Question

—This is a Homework question—

I’m having problems reading float values from a text file using fscanf.

Basically I’m trying to read float values from a file and store them in a dynamic array.
The input file has two floats per line. so a line might be “0.85 7.34” (w/o quotes). so I’m trying to use fscanf(fp, “%f %f”, &coordinates[i], &coordinates[i++]) to read in the 2 float values. when i print it shows as 0.00000. Below is the code I wrote and the output it produces.

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

int main (int argc, char *argv []) {

FILE * fp = fopen("nums", "r");

float *coordinates;
float *tmp;
int i = 0;
int ARRAY_SIZE = 5;
coordinates = malloc(5*sizeof(float));

while (fscanf(fp,"%f %f", &coordinates[i], &coordinates[i++]) > 1)
{

  printf("iteration# %d | coord1 = %f coord2 = %f \n", i, &coordinates[i-1], &coordinates[i]);

  if (i >= ARRAY_SIZE)
  {
    tmp = realloc(coordinates, (i*2)*sizeof(float));
    coordinates = tmp;
    ARRAY_SIZE = i*2;
  }
  i++;
}

for(i = 0; i < 8; i++)
  printf("%f\n", &coordinates[i]);


return 0;
}

OUTPUT:

iteration# 1 | coord1 = 0.000000 coord2 = 0.000000 
iteration# 3 | coord1 = 0.000000 coord2 = 0.000000 
iteration# 5 | coord1 = 0.000000 coord2 = 0.000000 
iteration# 7 | coord1 = 0.000000 coord2 = 0.000000 
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000
0.000000  

Answer 1

You don’t use the “address of”-operator & with printf. fscanf requires a pointer to the data so it knows can change the variables value, while printf does not.

Change:

printf("iteration# %d | coord1 = %f coord2 = %f \n", i, &coordinates[i-1], &coordinates[i]);

To:

printf("iteration# %d | coord1 = %f coord2 = %f \n", i, coordinates[i-1], coordinates[i]);