Review of Arrays, Pointers, and References

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1 Review of Arrays, Pointers, and References Instructor: Tsung-Che Chiang Department of Computer Science and Information Engineering National Taiwan Normal University

2 Types of Pointers and Arrays int num, arr[5], arr2[3][5], *ptr1, **ptr2, (*ptr3)[5]; ptr1 = num; ptr1 = &num; ptr1 = arr; ptr1 = arr2; ptr2 = num; ptr2 = &num; ptr2 = &ptr1; ptr2 = arr; ptr2 = arr2; ptr3 = num; ptr3 = &num; ptr3 = &ptr1; ptr3 = &ptr2; ptr3 = &arr; ptr3 = arr2; ptr3 = &arr2; //an integer //an array of 5 integers //an array of 3 arrays of 5 ints //a pointer to integer //a pointer to pointer to integer //a pointer to array of 5 integers //*ptr1 refers tonum //ptr1[i] refers toarr[i] //*ptr2 refers toptr1 //ptr3[0][i] refers toarr[i] //ptr3[i][j] refers toarr2[i][j] 2

3 Operations on Pointers & Arrays Addition & subtraction Only valid when accessing elements in arrays. Address-of (reference) Dereferencing int arr1[10]; &arr1 arr1+0 arr1+4 &arr1+1 arr1[0] *arr arr arr1[3] *(arr1+3) 3

4 Operations on Pointers & Arrays Addition & subtraction Only valid when accessing elements in arrays. Address-of (reference) Dereferencing &arr2 &arr2+1 int arr2[2][5]; arr2+0 &arr2[0] &arr2[0][0] &arr2[0][0]+1 arr2+1 &arr2[0] arr2 arr2[0] *arr2 arr1[1][2] 4

5 Pointers & Individual Variables int num1 = 10, num2 = 20, *ptr11, *ptr12, **ptr2; ptr11 = &num1; ptr12 = &num2; ptr2 = &ptr11; ptr2 ptr11 ptr12 10 num1 20 num2 *ptr11 = 30; *ptr2 = &num2; *ptr11 = 40; *ptr12 = 50; **ptr2 = 60; ptr2 ptr11 ptr12 30 num1 20 num2 ptr2 ptr11 ptr12 30 num1 20 num2 ptr2 ptr11 ptr12 30 num1 20 num

6 Pointer to Integer int arr1[5]=0, arr2[3][4]=0, *ptr1=0; ptr1 = &arr1[2]; *ptr1 = 20; *(ptr1+1) = 30; ptr1[2] = 40; ptr1 = &arr2[1][3]; *ptr1 = 50; ptr1[1] = 60; ptr ptr1 arr arr2 6

7 Pointer to Array (1/2) int arr2[3][4]=0, (*ptr2)[4]=0; ptr2 = arr2; ptr2[0][0] = 70; ptr2[1][1] = 80; ptr2 = arr2+1; ptr2[0][0] = 90; ptr2[1][1] = 60; ptr2 = &arr2[2]; ptr2[0][0] = 40; ptr arr2 7

8 Pointer to Array (2/2) int arr2[10][4]=0, (*ptr2)[4]=0; ptr2 = arr2; // or ptr2 = &arr2[0]; for (int i=0; i<10; i+=1) for (int j=0; j<4; j+=1) cout << ptr2[i][j] << ; ptr2 = arr2+2; // or ptr2 = &arr2[2]; for (int i=0; i<3; i+=1) for (int j=0; j<4; j+=1) cout << ptr2[i][j] << ; 8

9 Array of Pointers (1/3) int num=0, arr1[5]=0, arr2[3][4]=0, *ptr1=0, *parr3[4]=0; ptr1 = &num; parr3[0] = ptr1; *parr3[0] = 60; parr3[1] = &arr1[2]; *parr3[1] = 70; parr3[2] = arr1; parr3[2][1] = 80; ptr1 num arr parr3 parr3[3] = arr2[1]; parr3[3][1] = 90; arr

10 Array of Pointers (2/3) int arr2[10][4]=0, *parr3[3]=0; parr3[0] = arr2; parr3[1] = arr2+2; parr3[2] = arr2+5; for (int i=0; i<3; i+=1) for (int j=0; j<4; j+=1) cout << parr3[i][j] << ; cout << endl; 10

11 Array of Pointers (3/3) int arr1[5], arr2[6], arr3[10], *parr[3]=arr1, arr2, arr3, sarr[3] = 5, 6, 10; for (int i=0; i<3; i+=1) for (int j=0; j<sarr[i]; j+=1) cout << parr[i][j] << ; cout << endl; 11

12 Pointer to Pointer int arr2[3][4]=0, **ptr2=0, *parr3[4]=0; ptr2 = parr3; *ptr2 = &arr2[2][1]; ptr2[1] = &arr2[2][3]; ptr2 parr arr2 12

13 Pointer to Pointer void InputByUsers_C(const char msg[], int *p ) printf("%s...>", msg); scanf("%d", p ); void InputByUsers_CPP_Addr(const char msg[], int *p ) cout << msg << "...>"; cin >> *p ; int val; InputByUsers_C("Please input an integer", &val); printf("%d\n", val); InputByUsers_CPP_Addr("Please input an ineger", &val ); cout << val << endl; 13

