pass string to function in C

Question

I have a simple program which reads in a list of positions and velocities, though it is not compiling. I simply want to ask the user for the name of the position and velocity file and then output the array back in main().

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

#define loop(idx,last) for (idx = 0; idx < last ; idx++)

float readinput (char *posfile, char *velfile);


int main (void)
{
char posfile[100],velfile[100];
float pos[10000][3], vel[10000][3];
printf( "What is the name of your positions file (x y z): " );
scanf( "%s", &posfile );
printf( "What is the name of your velocity file (vx vy vz): " );
scanf( "%s", &velfile );
pos = readinput(posfile,velfile);
return 0;
}

float readinput (char *posfile, char *velfile)
{
float pos[10000][3], vel[10000][3];
float x,y,z;
float vx,vy,vz;
int i;
char line[256];

FILE *files;

files = fopen(posfile, "r");
loop(i,10000){
                  fscanf(files, "\n%f %f %f\t", &x, &y, &z);
                  pos[i][0] = x;
                  pos[i][1] = y;
                  pos[i][2] = z;
                  printf("\n%f %f %f\t",x,y,z);
              }
fclose(files);

files = fopen(velfile, "r");
loop(i,10000){
                 fscanf(files, "\n%f %f %f\t", &vx, &vy, &vz);
                 vel[i][0] = vx;
                 vel[i][1] = vy;
                 vel[i][2] = vz;
                 printf("\n%f %f %f\t",vx,vy,vz);
             }
fclose(files);
return pos;    
}

I do apologize, this is my first program.

  main.c: In function 'main':
main.c:18:5: error: incompatible types when assigning to type 'float[10000][3]' from type 'float'
 pos = readinput(posfile,velfile);
     ^
main.c: In function 'readinput':
main.c:51:1: error: incompatible types when returning type 'float (*)[3]' but 'float' was expected
 return pos;

If you say it is not compiling, you should post the errors rather than make people audit your entire code looking for what might be wrong. — Chris Stratton
– Chris Stratton, Commented May 8, 2013 at 17:37
For starters, your charfile and varfile are chars instead of char[] or char*. Strings in C are arrays of characters, a char in C is a single ascii byte. — Sniggerfardimungus
– Sniggerfardimungus, Commented May 8, 2013 at 17:38
Functions in C return at most one value (they can return no value -- or even fail to return)! — pmg
– pmg, Commented May 8, 2013 at 17:39

Mihai Maruseac · Accepted Answer · 2013-05-08 17:37:52Z

1

you've got it wrong. Type char has space for a single character only. You have to use a char * to have a string there.

int readinput (char *posfile, char *velfile)

And in main, make both posfile and velfile vectors:

char posfile,velfile;

And when reading their content, skip the &:

scanf( "%s", velfile );

answered May 8, 2013 at 17:37

Mihai Maruseac

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Comments

Kninnug · Accepted Answer · 2013-05-08 17:38:31Z

0

A string in C is an array of chars. You should change the arguments to readinput from char to char * or char[]:

int readinput(char * posfile, char * velfile)

Hint: next time tell why it isn't compiling.

answered May 8, 2013 at 17:38

Kninnug

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Comments

Mike · Accepted Answer · 2013-05-08 17:38:50Z

I'm not even sure where to start... you might want to invenst in a good C book.

In your main() function:

char posfile,velfile;
printf( "What is the name of your positions file (x y z): " );
scanf( "%s", &posfile );

A char is a single character like 'a', if you want to do a string you need either a dynmically or statically allocated array of chars:

char posfile[100];

for example.

In your readinput() function:

You're passing chars (single characters) when you wanted to pass char * (strings)

char posfile, char velfile

Your function returns an int type, but you're trying to return float types:

int readinput(char posfile, char velfile)
float pos[10000][3], vel[10000][3];

You can not return more than 1 values from a function in a return statement:

return pos,vel;

You are trying to return local data from your function back to main:

float pos[10000][3], vel[10000][3];
...
return pos,vel;

That's going to give you UB, you need to make them global, or define them in your main and pass them.

typ1232 · Accepted Answer · 2013-05-08 17:41:13Z

0

Should't

int readinput (char posfile, char velfile)

be

int readinput (char *posfile, char *velfile)

since I'm guessing you want a string, not just a character?

edited May 8, 2013 at 17:41

typ1232

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answered May 8, 2013 at 17:39

ps515

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Comments

V-X · Accepted Answer · 2013-05-08 18:11:36Z

0

The reported problem is in that you are trying to:

 pos = readinput(posfile,velfile);

and

 return pos;

you cannot return array as float and you cannot assign float to array.

except other issues with pointers in scanfs, you should change the

float readinput (char *posfile, char *velfile)
{
float pos[10000][3], vel[10000][3];
...

to

void readinput (char *posfile, char *velfile, float pos[][3], float vel[][3])
{    
...

and drop the local variables....

answered May 8, 2013 at 18:11

V-X

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Comments

BenMorel · Accepted Answer · 2013-12-13 22:42:20Z

0

another problem is posfile and velfile's type should not be char, how could you let one char to hold a file's name?

you should define them like this:

char posfile[256],velfile[256];

and change the reading code to:

scanf( "%s", posfile );

and

scanf( "%s", velfile);

Added: you are changing your code, so we have to follow you:

you should pass pos,vel as reference parameters to the function and remove the their definition inside readinput, and you don't need to return anything,

void readinput (char *posfile, char *velfile,float** pos, float** vel)

edited Dec 13, 2013 at 22:42

BenMorel

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answered May 8, 2013 at 17:42

diwatu

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