Report

Lecturer’s slides http://www.comp.nus.edu.sg/~cs1010/ WEEK 5 Class Activities © NUS CS1010 (AY2014/5 Semester 1) Week5 - 2 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 © NUS CS1010 (AY2014/5 Semester 1) Tracing Pointers (1/2) Trace the code below manually to obtain the outputs. Compare your outputs with your neighbours. int a = 8, b = 15, c = 23; int *p1, *p2, *p3; Week5_TracePointers.c 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); Week5 - 3 © NUS CS1010 (AY2014/5 Semester 1) Tracing Pointers (2/2) a 8 int a = 8, b = 15, c = 23; int *p1, *p2, *p3; Week5 - 4 p1 b p2 15 120 121 122 123 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); p3 c 23 15 7 -1 1: 15 23 23 2: 123 -1 -1 3: 8 123 -1 © NUS CS1010 (AY2014/5 Semester 1) Week5 - 5 Choose the Correct Codes Pick the correct codes to read a value into the float variable var. (A) (B) float var; scanf("%f", var) (C) float var; float *p; p = &var; scanf("%f", p) float var; scanf("%f", &var) (D) float var; float *p; p = &var; scanf("%f", &p) © NUS CS1010 (AY2014/5 Semester 1) Week5 - 6 Incrementing a Pointer If p is a pointer variable, what does it mean by p = p + 1 (or p++)? Unit 3 Exercise #1: int a, *ap; int takes up 4 bytes float b, *bp; float takes up 4 bytes char takes up 1 byte char c, *cp; double d, *dp; double takes up 8 bytes Week5_IncrementPointers.c ap = &a; bp = &b; cp = &c; dp = &d; printf("%p %p %p %p\n", ap, bp, cp, ffbff62c ffbff628 ap++; bp++; cp++; dp++; printf("%p %p %p %p\n", ap, bp, cp, ffbff630 ffbff62c ap += 3; printf("%p\n", ap); ffbff63c dp); ffbff627 ffbff618 dp); ffbff628 ffbff620 © NUS CS1010 (AY2014/5 Semester 1) Week5 - 7 Exercise #1: Reversing an Array Write a program Week5_ReverseArray.c to 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.c is given. Sample run: Enter size of array (<=10): 5 Enter 5 elements: 1 -2 3 8 6 After reversing: 6 8 3 -2 1 © NUS CS1010 (AY2014/5 Semester 1) Week5 - 8 Exercise #2: Missing Digits (1/3) Write a program Week5_MissingDigits.c to 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? © NUS CS1010 (AY2014/5 Semester 1) Week5 - 9 Exercise #2: Missing Digits (2/3) Where does the array come in? Hint… (Let you THINK first before giving out the hint) Input: 73105 0 false true Create an array called found, with 10 elements, each element represents a digit. 1 false true 2 false 3 false 4 false That is, found[0] is about digit 0, found[1] about digit 1, …, found[9] about digit 9. 5 false 6 false 7 false 8 false How do you use this array? 9 false true true true © NUS CS1010 (AY2014/5 Semester 1) Week5 - 10 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); Key idea 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"); return 0; } Week5_MissingDigits.c © NUS CS1010 (AY2014/5 Semester 1) Week5 - 11 Exercise #3: Modularising Exercise #2 Let’s re-write our program Week5_MissingDigits.c into 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)? © NUS CS1010 (AY2014/5 Semester 1) Week5 - 12 Exercise #4: Set Containment Consider two arrays, arrA and arrB, of int values, where their sizes are sizeA and sizeB respectively. Write a function int isSubset(int arrA[], int sizeA, int arrB[], int sizeB) to determine if the set arrA is 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.c is given. Complete the program. This is your take-home exercise. Also mounted on CodeCrunch © NUS CS1010 (AY2014/5 Semester 1) Things-To-Do Revise Deadline for Lab #2 Deadline: 13 September 2014, Saturday, 9am Preparation for next week Chapter 6: Numeric Arrays Multi-dimensional arrays Continue to do practice exercises on CodeCrunch Week5 - 13 © NUS CS1010 (AY2014/5 Semester 1) End of File Week5 - 14