How to read numbers from file and calculate the mean?

Question

I have a list of numbers in my *.txt file:

1 2  
3

called numbers.txt. I need to read them and calculate the mean, for my file it will be: (1 + 2 + 3) / 3 = 2; Although my code shows some wrong results, it read 3 two times. Why is that, how to solve it?

My code:

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

double fun(const char *filename)
{
    double sum = 0, mean = 0, tmp = 0;
    int i = 0;
    FILE *f;
    if((f = fopen(filename, "r")) == NULL)
    {
        exit(-1);
    }

    while(!feof(f))
    {
        fscanf(f, "%lf", &tmp);
        printf("tmp = %f \n", tmp);
        sum += tmp;
        ++ i;
    }

    i = i - 1;
    mean = sum / i;

    fclose(f);

    printf("i = %d\n", i);
    printf("sum = %f\n", sum);
    printf("mean = %f\n", mean);

    return mean;
}

int main(int argc, char **argv)
{
    fun("numbers.txt");

    return 0;
}

Stefano Sanfilippo · Answer 1 · 2014-01-19T12:24:09.967

You are not actually reading 3 twice.

The problem is that after the last number (3), there are other characters, namely whitespaces. So, the EOF flag is not raised after reading the last number and your while loop enters another iteration. No value is read by fscanf, but the previous one is still in tmp, which gets processed again.

What you should do, is check the return value of fscanf, which will tell you the number of parameters actually scanned and converted. It should be 1, because you are trying to scan one variable. Fix your code like:

while(!feof(f))
{
    if (fscanf(f, "%lf", &tmp) != 1) {
        // No numbers
        break;
    }

    printf("tmp = %f \n", tmp);
    sum += tmp;
    ++ i;
}

and remove the i = i - 1. You had to write it to compensate the fact that loop was iterating one more time than intended.

chux - Reinstate Monica · Answer 2 · 2014-01-19T12:17:18.657

3

Change from testing feof() to testing fscanf()

while(fscanf(f, "%lf", &tmp) != EOF) {
  printf("tmp = %f \n", tmp);
  sum += tmp;
  ++ i;
}

feof(f) is not true until a read pass the end of file was attempted. Thus the previous call of fscanf(f, "%lf", &tmp) returned EOF in OP's original code, not updating tmp. So tmp appeared to be 3 twice.

As @BLUEPIXY points out, the i = i - 1; is not needed. If anything a test of if (i==0) could be added for the rare case of no input.

edited Jan 19 '14 at 12:17

answered Jan 19 '14 at 12:11

chux - Reinstate Monica

143,097
13
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256

The best solution is to `int cnt; while((cnt = fscanf(f, "%lf", &tmp) != 1) { ` Then test `cnt`. `EOF` means EOF or IO Error, 0 means failed to convert to an `double`, 1: means 1 successful conversion. – chux - Reinstate Monica Jan 19 '14 at 12:21
The numbers in parentheses are not aligned in the previous comment. – BLUEPIXY Jan 19 '14 at 12:28

score 2 · Answer 3 · answered Jan 19 '14 at 12:14

It's read 3 two time because feof(f) enters the loop one more time than the you expects.

In this case, feof() is called before any data has been read, so it returns false. The loop is entered, fgetc() is called (and returns EOF), and count is incremented. Then feof() is called and returns true, causing the loop to abort.

How to read numbers from file and calculate the mean?

3 Answers3