Help with reading in a file

[NeedHelpWithC]

Can anyone help me with a code that reads numbers from a file and puts them into a 2-D Array?

Basically the file has 1000 rows and 2 columns.

For example, a section would look like:

47 56
20 15
20 16
35 76
32 12

So I guess can someone help me understand how to write a code that would put that data into an 2-d array? (if that's even possible) Maybe the first column of numbers into one variable and the second column into the other?

This is the code I used when I tried to even see if I could read the file.

int main(void)
{

    int first_number;
    int second_number;
    
    FILE* finput;
    
    finput = fopen("numbers.txt", "r");
    
    do
    {
        fscanf(finput, "%d %d ", &first_number, &second_number);
        printf("%d - %d\n", first_number, second_number);
        
    }while(!feof(finput));
    
    fclose(finput);
    
    system("PAUSE");
    return 0;
}

I know this isn't near what I wanted to do, but it sort of worked. Except for some reason it only read a portion of the file.

I think I might be hopeless

[tabstop]

Why do you think it only read a portion of the file, and which portion of the file do you think it read?

[brewbuck]

To read into a 2-D array, it helps to have such an array in your code

You will need something like this:

int data[ NUM_ROWS ][ NUM_COLS ];

And then the data on the M'th row, N'th column, should be stored and accessed as:

data[ M ][ N ];

In your case, NUM_ROWS == 1000, and NUM_COLS == 2.

So you'll need a loop which reads every line of the file. The line number can be indicated by a variable m, which starts at zero and increments for each line. For each line, you break it into two parts, and store these parts into slots data[ m ][ 0 ] and data[ m ][ 1 ].

[NeedHelpWithC]

Why do you think it only read a portion of the file, and which portion of the file do you think it read?

Hmm...maybe because I didn't specify what lines of data I wanted? I'm not sure how many numbers it read but it's definitely from the end of the file. I'd say maybe the last 200-300 numbers.

[tabstop]

Hmm...maybe because I didn't specify what lines of data I wanted? I'm not sure how many numbers it read but it's definitely from the end of the file. I'd say maybe the last 200-300 numbers.

So that means it read them all, and printed them all, but your terminal window only shows the last few. Run your program and redirect it to a (different) file and you'll see all the numbers.

[NeedHelpWithC]

To read into a 2-D array, it helps to have such an array in your code

You will need something like this:

int data[ NUM_ROWS ][ NUM_COLS ];

And then the data on the M'th row, N'th column, should be stored and accessed as:

data[ M ][ N ];

In your case, NUM_ROWS == 1000, and NUM_COLS == 2.

So you'll need a loop which reads every line of the file. The line number can be indicated by a variable m, which starts at zero and increments for each line. For each line, you break it into two parts, and store these parts into slots data[ m ][ 0 ] and data[ m ][ 1 ].

Maybe something close to this?

#include <stdio.h>

int main(void)
{
    int m;
    int n;
    int array[1000][2];
    
    FILE* finput;
    
    finput = fopen("numbers.txt", "r");
    
    for(m = 0; m < 1000; m++)
    {
        for(n = 0; n < m; n++)
        {
            fscanf(finput, "%d ", &array[m][n]);
            printf("%d\n", array);
        }
    }
    
    fclose(finput);
    
    system("PAUSE");
    return 0;
}

[brewbuck]

Almost correct, except that this:

        for(n = 0; n < m; n++)

Should be this:

        for(n = 0; n < 2; n++)

[NeedHelpWithC]

So that means it read them all, and printed them all, but your terminal window only shows the last few. Run your program and redirect it to a (different) file and you'll see all the numbers.

Cool, the file I wrote to did have all the numbers So what exactly does this mean? I can just trust that my program reads all the numbers but I cant expect it to print all 1000 numbers?

[NeedHelpWithC]

Almost correct, except that this:



Should be this:

        for(n = 0; n < 2; n++)

Oh okay, I see. It's still giving me weird numbers. Did I write the array part right? ("&array[m][n]")

I feel like that's wrong.

[tabstop]

Cool, the file I wrote to did have all the numbers So what exactly does this mean? I can just trust that my program reads all the numbers but I cant expect it to print all 1000 numbers?

That's right, a command prompt/shell/whatever won't have 1000 lines available to it. (There might be a way to extend the amount you have, but either I've forgotten it or never knew it.)

[tabstop]

Oh okay, I see. It's still giving me weird numbers. Did I write the array part right? ("&array[m][n]")

I feel like that's wrong.

That's fine -- your compiler should be telling you that printf("%d", array) is wrong -- since array is not an int.

[NeedHelpWithC]

That's fine -- your compiler should be telling you that printf("%d", array) is wrong -- since array is not an int.

it didn't lol.

[brewbuck]

it didn't lol.

Did you perhaps forget to #include <stdio.h>? Most (modern) compilers should warn when a printf specifier doesn't match its argument.

[NeedHelpWithC]

Did you perhaps forget to #include <stdio.h>? Most (modern) compilers should warn when a printf specifier doesn't match its argument.

Nope, that wasn't it. I used two nested for loops. One for scanning in the number and the next one to print them.

It looks like this now.

#include <stdio.h>

int main(void)
{
    int m;
    int n;
    int array[1000][2];
    
    FILE* finput;
    
    finput = fopen("numbers.txt", "r");
    
    for(m = 1; m < 1000; m++)
    {
        for(n = 0; n < 2; n++)
        {
            fscanf(finput, "%d ", &array[m][n]);
        }
    }
    
    for(m = 1; m < 1000; m++)
    {
        for(n = 0; n < 2; n++)
        {
            printf("%d", array[m][n]);
        }
        printf("\n");
    }
    
    fclose(finput);
    
    system("PAUSE");
    return 0;
}

[NeedHelpWithC]

Thank you for all the help tabstop and brewbuck!!!