C pointer to one integer
int* pt = (int*)malloc(sizeof(int));
pt[0] = 3;
printf("%d", pt[0]);
// free memory
free(pt);
C pointer to array of integer
int num = 4;
int* pt = (int*)malloc(sizeof(int) * num);
for(int i = 0; i < num; i++){
pt[i] = i;
}
for(int i = 0; i < num; i++){
printf("%d\n", pt[i]);
}
// free memory
free(pt);
There is no difference between free pointer to ONE Int or array of Int
with ONE Int is same as ONE element of array of Int
C allocate 2d array and free them.
int height = 3;
int width = 4;
// allocate memory for height x width 2d array.
int** pt2 = (int**)malloc(sizeof(int*)*height);
for(int i=0; i< height; i++){
pt2[i] = (int*)malloc(sizeof(int)*width);
}
for(int i = 0; i < height; i++){
for(int j = 0; j < width; j++){
pt2[i][j] = i + j;
}
}
// deallocate the memory
for(int i=0; i< height; i++){
free(pt2[i]);
}
if(pt2 != NULL)
free(pt2);
Copy int** arr to int** duplicated_arr
int** arr ⇒ int** duplicated_arr
int** arr int** duplicated_arr
+-+ +-+
| | → int ← - -+ | |
| | | | |
| | → int | | |
| | ↑ | | |
+-+ | | +-+
| | +-+
| | - → | | → int
| | |
+ - - - - - → | | → int
| |
+-+
free int** arr ⇒ 🗑
+-+
| |
| |
| |
| |
+-+
Copy int** to other int**
0. Given an original int** arr
1. We want to copy int** arr to int** duplicated_arr
2. We can allocate int** duplicated_arr first
3. Copy all int* from int** arr to int** duplicated_arr
3. free int** arr
int num = 3;
int** arr = (int**)malloc(sizeof(int*) * num);
int** duplicated_arr = (int**)malloc(sizeof(int*) * num);
for(int i = 0; i < num; i++){
arr[i] = (int*)malloc(sizeof(int));
arr[i][0] = i;
}
for(int i = 0; i < num; i++){
pl("arr[%d][0]=%d", i, arr[i][0]);
}
// copy int** arr => int** duplicated_arr
for(int i = 0; i < num; i++){
duplicated_arr[i] = arr[i];
}
free(arr);
for(int i = 0; i < num; i++){
pl("duplicated_arr[%d][0]=%d", i, duplicated_arr[i][0]);
}
for(int i = 0; i < num; i++){
free(duplicated_arr[i]);
}
Reallocate memory with old pointer or extend old pointer memory
1. Use realloc(oldpt, sizeof(int)*len);
#include <cstdlib>
void* realloc(void* ptr, size_t size);
+--------+ +----------+ +--+
| | -> | | | |
+--------+ +----------+ +--+
int len = 4;
↑
+ -----> 2*len
float* arr = (float*)malloc(sizeof(float)*len);
for(int i = 0; i < len; i++){
arr[i] = i;
}
printArrayX(arr, len);
arr = (float*) realloc(arr, sizeof(float)*(2*len));
↑ ↑
+ → Reuse pt + → Append len float to arr
for(int i = len - 1; i < 2*len; i++){
arr[i] = (10*i);
}
fw("2*len");
printArrayX(arr, 2*len);
free(arr);
Cpp new and delete in 2D
// two dimension array
int ncol = 4, nrow = 3;
int** arr = (int**) new int*[ncol];
// allocate row
for(int j=0; j < 2; j++){
arr[j] = (int*) new int[nrow];
}
//
for(int j=0; j < ncol; j++){
arr[j] = (int*) new int[nrow];
for(int i = 0; i < nrow; i++){
arr[j][i] = j * ncol + i;
}
}
printArray2dint(ncol, nrow, arr);
for(int i=0; i < ncol; i++)
delete[] arr[i];
delete[] arr;
3D array, 3d cube
printf("2 x 3 x 4 cube\n");
/*
\
\
- - - - >
/
/
/
*/
int arr[2][3][4];
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 3; j++) {
for(int k = 0; k < 4; k++) {
arr[i][j][k] = i + j + k;
}
}
}
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 3; j++) {
for(int k = 0; k < 4; k++) {
printf("%d ", arr[i][j][k]);
}
printf("\n");
}
printf("\n");
}
/*
2 x 3 x 4 cube
0 1 2 3
1 2 3 4
2 3 4 5
1 2 3 4
2 3 4 5
3 4 5 6
*/
Compare C++ allocate 3D array and C malloc 3D array
int* a1 = (int*) new int;
↑
⇳
↓
int* a1 = (int*) new int[1]
↑
+ -> 1 □ contains ℕ
int** a2 = (int**) new int*[10];
↑
+ -> 10 □ contains ℕ*
int*** a3 = (int***) new int**[10];
↑
+ -> 10 □ contains ℕ**
int* a1 = (int*) malloc(sizeof(int));
↑
⇳
↓
int* a1 = (int*) malloc(sizeof(int) * 1);
↑
+ -> 1 □ contains ℕ
int** a2 = (int**) malloc(sizeof(int*) * 10);
↑
+ -> 10 □ contains ℕ*
int*** a3 = (int**) malloc(sizeof(int**) * 10);
↑
+ -> 10 □ contains ℕ**
Allocate and Deallocate functions in C++
int* allocate(int size){
return new int[size];
}
void deallocate(int*& pt){
delete[] pt;
pt = NULL;
}
Why You can NOT find the length of array if you allocate memory dynamically
int* arr = new int[10];
int len = sizeof(arr)/sizeof(int); // not equal 10
int arr[10];
int len = sizeof(arr)/sizeof(int); // => equal 10
Dynamically allocate two dimension array in C
int height = 3;
int width = 4;
int* pt = (int*)malloc(sizeof(int)*width);
int** array = (int**)malloc(sizeof(int*)*height);
for(int i = 0; i < height; i++)
array[i] = (int*)malloc(sizeof(int)*width);
Allocate and Deallocate 3D array or tripple pointers in C/C++/ObjectiveC with malloc
