Attempting to Read Certain Values from a File into an Array in C

Question

I have a file,examplefile.txt, with data like this:

0.000 3.142
3.142 4.712
4.712 1.571

I am trying to read only the values in the second column. In an effort to do this I have been using the following code.

#include <stdio.h>
#include <float.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>

int main(int argc, const char * argv[])
{
    double tempvalue;
    int a,b;

    FILE *file;
    char *myfile=malloc(sizeof(char)*80);
    sprintf(myfile,"examplefile.txt");
    if (fopen(myfile,"r")==NULL)
    {

    }
    else
    {
        file=fopen(myfile,"r");
        for (a=0;a<3;a++)
        {
            for (b=0;b<2;b++)
            {
                printf("a=%d\n",a);
                fscanf(file,"%lf",&tempvalue);
                if (b==1)
                {
                    printf("%lf\n",tempvalue);
                }
            }
        }  
    }
}

However,this only returns the value 1.571 every time. I can't store all of the values because the actual file I'm using contains too many values to store. I have not been able to find a solution for this problem yet.

6 values is too many? Are you writing one on each of your fingers? Of one hand? — Blindy
– Blindy, Commented Feb 18, 2015 at 20:05
No it isn't. My actual file contains 100,000,000 values in two columns of 50,000,000 values apiece. — bandit1979
– bandit1979, Commented Feb 18, 2015 at 20:06

user590028 · Accepted Answer · 2015-02-18 20:49:37Z

2

I suggest reading both, and just ignoring the first. Something like this

#include <stdio.h>
int main() {
    float v1, v2;
    FILE* file = fopen(myfile, "r");
    if (file == NULL) {
        perror("open failed");
        return -1;
        }
    while(fscanf(file, "%f %f", &v1, &v2) == 2) {
        print("read %f\n", v2);
        }
    return 0;
    }

edited Feb 18, 2015 at 20:49

answered Feb 18, 2015 at 20:19

user590028

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1 Comment

WhozCraig Over a year ago

while(fscanf(file, "%f %f", &v1, &v2) == 2) would ensure you read two values successfully.

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