= 10; digit4 = num%10; num /= 10; digit3 = num%10; num /= 10; digit2 = num%10; num /= 10; digit1 = num%10; step1 = digit1*2 + digit2*7 + digit3*6 + digit4*5 + digit5*4 + digit6*3 + digit7*2; Week5 - 13

14. 1. Week 4 Ex3: NRIC Check code  Step 2: Divide step 1 result by 11 to obtain the remainder.  Eg: 104 % 11 = 5 step2 = step1 % 11;  Step 3: Subtract step 2 result from 11  Eg: 11 – 5 = 6 step3 = 11 - step2; Week5 - 14 CS1010 (AY2012/3 Semester 1)

15. 1. Week 4 Ex3: NRIC Check Code  Step 4: Match step 3 result in this table for the check code switch (step3) { case 1: code = 'A'; break; case 2: code = 'B'; break; case 3: code = 'C'; break; case 4: code = 'D'; break; case 5: code = 'E'; break; case 6: code = 'F'; break; case 7: code = 'G'; break; case 8: code = 'H'; break; case 9: code = 'I'; break; case 10: code = 'Z'; break; case 11: code = 'J'; } // end switch 1234567891011 ABCDEFGHIZJ Week5 - 15 CS1010 (AY2012/3 Semester 1)

16. Recall: Control Structures Week5 - 16 if-else, switch CS1010 (AY2012/3 Semester 1)

17. 2. LOOPS! (1/2) Week5 - 17 “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 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 (AY2012/3 Semester 1)

18. 2. LOOPS! (2/2) Week5 - 18 cond? Some statement(s) true false loop body Loop condition Week5 - 18 Each round of the loop is called an iteration. CS1010 (AY2012/3 Semester 1)

19. 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 Week5 - 19 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 (AY2012/3 Semester 1)

20. 2. Loop: Demo (2/3) int main(void) { int num; printf("Enter a number: "); scanf("%d", &num); if (num < 0) return 0; printf("You entered: %d\n", num); printf("Enter a number: "); scanf("%d", &num); if (num < 0) return 0; printf("You entered: %d\n", num); printf("Enter a number: "); scanf("%d", &num);.... } Week5 - 20 Loop condition 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 Loop body CS1010 (AY2012/3 Semester 1)

21. 2. Loop: Demo (3/3) Week5 - 21 num >= 0? printf … scanf … true false Week5 - 21CS1010 (AY2012/3 Semester 1) int main(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; } Week5_Read_print.c

22. 3. The while Loop CS1010 (AY2012/3 Semester 1)Week5 - 22 while ( condition ) { // loop body } cond? Loop body true false If condition is true, execute loop body; otherwise, terminate loop.

23. 3.1 The while Loop: Demo (1/3) 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 Week5 - 23 CS1010 (AY2012/3 Semester 1) Keep prompting the user to input a non- negative integer, and output that integer. Halt the loop when the input is negative, and output the maximum integer input so far.

24. 3.1 The while Loop: Demo (2/3) Week5 - 24 maxi = 0; num  input; while (num >= 0) { if (maxi < num) maxi = num; num  input; } print maxi; maxi = 0; num  input; if (num < 0) { print maxi; stop; } if (maxi < num) maxi = num ; num  input; if (num < 0) { print maxi; stop; } if (maxi < num) maxi = num; num  input;... Week5 - 24CS1010 (AY2012/3 Semester 1)

25. 3.1 The while Loop: Demo (3/3) Week5 - 25 CS1010 (AY2012/3 Semester 1) int main(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; } Week5_Find_max.c

26. 3.2 while Loop Condition (1/2)  When the loop condition is always false, the loop body is not executed. Week5 - 26 Output: ? Week5 - 26CS1010 (AY2012/3 Semester 1) a = 2; b = 7; while (a == b) { print a; a = a + 2; }

27. 3.2 while Loop Condition (2/2) Week5 - 27 Output: ? Week5 - 27CS1010 (AY2012/3 Semester 1) a = 2; b = 7; while (a != b) { print a; a = a + 2; } 

28. 3.3 Tracing while Loop (1/4)  Trace the following codes manually and write out their outputs (assume all variables are of type int) Week5 - 28CS1010 (AY2012/3 Semester 1) a = 1; while (a*a < 100) { printf("%d ", a); a *= 2; } printf("\n"); (a) 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); (b) 

29. 3.3 Tracing while Loop (2/4)  Example: Given a positive integer n, print out its digits from least significant to most significant.  Sample run: Week5 - 29 Enter a positive integer: 28943 3 4 9 8 2 CS1010 (AY2012/3 Semester 1)

30. 3.3 Tracing while Loop (3/4)  Example: Given a positive integer n, print out its digits from least significant to most significant. Week5 - 30CS1010 (AY2012/3 Semester 1) // Precond: n > 0 void print_digits(int n) { int digit; while (n > 0) { digit = n%10; printf("%d\n", digit); n /= 10; } Week5_Print_digits.c

31. // Precond: n > 0 void print_digits(int n) { int digit; while (n > 0) { digit = n%10; printf("%d\n", digit); n /= 10; } Week5_Print_digits.c 3.3 Tracing while Loop (4/4) Week5 - 31  n initially28943 n @ point29843 digit @ point*** What are the values of n and digit after exiting the loop? CS1010 (AY2012/3 Semester 1)

32. 4. The do-while Loop (1/2) Week5 - 32 do { // loop body } while ( condition ); Execute loop body at least once. cond? Loop body true false CS1010 (AY2012/3 Semester 1)

