GCC Compilation Error on array assignment - c

Question

I am trying to convert a string into its equivalent matrix form in C. The matrix would have 3 rows and as many columns as required. The following code doesn't compile, and I haven't figured out what's going wrong.
The error that GCC throws is:
app.c:10:25: error: subscripted value is not an array, pointer, or vector
printf("%d\n", arr[i][k]);
~~~^~
1 error generated.
Main file (app.c):
#include <stdio.h>
#include "converter.h"
int main() {
char source[] = "This is the source. "; // placeholder text
int arr = convert(source);
for (int i = 0; i < 21; i++) {
for (int k = 0; k < 3; k++) {
printf("%d\n", arr[i][k]); // error occurs at this line.
}
}
return 0;
}
converter.c file:
// Converts an input string to its respective ASCII matrix.
#include <string.h>
#include <stdio.h>
#include "converter.h"
// Converts the entire string into an multi-dimensional array.
int convert(char text[]){
// copy the input text into a local store.
char store[strlen(text)];
strcpy(store, text);
// make sure the length of the input string is a multiple of 3 or make it so.
int excess = strlen(store)%3;
char excess_spaces[3] = " ";
if (excess != 0) {
strncat(store, excess_spaces, 3-excess);
}
// covert the source into an array
int arr[3][strlen(store)/3];
int steps = strlen(store)/3;
for (int i = 0; i < steps; i++) {
int t[3];
for (int k = 0; k < 3; k++) {
t[k] = (int) store[3*i+k];
arr[k][i] = t[k];
}
}
return arr;
}
converter.h file:
int convert(char text[]);

Answer 1

There are multiple issues in this code.
The allocating storage for string, one must include one byte for a null terminator. Replace:
char store[strlen(text)];
with:
char store[strlen(text) + 1];
Additionally store must be big enough to contain the excess which is up to 3 spaces.
char store[strlen(text) + 3 + 1];
In C you cannot use an array as a value. It is converted to a pointer to it's first element in pretty must every context. Therefore it is not possible to return an array directly. It could be workaround by wrapping an array with a struct but it a topic for another day.
As result return arr will be equivalent to return &arr[0] which is int (*)[XXX] a pointer to int array of size XXX.
Never ever return a pointer to an object with automatic storage. It's Undefined Behaviour. I know that the intention was returning an array not a pointer to it. Create an object with dynamic storage with malloc-like function to safely return a pointer.
Returning Variable Length Array (VLA) by value is not possible because Variably Modified (VM) types cannot be defined at file scope.
It looks that indices are swapped in:
printf("%d\n", arr[i][k]);
I guess it should be arr[k][i].
Now... let's solve it.
Returning VLA is tricky. One solution is to pass a pointer to VLA as an argument. See https://stackoverflow.com/a/14088851/4989451.
The issue with this solution is that the caller must be able to compute the dimensions.
The other way it to wrap the result of the convert() to a struct. Note that the function and the struct can share the name. The result with have the sizes of VLA as n and m members and the pointer to the data as arr. The caller need to cast it to proper VM type.
To cumbersome casts between the non-trivial pointer types, one can cast via void*.
When all work with the array is done, release it memory with free().
// Converts an input string to its respective ASCII matrix.
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
// Converts the entire string into an multi-dimensional array.
struct convert {
int n, m;
int *arr;
} convert(char text[]){
// copy the input text into a local store.
size_t textlen = strlen(text);
char store[textlen + 3 + 1];
strcpy(store, text);
// make sure the length of the input string is a multiple of 3 or make it so.
int excess = textlen % 3;
char excess_spaces[3] = " ";
if (excess != 0) {
strncat(store, excess_spaces, 3-excess);
}
size_t storelen = strlen(store);
// allocate VLA with dynamic storage
int (*arr)[storelen / 3] = malloc(3 * sizeof *arr);
// covert the source into an array
int steps = storelen / 3;
for (int i = 0; i < steps; i++) {
int t[3];
for (int k = 0; k < 3; k++) {
t[k] = (int) store[3*i+k];
arr[k][i] = t[k];
}
}
return (struct convert){ .n = 3, .m = steps, .arr = (int*)arr };
}
int main() {
char source[] = "This is the source. "; // placeholder text
struct convert res = convert(source);
int n = res.n, m = res.m;
int (*arr)[m] = (void*)res.arr;
for (int i = 0; i < n; i++, puts("")) {
for (int k = 0; k < m; k++) {
printf("%d ", arr[i][k]); // error occurs at this line.
}
}
free(arr);
return 0;
}