Print nth Column in Tab Separated Data File

[edwardholmes91]

Hello,

I'm trying to print out a column in a data file, this is the contents of the data file:

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

This is the source code:

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


void main()
{
    /* Declare Local Variables */
    FILE *fptr;                // File pointer for reading data file
    int numTraces = 4;        // Number of traces in file to be analysed
    int numSamples = 7661;    // Number of samples in file to be analysed
    int i = 0;
    int trace;                // Trace number to analyse
    float sample;            // Sample being scanned into program
    char prompt;            // Prompt to hold program




    /* Input column to analyse */
    printf("Please enter trace number to analyse (1 - %d): ",numTraces);
    scanf("\n%d", &trace);
    
    /* Open program.dat file */
        fptr = fopen("data.txt", "r");        // For read access only
        if (fptr == NULL)
        {
            printf("Unable to find file: [data.txt]\n\n\r");    // Error if the program.dat file can't be found


            /* Wait for enter key to close */
            printf("Press Enter to close the program.");
            getchar();
            exit(1);
        }


        i = trace;        // Initialise i to 0 + trace number to analyse


        /* Scan Information from File */
        while (fscanf(fptr, "%f", &sample) != EOF)    // Loop until end of file marker, i.e. no more values to calculate
        {
            if (i % numTraces == 0)
            {
                printf("%f\n\r", sample);
            }
            i++;
        }


        /* Clean up */
        fclose(fptr);


        /* Wait for enter key to close */
        printf("Press any key followed by Enter to close the program.");
        scanf("\n%c", &prompt);
        exit(1);


}

So the user enters a number between 1 and 4 and then it should print on each line the numbers from that column.

It's sort of working... by this I mean when you ask for:

• column 1 it prints column 4
• column 2 it prints column 3
• column 3 it prints column 2
• column 4 it prints column 1

It's like it's transposing it somehow? I'm sure it's something simple, prehaps to do with the if statement that wraps around the printf statement?

I look forward to hearing from you.

Kind Regards

Edward

[Matticus]

I'm sure it's something simple, prehaps to do with the if statement that wraps around the printf statement?

Why not inspect that part of the code and see?

while (fscanf(fptr, "%f", &sample) != EOF)    // Loop until end of file marker, i.e. no more values to calculate
{
    printf("> i %% numTraces = %d\n", i % numTraces);
    if (i % numTraces == 0)
    {
        printf("%f\n\r", sample);
    }
    i++;
}

Please enter trace number to analyse (1 - 4): 1
> i % numTraces = 1
> i % numTraces = 2
> i % numTraces = 3
> i % numTraces = 0
4.000000
> i % numTraces = 1
> i % numTraces = 2
> i % numTraces = 3
> i % numTraces = 0
8.000000
> i % numTraces = 1
> i % numTraces = 2
> i % numTraces = 3
> i % numTraces = 0
12.000000
Press any key followed by Enter to close the program.0

Also, "main()" returns int, not void: FAQ > main() / void main() / int main() / int main(void) / int main(int argc, char *argv[]) - Cprogramming.com

[edwardholmes91]

Thank you for your reply. I can see what's happening, but I'm still not sure how I can modify the code to give the result that I'm looking for.

[Matticus]

So get out a pen and paper and start figuring out the logic.

When you have an idea, start a new project and test just that logic. When you get it working, go back to your real program and update it with the working logic. If you can't get it working, post the simplified test version and we can help you with that.

[edwardholmes91]

The actual data has 13 columns including an index (first column).

So knowing this, I was able to use a switch case statement to convert the column number that the user entered to give the correct result:

switch (trace)	{
	case 1:	 trace = 12; break;
	case 2:  trace = 11; break;
	case 3:  trace = 10; break;
	case 4:  trace = 9;	 break;
	case 5:  trace = 8;  break;
	case 6:  trace = 7;  break;
	case 7:  trace = 6;  break;
	case 8:  trace = 5;  break;
	case 9:  trace = 4;  break;
	case 10: trace = 3;  break;
	case 11: trace = 2;  break;
	case 12: trace = 1;  break;
	default:
		printf("Incorrect entry, program will now exit.");
		exit(1);
	}

It might be crude but seems to work!

[algorism]

A rather wordy way to simply calculate:

trace = 13 - trace;

[Matticus]

Well, good that you got it working. Of course, scaling the columns up or down might be more trouble with that solution than with a more clever one.

All you really had to do was initialize "i" to 1, and in the loop, compare "i" and "trace" for equality to determine whether to print. Then, when "i" was incremented, roll it back to 1 if the number of columns was exceeded.

[edwardholmes91]

A rather wordy way to simply calculate:

trace = 13 - trace;

Hello algorism,

Thanks, that's exactly what I was trying to achieve! I used this code to replace that ugly big switch case statement:

trace = numTraces - trace;