C program to extract different substrings from array of strings - c

Question

I want to extract different substrings from a file with array of strings. My file resembles this.
abcdxxx
efghijkyyy
lmzzz
ncdslanclsppp
kdfmsqqq
cbskdnsrrr
From the above file I want to extract xxx, yyy, zzz, ppp, qqq, rrr (basically last 3 characters) and store into an array. I refered this link How to extract a substring from a string in C? but not felt feasible because the content in my file is dynamic and might change for next execution. Can someone give a brief on this?
Here is my approach
FILE* fp1 = fopen("test.txt","r");
if(fp1 == NULL)
{
printf("Failed to open file\n");
return 1;
}
char array[100];
while(fscanf(fp1,"%[^\n]",array)!=NULL);
for(i=1;i<=6;i++)
{
array[i] += 4;
}

Answer 1

the content in my file is dynamic and might change for next execution
Then you need realloc or a linked list:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
FILE *f;
char **arr = NULL;
char s[100];
size_t i, n = 0;
f = fopen("text.txt", "r");
if (f == NULL) {
perror("fopen");
exit(EXIT_FAILURE);
}
while (fgets(s, sizeof s, f) != NULL) {
arr = realloc(arr, sizeof(*arr) * (n + 1));
arr[n] = calloc(4, 1);
memcpy(arr[n++], s + strlen(s) - 4, 3);
}
fclose(f);
for (i = 0; i < n; i++) {
printf("%s\n", arr[i]);
free(arr[i]);
}
free(arr);
return 0;
}
Output:
xxx
yyy
zzz
ppp
qqq
rrr
If you always want the last 3 characters you can simplify:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
FILE *f;
char (*arr)[4] = NULL;
char s[100];
size_t i, n = 0;
f = fopen("text.txt", "r");
if (f == NULL) {
perror("fopen");
exit(EXIT_FAILURE);
}
while (fgets(s, sizeof s, f) != NULL) {
arr = realloc(arr, sizeof(*arr) * (n + 1));
memcpy(arr[n], s + strlen(s) - 4, 3);
arr[n++][3] = '\0';
}
fclose(f);
for (i = 0; i < n; i++) {
printf("%s\n", arr[i]);
}
free(arr);
return 0;
}