How to use fprintf for writing data to a file

Question

I want to write data from a C program to a file, so that Excel can read the file to plot a graph of the data. But I'm not sure of the exact syntax to use for fprintf. I have stdlib.h declared in the very top of my program. I declared "File *fp;" in main but I'm getting that File and fp are undeclared. What could be the problem?

**EDIT: My program compiles and runs but now my output file doesn't contain any data This is what I have at the end of a while loop that does some computations..

 fp = fopen( "out_file.txt", "w" ); // Open file for writing

 fprintf(fp, "x = %f, y = %f, vx = %f, vy = %f, time = %f, ", x,y,vx,vy,time);

It's `FILE` not `File`. If that's not it you need to post more info. — Piotr Praszmo, May 20 '11 at 15:41
code snippets from web sites are not going to get you too far - a book provides far better information (and more depth besides) — KevinDTimm, May 20 '11 at 15:54
and, just out of curiosity, any error checking that the file was actually opened? (See what you might have learned from a book) — KevinDTimm, May 20 '11 at 16:01
my program works correctly. and yes a book probably would have been more helpful — kachilous, May 20 '11 at 16:02

score 8 · Accepted Answer · answered May 20 '11 at 15:56

8

Your logic should look something like this:

fp = fopen( "out_file.txt", "w" ); // Open file for writing

while ( some condition )
{

    ... some calculations

    fprintf(fp, "x = %f, y = %f, vx = %f, vy = %f, time = %f, ", x,y,vx,vy,time);
}

fclose(fp);

answered May 20 '11 at 15:56

Ryan

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thanks!! now this is sort of unrelated, but would I be able to simply import this in to Excel so that I could get a graph of the data? – kachilous May 20 '11 at 16:00
2

You'd have to change your file format a little bit for Excel to easily read it. Either CSV or tab delimited may be the easiest, e.g. `fprintf(fp, "%f\t%f\t%f\t%f\t%f\n", x,y,vx,vy,time);` – Ryan May 20 '11 at 16:04
Your welcome. If this is what you needed, please mark the question as answered. – Ryan May 20 '11 at 16:17

score 2 · Answer 2 · answered May 20 '11 at 15:40

2

The stdio file type is FILE (all uppercase).

answered May 20 '11 at 15:40

andrewdski

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BlackBear · Answer 3 · 2011-05-20T15:57:49.340

1

#include <stdio.h>

should be enough, but be careful because the structure's name is FILE (all uppercase) and not File. Finally, dont forget to close the file calling fclose()

edited May 20 '11 at 15:57

answered May 20 '11 at 15:40

BlackBear

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thanks but now the statement: fprintf("x = %f, y = %f, vx = %f, vy = %f, time = %f, ", x,y,vx,vy,time); is giving me an error "incompatible pointer type and incompatible argument" – kachilous May 20 '11 at 15:44
2

@kachilous: because prinff wants as first argument a pointer to the FILE structure: fprintf(file, "%d", 45) – BlackBear May 20 '11 at 15:45
@BlackBear - it compiles and runs now but my output file does not contain any data. I have a while loop that does some computations and then at the end fp = fopen( "out_file.txt", "w" ); // Open file for writing fprintf(fp, "x = %f, y = %f, vx = %f, vy = %f, time = %f, ", x,y,vx,vy,time); – kachilous May 20 '11 at 15:49
2

@kachilous - keep updating your question - it's too hard to read the inline source. also, do a printf exactly like your fprintf (minus the fp) so that you can tell if your loop is being reach. (BTW, You probably need to fclose or fflush your fp) – KevinDTimm May 20 '11 at 15:50
@kachilous have you closed the file? @KevinDTimm: ops, right :) – BlackBear May 20 '11 at 15:51

How to use fprintf for writing data to a file

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