Using fgets() and strtok() to read in a file line-by-line in C?

Question

I\'m trying to use fgets and strtok() to read in a file line by line, and create a linked list of each different line of information.

Right now, I\'m only just putt

Answer 1

There are several issues with this code. I've quickly modified your code to do what is expected.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXSTRINGSIZE 10
#define LINESIZE 128

struct person{
    char firstName[MAXSTRINGSIZE];
    char lastName[MAXSTRINGSIZE];
    char gender[MAXSTRINGSIZE];
    int birthYear;
    struct person *next;
} *first, *current;


int main (void){
    FILE *myfile = fopen ( "Assignment1file.txt", "r" );
    int i=0;
    int j=0;
    int k=0;
    int l=0;
    char *result[10][4];
    char line[LINESIZE];
    char *value;

    for(i=0; i<=9; i++){
        for(j=0;j<=4;j++){
            result[i][j] = NULL;
        }
    }
    i=0;

    // loop through each entry in Assignment1file
    while(fgets(line, sizeof(line), myfile)){
        //load last name
        value = strtok(line, ", ");
        result[i][0] = strdup(value);
    printf("last: %s\n", value);


        //load first time
        value = strtok(NULL, ", ");
        result[i][1] = strdup(value);
    printf("first: %s\n", value);

        // load gender
        value = strtok(NULL, ", ");
        result[i][2] = strdup(value);
    printf("gender: %s\n", value);

        // load birth year
        value = strtok(NULL, " \n");
        result[i][3] = strdup(value);
    printf("birth year: %s\n", value);

        //go to next line
        i++;
    }   

    // read out the array
    for(k=0; k<5; k++){
        for(j=0;j<4;j++){
            printf("%s\n", result[k][j]);
        }
    }

    fclose(myfile);
    return 0;
}

People have already commented on the details of this change.

Answer 2

You need to copy the tokens into a separate storage if you need them.

strtok() will modify the buffer that you reads the line in, and replace the delimiters with NUL character, and return a pointer to certain position in the buffer (that is the start of the current token).

When you read in the next line, the buffer will be filled with new data, hence, all the pointers that you have saved are useless, since the previous data is now gone.

Quote from documentation:

To determine the beginning and the end of a token, the function first scans from the starting location for the first character not contained in delimiters (which becomes the beginning of the token). And then scans starting from this beginning of the token for the first character contained in delimiters, which becomes the end of the token.

This end of the token is automatically replaced by a null-character by the function, and the beginning of the token is returned by the function.

And (emphasis mine):

str
C string to truncate. The contents of this string are modified and broken into smaller strings (tokens).
Alternatively, a null pointer may be specified, in which case the function continues scanning where a previous successful call to the function ended.

delimiters:
C string containing the delimiters.
These may vary from one call to another.

Answer 3

strtok() returns pointers inside line[] so when you read the next line all the pointers you saved are now pointing to places where the last line of the file is stored.

You could allocate memory for each bit of string like so:

//load last name
value = strtok(line, ",");
result[i][0] = malloc(strlen(value) + 1);
strcpy(result[i][0], value);

As an aside, you don't need the loops at the start to set everything to NULL, you could do this instead:

char *result[10][4] = {0};

Answer 4

strtok will modify the original string. Hence, previous pointers you stored are no longer there after each iteration.

Simple solution is to use: strdup to allocate and copy the values.

Just modify your assignment of value everywhere:

result[i][0] = value;

To:

result[i][2] = strdup(value);