Working with two files in C

Question

I am reading a file named "text.txt" that contains a series of numbers with a space in between. I want to write in a new file named "text_nr.txt", all the numbers in "text.txt" but without repetition! My code runs in an infinite loop at the second while loop, but I can't understand why.

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


int main()
{
    int n=0,m=0,nr;

    FILE *fp1;
    FILE *fp2;
    fp1 = fopen("text.txt","r");
    fp2 = fopen("text_nr.txt","a+");

    while(fscanf(fp1,"%d",&n) != EOF){
        nr=0;
        while(fscanf(fp2,"%d",&m) != EOF){
            printf("%d  %d ",m,n);
            if(n != m) continue;
            else{
                nr++;
                break;
            }
        }   
        if(nr == 0){
            fprintf(fp2,"%d ",n);
        }
    }

    fclose(fp1);
    fclose(fp2);

    return 0;      
}

At some point in the input, the format to fscanf doesn't match, so fscanf returns 0 but does not advance the file pointer. So it keeps reading from the same spot and returns 0 each time. — William Pursell
– William Pursell, Commented May 29, 2016 at 12:14
@WilliamPursell It does that at first time it starts to run. I have just started working with files and i don't know why that happens. Can you explain a bit further so i can understand and fix this problem ? — Al.Ka
– Al.Ka, Commented May 29, 2016 at 12:17
My general approach to these types of problems is to never use scanf, so I'm not the one to help! IMO, scanf is complicated, and you will learn more if you use fread and strtol instead. — William Pursell
– William Pursell, Commented May 29, 2016 at 12:19

Community · Accepted Answer · 2017-05-23 11:58:42Z

1

Functions like fscanf() return various values like EOF (end of file or error ) or the number of specifiers matched, 0, 1, ...

Rather than test for one value code needs to stop on, test against all undesired values.

// while(fscanf(fp1,"%d",&n) != EOF){
while(fscanf(fp1,"%d",&n) == 1){
    nr=0;
    // while(fscanf(fp2,"%d",&m) != EOF){
    while(fscanf(fp2,"%d",&m) == 1){

@William Pursell explained it well

At some point in the input, the format to fscanf doesn't match, so fscanf returns 0 but does not advance the file pointer. So it keeps reading from the same spot and returns 0 each time.

Infinite loop.

edited May 23, 2017 at 11:58

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answered May 29, 2016 at 13:46

chux

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1 Comment

David C. Rankin Over a year ago

Creative license with 'line' -> 'like' in first line :)

chux · Accepted Answer · 2016-05-29 13:38:43Z

1

Your inner loop will not reach the end of file since it won't match your search string if the file starts empty.

You could do the fscanf and check if it's 0 (couldn't find anything) or EOF (hit the end).

edited May 29, 2016 at 13:38

chux

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answered May 29, 2016 at 12:23

Wayne Booth

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4 Comments

Al.Ka Over a year ago

Doesn't it hit EOF at the first time it starts to read it if the file is empty?

Wayne Booth Over a year ago

Even if your at the start of an empty file, you've not hit the EOF flag, so no. (Updated the answer for clarity)

Al.Ka Over a year ago

Even if the second file is not empty it still runs in an infinite loop. Itried to put some numbers in there

Wayne Booth Over a year ago

Wha do you get if you change "!= EOF" to "== 1" ?

Mike Kor · Accepted Answer · 2016-05-29 13:03:20Z

0

Try to use fseek function to move stream pointer inside output file.

 while(fscanf(fp1,"%d",&n) != EOF){

    fseek ( fp2, 0 , SEEK_SET );
    bool flag = false;

    while(fscanf_s(fp2,"%d",&m) != EOF){
        printf("%d  %d ",m,n);

        if(n == m) 
        {
            printf("%d = %d ", m, n);
            flag = true;


            break;
        }
    }   
    if(!flag)
    {
        fseek ( fp2, 0 , SEEK_END );
        fprintf(fp2,"%d ",n);
    }
}

EDIT

Another method is to store written values in array. For this you can use vector.

vector<int> vData;

while(fscanf(fp1,"%d",&n) != EOF){


    bool flag = false;

    for(int i = 0; i < vData.size(); i++)
    {
        if(vData[i] == n)
        {
            flag = true;
            break;
        }
    }

    if(!flag)
    {
        vData.push(n);
        fprintf(fp2,"%d ",n);
    }
}

edited May 29, 2016 at 13:03

answered May 29, 2016 at 12:27

Mike Kor

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3 Comments

Al.Ka Over a year ago

It still runs in an infinite loop!

Mike Kor Over a year ago

I've made some changes. What numbers are you comparing?

Al.Ka Over a year ago

This doesn't chane anything, the program fails here: while(fscanf(fp2,"%d",&m) != EOF){ printf("%d %d ",m,n);

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