I am trying to make a program that first creates an array in another function, returns it and then calls another function that shuffles the contents of the array and returns it. However I am struggling to do this in C since I do not quite understand the array pointer system that has to be used here.
So far my code doesnt return the values 1-20 from makeArray() but instead returns an array full of 0s and I have a feeling it has to do with the c's array pointer system.
Any help would greatly be appreciated! Thank you in advance
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int arrShuffle();
int arrShuffle(int * arr) {
int arr[21];
// shuffle array
for(int j=0; j<20; j ) {
int randInd = (rand() % 20) 1;
int temp = arr[j];
arr[j] = arr[randInd];
arr[randInd] = temp;
}
return arr;
}
int makeArray() {
int arr[21];
// make array of 1-20
for(int i=0; i < 20; i ) {
arr[i] = i 1;
}
return arr;
}
void main() {
int *orgArr;
int *modArr;
srand(time(NULL));
orgArr = makeArray();
for(int i=0; i < 20; i ) {
printf("OrgArr: %d\n", orgArr);
}
modArr = arrShuffle(orgArr);
}
CodePudding user response:
You cannot use variables with automatic storage (aka local ones). You must allocate the array so the memory remains valid after the function ends:
int* makeArray() {
int *arr = calloc(21, sizeof *a);
// make array of 1-20
for(int i=0; i < 20; i ) {
arr[i] = i 1;
}
return arr;
}
Remember to release the array when it is no longer used:
int main() {
int *orgArr;
...
orgArr = makeArray();
...
free(orgArr);
}
CodePudding user response:
As tstanisl
pointed out in their answer, a possible solution is to use dynamic memory allocation. My answer, instead, will give you yet another solution: using an array passed by the caller.
NOTE: both solutions are valid and their usefulness depends on the specific needs of your program. There's no "right" universal solution.
void makeArray(int arr[], size_t len) {
for (size_t i = 0; i < len; i = 1) {
arr[i] = (int) (i 1);
}
}
void cloneAndModifyArray(const int orig[], int new[], size_t len) {
for (size_t i = 0; i < len; i = 1) {
new[i] = orig[i] * 2; // or some other modification
}
}
And you use it like this:
#define ARR_LEN (100)
int main(void) {
int arr[ARR_LEN];
makeArray(arr, ARR_LEN);
int modified_arr[ARR_LEN];
cloneAndModifyArray(arr, modified_arr, ARR_LEN);
return 0;
}