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

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

0. Revision(1/3) General form of if statements There can be any number of else-if fragments. The else fragment is optional but can only appear last. The curly brackets around a set of statements are required unless there is only one statement in the set. if (condition1 ) { statements; // To be executed if condition1 is true } elseif (condition2 ) { statements; // To be executed if condition2 is true // and all previous conditions are false } elseif (condition3 ) { statements; // To be executed if condition3 is true // and all previous conditions are false } … else { statements; // To be executed if and all previous // conditions are false } CS1010 (AY2013/4 Semester 1) Week5 - 2

0. Revision (2/3) A condition is an expression evaluated to true or false. Composed of expressions combined with relational operators and logical operators Relational Operators: <, <=, >, >=, ==, != Left associative Precedence: higher than && and || but lower than !. Logical Operators: &&, ||, ! Left associative Precedence (from high to low): !, &&, || Short-circuit evaluation: expr1 || expr2 : If expr1 is true, skip evaluating expr2 expr1 && expr2: If expr1 is false, skip evaluating expr2 Do not confuse with “=”! CS1010 (AY2013/4 Semester 1) Week5 - 3

0. Revision (3/3) General form of switch statements The value of expression must be an integer or a character “break” prevents statements in subsequent cases from being executed. switch(expression) { case value1: statements; // To be executed if expression == value1 break; case value2: statements; // To be executed if expression == value2 break; ... default: statements; // To be executed if expression does not // match any case values } CS1010 (AY2013/4 Semester 1) Week5 - 4

Week 4 Ex3: NRIC Check Code (1/3) Algorithm for NRIC check code S/T + 7 digits +1 letter check code E.g.: S8730215F (valid), S8730216F(invalid, check code should be D) Step 1: Multiply the digits with corresponding weights 2,7,6,5,4,3,2 and add them up. E.g.: 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. E.g.: 104 % 11 = 5 CS1010 (AY2013/4 Semester 1) Week5 - 5

Week 4 Ex3: NRIC Check Code (2/3) Algorithm for NRIC check code (cont…) Step 3: Subtract step 2 result from 11 E.g.: 11 – 5 = 6 Step 4: Match step 3 result in this table for the check code E.g.: 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

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 (the check code). You need to use the char type. (Explore this on your own.) A character constant is enclosed in single quotes (e.g.: '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 but you can always improve it later. This is your take-home exercise. This exercise is available in CodeCrunch. CS1010 (AY2013/4 Semester 1) Week5 - 7

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

Outline 0. Revision • Loops! • The while Loop • The do-while Loop • The for Loop • Exercise #1: Sum of Multiples of 3 • Exercise #2: Asterisks • Common Errors • Some Notes of Caution 9. Exercise #3: Tracing Nested Loops 10. Using break in Loop 11. Using continue in Loop 12. Exercise #4: Prime Number CS1010 (AY2013/4 Semester 1)

Recall: Control Structures Sequence Selection if-else, switch Repetition Loops! CS1010 (AY2013/4 Semester 1) Week5 - 10

1. 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 - 11

1. 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 - 12

1. Loop: Demo (1/3) • Keep prompting the user to input a non-negative integer, and output that integer. • Halt 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 (not) met • You do not know beforehand how many iterations there will be. CS1010 (AY2013/4 Semester 1) Week5 - 13

1. Loop: Demo (2/3) intmain(void) { intnum; printf("Enter a number: "); scanf("%d", &num); if (num >= 0) { printf("You entered: %d\n", num); printf("Enter a number: "); scanf("%d", &num); } else return0; if (num >= 0) { printf("You entered: %d\n", num); printf("Enter a number: "); scanf("%d", &num); } else return0; .... } 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 condition Loop body CS1010 (AY2013/4 Semester 1) Week5 - 14

1. 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; } Loop condition false num >= 0? true Loop body printf … printf … scanf… CS1010 (AY2013/4 Semester 1) Week5 - 15

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

2.1 The whileLoop: Demo (1/2) 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 • Keep prompting the user to input a non-negative integer • Halt when the input is negative. • Output the maximum number so far maxi = 0; num  input; while (num >= 0) { if (maxi < num) maxi = num; num  input; } print maxi; CS1010 (AY2013/4 Semester 1) Week5 - 17

2.1 The whileLoop: Demo (2/2) 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 - 18

2.2 whileLoop Condition (1/2) When the loop condition is false at the beginning, the loop body is not executed. Output: ? int a =2; int b =7; while (a == b) { printf(“%d\n”, a); a = a + 2; } false cond? true Loop body CS1010 (AY2013/4 Semester 1) Week5 - 19 

