Arranging numbers in a file using C

Question

I need to take the numbers listed in the two files "numbers1.txt" and "numbers2.txt" (where the numbers are listed in ascending order) and move them to a file named "output.txt" where they are arranged in ascending order.

Here is what I have so far:

void appendToOutput(FILE *numFile1, FILE *numFile2, FILE *outputFile)
{
int num1 = 0;
int num2 = 0;

do {
    fscanf(numFile1, "%d", &num1);
    fscanf(numFile2, "%d", &num2);
    while ((num1 < num2) && !feof(numFile1))
    {
        fprintf(outputFile, "%d\n", num1);
        fscanf(numFile1, "%d", &num1);
    }
    while ((num2 < num1) && !feof(numFile2))
    {
        fprintf(outputFile, "%d\n", num2);
        fscanf(numFile2, "%d", &num2);
    }
    if (num1 == num2)
    {
        fprintf(outputFile, "%d\n%d\n", num1, num2);
    }
} while (!feof(numFile1) || !feof(numFile2));

return;
}

my files look like the following:

numbers1.txt

1
2
3
4
5
6
7
8
9
10
11
12

numbers2.txt:

2
4
6
8
10
12
14
16
18
20
22
24

The issue I am having is that the output file ends up looking like this: output.txt

1
2
2
3
4
4
5
6
6
7
8
8
9
10
10
11
12
12

So my issue is that the program is not continuing to read/write numbers from numbers2.txt even though it has not yet hit the end of it's file. I've looked through it and I can't seem to find out why it's stopping, so help would be appreciated!

Answer 1

There are a few problems with the current code

1. Initialization of the 2 numbers should be performed before the main loop
2. Use of feof() here
3. Test of strict < insteaf of <= (could lock your algo)
4. Keep reading a file if the other one becomes empty

Regarding 2., fscanf returns a number >0 if it could read the number, so you can test this instead of checking feof (that says that EOF (a stop condition) happened before, and this might trigger one more unwanted iteration).

Suggested fixed code:

int read1 = fscanf(numFile1, "%d", &num1);
int read2 = fscanf(numFile2, "%d", &num2);

while (read1 > 0 || read2 > 0) {
    while (read1 > 0 && (read2<=0 || num1 <= num2))
    {
        fprintf(outputFile, "%d\n", num1);
        read1 = fscanf(numFile1, "%d", &num1);
    }
    while (read2 > 0 && (read1<=0 || num2 <= num1))
    {
        fprintf(outputFile, "%d\n", num2);
        read2 = fscanf(numFile2, "%d", &num2);
    }
} 

Further explanations

• readX<=0 || numY <= numX addresses 4.
• readX = fscanf(...) addresses2.