char array assignment

Question

i am trying to read lines from a text file into char array, but there is something wrong. Please see the code and let me know what am i doing wrong. thanks.

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    int i=0,j;
    char* string[100];
    char line[100];
    FILE *file; 
    file = fopen("patt", "r"); 
    while(fgets(line, sizeof line, file)!=NULL) {
        printf("%d %s",i, line);
        string[i]=line;
        i++;
    }
    for (j=0 ; j<i ; j++) {
        printf("string[%d] %s",j, string[j]);
    }
    fclose(file);
    return 0;
}

the input file patt has the following contents.

rec
cent
ece
ce
recent
nt

On executing the codes above i get this

0 rec
1 cent
2 ece
3 ce
4 recent
5 nt
string[0] nt
string[1] nt
string[2] nt
string[3] nt
string[4] nt
string[5] nt

what i expect is this

0 rec
1 cent
2 ece
3 ce
4 recent
5 nt
string[0] rec
string[1] cent
string[2] ece
string[3] ce
string[4] recent
string[5] nt

Answer 1

You are repeatedly writing to the same array.

while(fgets(line, sizeof line, file)!=NULL) {

In this line you repeatedly, write to the array line.

---

I think you are misunderstanding what char* string[100]; does. You are using it as an array of arrays, while it actually is an array of 100 pointers.

To correct the problem, you need to first allocate a new memory block in every iteration :

 string[i] = malloc(strlen(line)+1);

Then you need to copy the contents of line to string[i] :

strcpy(string[i], line); 

Also, at the end of the program you will need to use free to release the memory.

Answer 2

Your string[i]=line; means you're repeatedly storing a pointer to the same buffer (line) into each successive item in string. Something like this:

To make things work, you need to allocate a new buffer for each of those to point at, something like: string[i] = dupe_string(line);, where dupe_string will be something like:

char *dupe_string(char const *in) { 
    char *dupe;
    if (NULL != (dupe = malloc(strlen(in) + 1)))
        strcpy(dupe, in);
    return dupe;
}

Note that since this uses malloc to allocate space for each string, you'll eventually need to call free for each of those to avoid having a memory leak.

Answer 3

To help you out understanding your code:

• You are writing to string[i] the address of the line every time, while the content of the array line keeps changing throughout the loop. This results in, all string[i] to contain the address of the same variable i.e. line. At the end of the loop, the last line read from the file is stored in line array. Now while you are printing string[i], you are printing the data present in the variable line.Since, all the instance of string[i] contain the same address of line, they all are printing the same value i.e. nt in your output.

int main(void) {
    int i=0,j;
    char* string[100];
    char line[100];
    FILE *file; 
    file = fopen("patt", "r"); 
    while(fgets(line, sizeof(line), file)!=NULL) {//You missed the ()'s in the sizeof
        printf("%d %s",i, line);
        string[i]=line;
        i++;
    }

*What you should have instead done is:

int main(void) {
int i=0,j;
char* string[100];
char line[100];
FILE *file; 
file = fopen("patt", "r"); 
while(fgets(line, sizeof(line), file)!=NULL) {//You missed the ()'s in the sizeof
    printf("%d %s",i, line);
    string[i]=malloc(strlen(line)+1); ///You need to allocate some memory to char* string[i] here
    if(string[i] == NULL)//Do a NULL check if malloc failed
      //Handle NULL here
    strncpy(string[i],line,strlen(line)+1);//Use strncpy to copy line into the malloced memory in string[i]
    i++;
}

Now, since you have malloced the memory for storing you data you need to free the memory too. So, instead of this in your code:

    for (j=0 ; j<i ; j++) {
        printf("string[%d] %s",j, string[j]);
    }
    fclose(file);
    return 0;
}

Do this:

    for (j=0 ; j<i ; j++) {
    printf("string[%d] %s",j, string[j]);
    free(string[j]);
    }
   fclose(file);
   return 0;
}

Now, that should give you what you wanted.