Thread: File I/O Merging Files Program

Hi all this is my first post so forgive me if it's incorrectly posted and let me know what I can improve upon in further posts.

My assignment is to:

Write a program that merges the numbers in two files and writes the results to a third file. The program reads input from two different files and writes the output to a third file. Both files contain integers in ascending order(smallest to largest). After the program is executed, the output file contains the numbers from the two input files in one longer list, also in ascending order.

Question:
I've searched online and I think my function is correct, but I'm confused on how to call output is it "w" for writing or "w+" for reading and writing to the screen to check that the numbers have been merged correctly? Also, how do I display the output file of my function on screen?

PHP Code:

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


void file_merger(FILE *input_1, FILE *input_2, FILE *output);
int main (int argc, char * argv[])
{
    FILE *input_1, *input_2, *output;
    input_1 = fopen("numbers1.txt", "r");
    input_2 = fopen("numbers2.txt", "r");
    output = fopen("output.txt", "w+");


    if(input_1 == NULL || input_2 == NULL)
    {
        fprintf(stderr, "File failed to open.\n");
        exit(1);
     }


    fclose(input_1);
    fclose(input_2);
    fclose(output);
    return 0;
}
void file_merger(FILE *input_1, FILE *input_2, FILE *output)
{
    int i1, i2, value1, value2;
    i1 = fscanf(input_1, "%d", &value1);
    i2 = fscanf(input_2, "%d", &value2);


    while(i1 == 1 || i2 == 1)
    {
        if(value1 <= value2)
        {
            fprintf(output, "%d\n", value1);
            i1 = fscanf(input_1, "%d", &value1);
        }


        if(value2 <= value1)
        {
            fprintf(output, "%d\n", value2);
            i2 = fscanf(input_2, "%d", &value2);
        }


        while(i1 == 1)
        {
            fprintf(output, "%d\n", value1);
            i1 = fscanf(input_1, "%d", &value1);
        }
        while(i2 == 1)
        {
            fprintf(output, "%d\n", value2);
            i2 = fscanf(input_2, "%d", &value2);
        }
    }
} 


Excellent post!


Personally, I'd open the file with "w" and then, if you really need to print it out, close it and reopen it with "r". Of course, if you simply need to check the output file for correctness, you could open it in a text editor.


Your merge logic is not quite right. You need to move the second and third white loops AFTER the initial while loop. The idea is that the initial loop keeps going as long as BOTH files have data. The other two loops are for emptying out whichever file still has data remaining. So your condition on the first while loop should be an &&, not an ||.

Originally Posted by Adak

You want to use "w+r" mode.

Sadly there is no "w+r" mode.
You mean "w+".

However, I still prefer my idea.

Originally Posted by oogabooga

Excellent post!


Personally, I'd open the file with "w" and then, if you really need to print it out, close it and reopen it with "r". Of course, if you simply need to check the output file for correctness, you could open it in a text editor.


Your merge logic is not quite right. You need to move the second and third white loops AFTER the initial while loop. The idea is that the initial loop keeps going as long as BOTH files have data. The other two loops are for emptying out whichever file still has data remaining. So your condition on the first while loop should be an &&, not an ||.

Thank you for your help your post has given me two more questions.
1. Is the code below the correct way to call this function because when I opened the output file it read all of the first list and then all of the second list?
2. If I'm on a linux cluster that my school has setup could I run my program and then check to see if I have the right output by opening the output.txt file in an editor such as vi?

Code:

int main (int argc, char * argv[])
{
    FILE *input_1, *input_2, *output;
    input_1 = fopen("numbers1.txt", "r");
    input_2 = fopen("numbers2.txt", "r");
    output = fopen("output.txt", "w+");


    if(input_1 == NULL || input_2 == NULL)
    {
        fprintf(stderr, "File failed to open.\n");
        exit(1);
     }
    
    file_merger(input_1, input_2, output);

    fclose(input_1);
    fclose(input_2);
    fclose(output);
    return 0;
}

I don't see a call to the function in main.

You seem to be missing

Code:

    file_merger(input_1, input_2, output);

And, yes, you could look at it in vi.

You haven't yet moved the second and third while loops outside of the first one. Re-read Adak's post which (as usual) is clearer than mine.

Oh, sorry, I missed that brace.

But your problem is that you've accidentally changed from comparing the values to comparing the file pointers! I.e., this

Code:

if(input_1 <= input_2)

should be this

Code:

if(value1 <= value2)