CS1010 Week 5: Pointers & Arrays Class Activities

1. Lecturer’s slides http://www.comp.nus.edu.sg/~cs1010/ WEEK 5 Class Activities

2. CS1010 (AY2014/5 Semester 1) Week 5: Pointers & Arrays Pointers • Tracing Pointers • Choose the Correct Codes • Incrementing a Pointer Arrays • Exercise #1: Reversing an Array • Exercise #2: Missing Digits • Exercise #3: Modularising “Missing Digits” program • Exercise #4: Set Containment – Take home if time runs out

3. CS1010 (AY2014/5 Semester 1) Tracing Pointers (1/2) • Trace the code below manually to obtain the outputs. • Compare your outputs with your neighbours. Week5_TracePointers.c int a = 8, b = 15, c = 23; int*p1, *p2, *p3; p1 = &b; p2 = &c; p3 = p2; printf("1: %d %d %d\n", *p1, *p2, *p3); *p1 *= a; while(*p2 > 0) { *p2 -= a; (*p1)++; } printf("2: %d %d %d\n", *p1, *p2, *p3); printf("3: %d %d %d\n", a, b, c);

4. CS1010 (AY2014/5 Semester 1) Tracing Pointers (2/2) 8 a b c 15 23 15 120 p1 p3 p2 int a = 8, b = 15, c = 23; int*p1, *p2, *p3; p1 = &b; p2 = &c; p3 = p2; printf("1: %d %d %d\n", *p1, *p2, *p3); *p1 *= a; while(*p2 > 0) { *p2 -= a; (*p1)++; } printf("2: %d %d %d\n", *p1, *p2, *p3); printf("3: %d %d %d\n", a, b, c); 7 121 -1 122 123 1: 15 23 23 2: 123 -1 -1 3: 8 123 -1

5. CS1010 (AY2014/5 Semester 1) Choose the Correct Codes • Pick the correct codes to read a value into the float variable var. (A) (B) float var; scanf("%f", var) float var; scanf("%f", &var) (C) (D) float var; float *p; p = &var; scanf("%f", p) float var; float *p; p = &var; scanf("%f", &p)

6. CS1010 (AY2014/5 Semester 1) Incrementing a Pointer • If p is a pointer variable, what does it mean by p = p + 1 (or p++)? Unit 3 Exercise #1: int takes up 4 bytes float takes up 4 bytes char takes up 1 byte double takes up 8 bytes Week5_IncrementPointers.c inta, *ap; floatb, *bp; charc, *cp; doubled, *dp; ap = &a; bp = &b; cp = &c; dp = &d; printf("%p %p %p %p\n", ap, bp, cp, dp); ap++; bp++; cp++; dp++; printf("%p %p %p %p\n", ap, bp, cp, dp); ap += 3; printf("%p\n", ap); ffbff62c ffbff628 ffbff627 ffbff618 ffbff630 ffbff62c ffbff628 ffbff620 ffbff63c

7. CS1010 (AY2014/5 Semester 1) Exercise #1: Reversing an Array • Write a program Week5_ReverseArray.cto read a list of numbers (at most 10 of them) into the array, reverse the array and print its elements. • We will write everything in the main() function for now. • An incomplete program Week5_ReverseArray.cis given. • Sample run: Enter size of array (<=10): 5 Enter 5 elements: 1 -2 3 8 6 After reversing: 6 8 3 -2 1

8. CS1010 (AY2014/5 Semester 1) Exercise #2: Missing Digits (1/3) • Write a program Week5_MissingDigits.cto read in a positive integer and list out all the digits that do not appear in the input number. • We will write everything in the main() function. (You will modularise it in exercise #3.) • Sample run: Enter a number: 73015 Missing digits in 73015: 2 4 6 8 9 • Recall: How do we extract individual digits from an integer?

9. CS1010 (AY2014/5 Semester 1) Exercise #2: Missing Digits (2/3) • Where does the array come in? • Hint…  (Let you THINK first before giving out the hint) Input: 73105 true 0 9 1 6 2 3 4 7 5 8 false false false false false false false false false false Create an array called found, with 10 elements, each element represents a digit. That is, found[0] is about digit 0, found[1] about digit 1, …, found[9] about digit 9. How do you use this array? true true true true

10. CS1010 (AY2014/5 Semester 1) Exercise #2: Missing Digits (3/3) Show this only after students have attempted it themselves. int main(void) { int number, i; int found[10] = {0} // found[i]=0 means digit i is missing printf("Enter a number: "); scanf("%d", &number); printf("Missing digits in%d:", number); while (number > 0) { found[number%10] = 1; // found digit in input number number /= 10; } for (i = 0; i < 10; i++) { if (!found[i]) printf("%d ", i); } printf("\n"); return0; } Key idea Week5_MissingDigits.c

11. CS1010 (AY2014/5 Semester 1) Exercise #3: Modularising Exercise #2 • Let’s re-write our program Week5_MissingDigits.cinto Week5_MissingDigitsModular.c • Objective: Passing array to a function • The program should contain a function called analyseNumber() that takes in a number and analyse what are the missing digits in that number • What is/are the parameter(s)? • The program should also contain a function called printMissingDigits() to print out all the missing digits • What is/are the parameter(s)?

12. CS1010 (AY2014/5 Semester 1) Exercise #4: Set Containment • Consider two arrays, arrA and arrB, of int values, where their sizes are sizeA and sizeBrespectively. • Write a functionint isSubset(int arrA[], int sizeA, int arrB[], int sizeB) to determine if the set arrAis a subset of the set arrB. • The function returns 1 if arrA is a subset of arrB, or 0 otherwise. You may assume there are no duplicate numbers in each set. • Example: If arrA[ ] = {14, 5, 1, 9} and arrB[ ] = {2, 9, 3, 14, 5, 6, 1} • isSubset(arrA, 4, arrB, 7)returns 1 • isSubset(arrA, 4, arrB, 6)returns 0 • An incomplete program Week5_SetContainment.cis given. Complete the program. This is your take-home exercise. Also mounted on CodeCrunch

13. CS1010 (AY2014/5 Semester 1) Things-To-Do • Revise • Chapter 6: Numeric Arrays • Deadline for Lab #2 • Deadline: 13 September 2014, Saturday, 9am • Preparation for next week • Multi-dimensional arrays • Continue to do practice exercises on CodeCrunch

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