33. 4. The do-while Loop (2/2)  Example: Count the number of digits in an integer. Week5 - 33 do { // loop body } while ( condition ); CS1010 (AY2012/3 Semester 1) // Precond: n > 0 int count_digits(int n) { int counter = 0; do { counter++; n /= 10; } while (n > 0); return counter; } Week5_Count_digits.c

34. 5. The for Loop (1/2) Week5 - 34 for ( initialization; condition; update ) { // loop body } Initialization: initialize the loop variable Condition: repeat loop while the condition on loop variable is true Update: change value of loop variable CS1010 (AY2012/3 Semester 1)

35. 5. The for Loop (2/2)  Example: Print numbers 1 to 10 Week5 - 35 int n; for (n=1; n<=10; n++) { printf("%3d", n); } Steps: 1.n=1; 2.if (n<=10) { printf(…); n++; Go to step 2 } 3.Exit the loop CS1010 (AY2012/3 Semester 1)

36. 5.1 The for Loop: Odd Integers (1/2) Week5 - 36CS1010 (AY2012/3 Semester 1) #include void print_odd_integers(int); int main(void) { int num; printf("Enter a positive integer: "); scanf("%d", &num); print_odd_integers(num); return 0; } // Precond: n > 0 void print_odd_integers(int n) { int i; for (i=1; i<=n; i+=2) printf("%d ", i); printf("\n"); } Week5_OddIntegers_v1.c

37. 5.1 The for Loop: Odd Integers (2/2) Week5 - 37CS1010 (AY2012/3 Semester 1) // Precond: n > 0 void print_odd_integers(int n) { int i; for (i=1; i<=n; i++) if (i%2 != 0) printf("%d ", i); printf("\n"); } Week5_OddIntegers_v2.c // Precond: n > 0 void print_odd_integers(int n) { for ( ; n > 0; n--) if (n%2 != 0) printf("%d ", n); printf("\n"); } Week5_OddIntegers_v3.c Values printed from largest to smallest. Empty statement

38. 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 ‘while’ loop just for exercise.  Call this program Week5_SumMultiples3.c  Sample run: Week5 - 38 Enter a positive integer: 50 Sum = 408 CS1010 (AY2012/3 Semester 1)

39. 6. Exercise #1: Sum of Multiples of 3 (2/2)  How about using a while loop instead?  Pseudo-code using a while loop: Week5 - 39CS1010 (AY2012/3 Semester 1) 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

40. 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: Week5 - 40 Enter n: 3 ***** Done! Enter n: 6 *********** Done! Enter n: 10 ******************* Done! Enter n: -2 Done! Think! What is the relationship between n and the number of *? CS1010 (AY2012/3 Semester 1)

41. 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). Week5 - 41CS1010 (AY2012/3 Semester 1) 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;

42. 8. Common Errors (1/2)  What are the outputs for the following programs? (Do not code and run them. Trace the programs manually.) Week5 - 42  CS1010 (AY2012/3 Semester 1) int i; for (i=0; i<10; i++); printf("%d\n", i); Week5_CommonErrors1.c int i = 0; while (i<10); { printf("%d\n", i); i++; } Week5_CommonErrors2.c

43. 8. Common Errors (2/2)  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 - 43CS1010 (AY2012/3 Semester 1)  int z = 3; while (z = 1) { printf("z = %d\n", z); z = 99; } Week5_CommonErrors3.c

44. 9. Some Notes of Caution (1/2)  Involving real numbers  Trace the program manually without running it. Week5 - 44  CS1010 (AY2012/3 Semester 1) 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

45. 9. Some Notes of Caution (2/2)  Involving ‘wrap-around’  Trace the program manually without running it. Week5 - 45  CS1010 (AY2012/3 Semester 1) int a = 2147483646; int i; for (i=1; i<=5; i++) { printf("%d\n", a); a++; } Week5_Caution2.c

46. 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 Week5 - 46CS1010 (AY2012/3 Semester 1)

47. 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 Week5 - 47CS1010 (AY2012/3 Semester 1)

48. 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’. Week5 - 48CS1010 (AY2012/3 Semester 1) // with break int n, i = 1, sum = 0; while (i <= 5) { scanf("%d", &n); if (n < 0) break; sum += n; i++; } // without break int n, i = 1, sum = 0; int isValid = 1; while ((i <= 5) && isValid){ scanf("%d", &n); if (n < 0) isValid = 0; else { sum += n; i++; }

49. 12. Using continue in Loop  Test out Week5_ContinueInLoop.c  ‘continue’ is used even less often than ‘break’ Week5 - 49CS1010 (AY2012/3 Semester 1)

50. 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: Week5 - 50 Enter a positive integer: 131 131 is a prime. Enter a positive integer: 713 713 is not a prime. CS1010 (AY2012/3 Semester 1)

51. 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. Week5 - 51CS1010 (AY2012/3 Semester 1)

52. Summary for Today (1/2) Week5 - 52 Repetition statements (loops)  for, while, do-while  Nested loops  break and continue CS1010 (AY2012/3 Semester 1)

53. Summary for Today (2/2) Week5 - 53 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 (AY2012/3 Semester 1)

54. Announcements/Things-to-do Revise Chapter 4 (Lessons 4.7 – 4.11) Deadline for Lab #2  Deadline: 15 th September 2012, Saturday, 12noon Practical Exam 1 (PE1)  22 nd September 2012, Saturday  See web page for details: http://www.comp.nus.edu.sg/~cs1010/3_ca/pe.html 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 Week5 - 54CS1010 (AY2012/3 Semester 1)

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