Reading in strings from a file and storing them in an array as an integer in C

Question

I'm trying to read in a file with a couple hundred integers, some positive, some negative and store them in an array. They have to be read in as a string using strtok, though. I keep getting a segmentation fault and I'm not sure why. The count is to figure out how many total integers are in the file.

/*Input file looks like this:
718321747   -1828022042
-1665405912 -175307986
-53757018 -1551069786 525902369
-1945908378 853648883
*/

int main(int argc, char* argv[])
{
    char buffer[50];
    char* token;
    int count = 0;
    int num = 0;
    int arr[MAX_SIZE];
    if (argc != 2)
    {
        printf("Invalid number of arguments\n");
        return 0;
    }
    FILE* fptr = fopen(argv[1], "r");
    //open file
    if (fptr == NULL)
    {
        printf("Unable to open file\n");
        return 0;
    }
    while(fgets(buffer, 50, fptr))
    //to get the file line by line
    {
        token = strtok(buffer, "\n\t ");
        //find first token
            num = atoi(token);
            //convert it to an int
            arr[count] = num;
            //store in array
            count++;

            while(token != NULL)
            //get rest of tokens and convert to int
            {
                token = strtok(buffer, "\n\t ");
                num = atoi(token);
                arr[count] = num;
                count++;
            }
    }
return 0;
}

Shouldn't that 2nd call to strtok() have NULL as the first parameter? — John3136
– John3136, Commented Jan 27, 2015 at 0:32

Iharob Al Asimi · Accepted Answer · 2015-01-27 00:46:19Z

2

You never check if the token was found in the string, you must check that strtok() didn't return NULL before trying to call atoi().

Then you keep scanning the same string with strtok() passing the string in each iteration, that's also wrong, you should pass NULL after the first time.

I would also recommend to use strtol() instead of atoi() to check if the conversion was successful.

Check this code, i fixed it

#include <stdio.h>
#include <stdlib.h>

#define MAX_SIZE 1000 /* ? whatever value you think is good. */

int main(int argc, char* argv[])
{
    char buffer[50];
    int  count = 0;
    int  arr[MAX_SIZE];

    if (argc != 2)
    {
        printf("Invalid number of arguments\n");
        return 0;
    }

    FILE* fptr = fopen(argv[1], "r");
    //open file
    if (fptr == NULL)
    {
        printf("Unable to open file\n");
        return 0;
    }

    //to get the file line by line
    while ((fgets(buffer, 50, fptr) != NULL) && (count < MAX_SIZE))
    {
        char *pointer;
        char *token;

        pointer = buffer;
        while (((token = strtok(pointer, "\n\t ")) != NULL) && (count < MAX_SIZE))
        {
            char *endptr;

            arr[count] = strtol(token, &endptr, 10);
            printf("%d\n", arr[count]);
            if (*endptr != '\0')
                printf("error: could not convert %s to integer\n", token);
            else
                count++;
            pointer = NULL;
        }
    }
    return 0;
}

I am not sure it will work for you because I haven't seen the structure of your input data, but I am sure it will not cause a segmentation fault.

edited Jan 27, 2015 at 0:46

answered Jan 27, 2015 at 0:32

Iharob Al Asimi

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2 Comments

iamklutch4life Over a year ago

Ok so now, it doesn't seg fault, but its only reading the first line of the input file. the structure of the data is ints separated by new lines, spaces, and tabs

Iharob Al Asimi Over a year ago

@BrianHurley You should post a sample of the file's content. For this question, this is the answer and hence you have to post a new question asking why the output is not as expected.

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