Finding duplicates from an array of pointers

Question

I wanted to find duplicates present in the array of pointers. The code is shown below. When I run this application, it faces a segmentation problem. But when I extract this function, I can run it just fine. someone will tell me what can`

when I detect duplicates, I just put these lines in a file called output.txt.

I found that when strcmp is used, it gives this segmentation error. but when I extract this function and run it on some kind of test code, it works fine.

main() { char *a[20]; DIR *dip; int i = 0; dip = opendir("src/my_folder"); char *condition_var; while ((dit = readdir(dip)) != NULL) { condition_var = dit->name; a[i] = condition_var i++; } findduplicates(a,i); } char *findduplicates(char *arr[3],int count) { int i = 0; int j = 0; int val = 0; FILE *output = fopen("output.txt","w"); for(i = 0;i<count;i++) { j = i+1; for(;j<count;j++) { if(strcmp(arr[i],arr[j])==0) { printf("The index of a duplicate elemnt is %d\n",j); arr[j] = " "; } } } int k = 0; while(k<3) { printf("the aarray is %s\n",arr[k]); fputs(arr[k],output); fputs("\n",output); k++; } 

} `

advanced thanks Maddy

Answer 1

 char *a[20]; // an array of char pointers. 20 elements in the array. char *findduplicates(char *arr[3],int count) // arr is an array of char pointers, with 3 elements in the array. findduplicates(a, count); // this will just pass 3 of the 20 pointers // thus when you try to access a[3], it will probably seg-fault. 

try this instead:

 char *findduplicates(char **arr,int count) 

[edit] Maybe I'm wrong. I wrote a small test program, and it will not work, even if the function parameter is declared as char *a[0] . But try my suggestion anyway. Memory errors do not always cause a malfunction ...

Answer 2

I wrote something that will find duplicates in an arbitrary array of strings and print the names. I cannot say from your question if you need indexes from the original array. This version will sort the array, and the original indexes are "lost." But if you only need to know the names that are duplicates, the following should work:

 #include <string.h> #include <stdio.h> #include <malloc.h> #include <stdlib.h> #define TRUE 1 #define FALSE 0 /* Compare function for quicksort. Shamelessly taken directly from linux man * page */ static int cmpstringp(const void *p1, const void *p2) { return strcmp(* (char * const *) p1, * (char * const *) p2); } void find_duplicates(char *strings[], size_t size) { size_t i, j, last_occurrence; int is_unique; for(i = 0; i < size; ++i) { /* This variable tells us if the current element is unique */ is_unique = TRUE; for(j = i; j < size; ++j) { /* Make sure we aren't comparing the element to itself */ if((j != i) && (strcmp(strings[i], strings[j]) == 0)) { /* Since the array is sorted, we store the index of the last * duplicate and continue our search from there. */ last_occurrence = j; is_unique = FALSE; } } /* The element was not unique, print the name */ if(is_unique == FALSE) { printf("Duplicate: %s\n", strings[i]); /* Set the counter to the next element we must check for * "uniqueness" */ i = last_occurrence; } } } int main(int argc, char *argv[]) { /* Test array */ char *test_array[] = { "test1", "test2", "test3", "test1", "test4", "test1", "test4" }; /* Sort it using quicksort to make the process of finding duplicates * easier */ qsort(test_array, 7, sizeof(char*), cmpstringp); find_duplicates(test_array, 7); return 0; } 

Answer 3

I think this part will be the root of the problem:

  while ((dit = readdir(dip)) != NULL) { condition_var = dit->name; a[i] = condition_var; i++; } 

I'm not sure how readdir () works, but I would assume that it returns a temporary structure pointer to dit each time. a [i] saves the var_condition as a pointer to a name. It is possible that another readdir () call will be made later and will use the same memory space to return the temporary structure by overwriting the memory space pointed to by the previous var_ condition.

I would suggest a safer way to do this part, which can be a little more complicated.

  while ((dit = readdir(dip)) != NULL) { int len = strlen(dit->name); condition_var = malloc(len+1); memncpy(condition_var, dit->name, len+1); //condition_var = dit->name; a[i] = condition_var; i++; } 

Remember to free each mem block that you malloced at the end of the program. (Or you can just leave it all on the OS to build mem - not recommended). The code should look like this:

  int j; // use another loop indicator if you already declared j for other uses. for(j = 0; j<i; j++) // i is the length of a[]. { free(a[j]); // no worry about a[j] being NULL. free() does nothing on a NULL pointer. a[j] = NULL; }