14 Definitions of Parameters and Nonparameters (1/2) void print1darray_1(int ptr1[], int size) for (int i=0; i<size; i+=1) cout << ptr1[i]; void print1darray_2 (int *ptr2, int size) for (int i=0; i<size; i+=1) cout << *(ptr2+i); void print1darray_3 (int ptr3[10], int size) for (int i=0; i<size; i+=1, ptr3+=1) cout << *ptr3; void print1darray_4 (int ptr4[1000], int size) for (int i=0; i<size; i+=1) cout << ptr4[i]; int arr1[], *arr2=0, arr3[10]=0, arr4[1000]=0; ptr1,ptr2,ptr3, andptr4 are all pointers to integer. // syntax error, butint arr1[] = 1, 2, 3; defines an array of 3 integers. //arr2 is a pointer to integer //arr3 is an array of 10 integers //arr4 is an array of 1000 integers print1darray_1(arr3, 10); print1darray_2(arr4, 1000); 14

15 Definitions of Parameters and Nonparameters (2/2) void print2darray_1(int ptr5[3][5], int rows) for (int i=0; i<rows; i+=1) print1darray_1(ptr5[i], 5); void print2darray_2(int (*ptr6)[5], int rows) for (int i=0; i<rows; i+=1) for (int j=0; j<5; j+=1) cout << ptr6[i][j]; void print2darray_3(int ptr7[][5], int rows) for (int i=0; i<rows; i+=1) for (int j=0; j<5; j+=1) cout << *(*(ptr7+i)+j); ptr5, ptr6, and ptr7 are all pointers to array of 5 integers. int arr5[3][5], arr6[4][5], arr7[4][4]; print2darray_1(arr5, 3); print2darray_2(arr6, 4); print2darray_3(arr7, 4); // syntax error, incompatible type 15

16 Pointer to Struct/Class struct Student static const int ScoreNum = 3; int id, scores[scorenum]; char name[20]; ; void InputByUsers(Student *s) printf("please input id and names...>"); scanf("%d %s", &s->id, s->name); printf("please input %d scores...>", Student::ScoreNum); for (int i=0; i<student::scorenum; i+=1) scanf("%d", &s->scores[i] ); Student stu; InputByUsers(&stu); 16

17 Pointer to Struct/Class void InputByUsers_Batch(Student stu[], int size) for (int s=0; s<size; s+=1) printf("student %d:\n", s+1); printf("please input id and names...>"); scanf("%d %s", &stu[s].id, stu[s].name); printf("please input %d scores...>", Student::ScoreNum); for (int i=0; i<student::scorenum; i+=1) scanf("%d", &stu[s].scores[i] ); const int StudentNum = 10; Student stu[studentnum]; InputByUsers_Batch(stu, StudentNum); 17

18 Pointer to Struct/Class void InputByUsers_Batch(Student stu[], int size) for (int s=0; s<size; s+=1) printf("student %d:\n", s+1); InputByUsers(&stu[s]); const int StudentNum = 10; Student stu[studentnum]; InputByUsers_Batch(stu, StudentNum); 18

19 Pointers vs. References int TsaiYiLin; int *agent = &TsaiYiLin, &Jolin = TsaiYiLin; TsaiYiLin = 7; Jolin = 9; *agent = 11; agent TsaiYiLin/Jolin void passbyreference(int &Tsai10) Tsai10 = 10; cout << Tsai10; void passbypointer(int *agent) *agent = 90; cout << *agent; void passbyvalue(int photo) photo = -1; cout << photo; passbyreference(tsaiyilin); //TsaiYiLin changes to10. passbypointer(&jolin); //TsaiYiLin changes to90. passbyvalue(jolin); //TsaiYiLin does not change. 19

20 Pointers vs. References int TsaiYiLin, ChaoJayLun; int *agent = &TsaiYiLin, *agent2, // Ok. Pointers can be left uninitialized. &Jolin = TsaiYiLin, &Jay; // Error. References can NOT be left uninitialized. agent = &ChaoJayLun; // Pointers can be modified to point to other objects after definition. Jolin = ChaoJayLun; // References will not refer to other objects after definition. // You are modifying the object to which the reference refers. 20

21 Pointers vs. References void InputByUsers_CPP_Addr(const char *msg, int *p) cout << msg << "...>"; cin >> *p; void InputByUsers_CPP_Ref(const char *msg, int &p) cout << msg << "...>"; cin >> p; int val; InputByUsers_CPP_Addr("Please input an integer", &val); cout << val << endl; InputByUsers_CPP_Ref("Please input an integer", val); cout << val << endl; 21

Pointers. 1 Background. 1.1 Variables and Memory. 1.2 Motivating Pointers Massachusetts Institute of Technology

Pointers. 1 Background. 1.1 Variables and Memory. 1.2 Motivating Pointers Massachusetts Institute of Technology Introduction to C++ Massachusetts Institute of Technology ocw.mit.edu 6.096 Pointers 1 Background 1.1 Variables and Memory When you declare a variable, the computer associates the variable name with a