2.2 whileLoop Condition (2/2) When the loop condition is always true, the loop body is executed forever – infinite loop. Output: ? int a =2; int b =7; while (a != b) { printf(“%d\n”, a); a = a + 2; } false cond? true Loop body CS1010 (AY2013/4 Semester 1) Week5 - 20 

2.3 Tracing whileLoop (1/3) • 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) 

2.3 Tracing whileLoop (2/3) • 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)

2.3 Tracing whileLoop (3/3) 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; } } For the number 28943, what are the values of n and digit before the loop and after each iteration? CS1010 (AY2013/4 Semester 1) 

3. 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)

3. 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. What happens when n is 0? CS1010 (AY2013/4 Semester 1)

4. The for Loop for ( initialization; condition; update ) { // loop body } Update: change value oftheloop variable Initialization: initialize the loop variable Condition: repeat loop while the condition is true Initialization int n; for (n=1; n<=10; n++) { printf("%d\n", n); } true Loop body cond? false Loop variable: a variable which is involved in the loop condition. Update CS1010 (AY2013/4 Semester 1)

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"); } 4.1 The for Loop: Odd Integers (1/2) Initialization: i = 1 Condition: i <= n Body:printf(…) Update: i+=2 CS1010 (AY2013/4 Semester 1) 

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"); } 4.1 The for Loop: Odd Integers (2/2) First version inti; for (i=1; i<=n; i+=2) printf("%d ", i); printf("\n"); Values printed from largest to smallest. Empty statement CS1010 (AY2013/4 Semester 1)

5. 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 loops just for exercise. • Call this program Week5_SumMultiples3.c • Sample run: Enter a positive integer: 50 Sum = 408 CS1010 (AY2013/4 Semester 1)

5. Exercise #1: Sum of Multiples of 3 (2/2) 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 • Pseudo-code using a while loop: CS1010 (AY2013/4 Semester 1)

6. 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: 10 ******************* Done! Enter n: -2 Done! CS1010 (AY2013/4 Semester 1)

6. 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)

7. Common Errors (1/2) Week5_CommonErrors1.c • 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_CommonErrors2.c inti = 0; while (i<10); { printf("%d\n", i); i++; } • Putting ‘;’ where it should not be CS1010 (AY2013/4 Semester 1) 

7. Common Errors (2/2) Week5_CommonErrors3.c intz = 3; while (z = 1) { printf("z = %d\n", z); z = 99; } • Using ‘=’ where it should be ‘==’ • Missing a statement that eventually causes the loop to terminate • Off-by-one error: The loop stops one iteration too early or too late. CS1010 (AY2013/4 Semester 1) 

8. 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) 

8. 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) 

9. 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)

9. Exercise #3: Tracing Nested Loops Week5_NestedLoop1.c 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++; } … CS1010 (AY2013/4 Semester 1) 

10. Using break in Loop (1/2) • Exit the immediate loop • Can be used in switch statement and loops 1 1 Ya 1 2 2 1 Ya 2 2 3 1 Ya 3 2 • for (i=1; i<=3; i++){ • printf ("%d\n", i); • printf ("Ya\n"); • } 1 Ya 2 Ya 3 Ya for (i=1; i<=3; i++) { for (j=1; j<=3; j++) { printf ("%d,%d\n", i, j); if (j==2) break; printf ("Ya\n"); } } for (i=1; i<=3; i++){ printf ("%d\n", i); if (i==2) break; printf ("Ya\n"); } 1 Ya 2 CS1010 (AY2013/4 Semester 1)

10. 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)

11. Using continue in Loop • Skip the rest of the current iteration • Can be used in loops but even less often than ‘break’ 1 1 Ya 1 2 1 3 Ya 2 1 Ya 2 2 2 3 Ya 3 1 Ya 3 2 3 3 Ya • for (i=1; i<=3; i++){ • printf ("%d\n", i); • printf ("Ya\n"); • } 1 Ya 2 Ya 3 Ya for (i=1; i<=3; i++) { for (j=1; j<=3; j++) { printf ("%d,%d\n", i, j); if (j==2) continue; printf ("Ya\n"); } } for (i=1; i<=3; i++){ printf ("%d\n", i); if (i==2) continue; printf ("Ya\n"); } 1 Ya 2 3 Ya CS1010 (AY2013/4 Semester 1)

12. 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)

12. 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)

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

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: • Design • Logic should be clear • Algorithm should be efficient • Style • Names of variables/functions should be descriptive • Programs should be indented and commented appropriately CS1010 (AY2013/4 Semester 1)

Announcements/Things-to-do • Revise Chapter 4 (Lessons 4.7 – 4.11) • Lab 2 • This Saturday, 14 Sep, 9am • PE1 • Up to and including repetition statements • Next Saturday, 21 Sep • To prepare for next week’s lecture: • Read Chapter 5 Functions CS1010 (AY2013/4 Semester 1)

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