Thread: Creating a file with random numbers question??

first things first, you need to learn how to generate a good set of
random numbers. you need to seed rand first like this:

Code:

#include <time.h>

int main (void)
{
      int i, j, k;
.
.
.

      srand (time(0));

      printf ("How many random numbers would you like: ");
      scanf ("%d", &i);

      for (j=0; j<i; j++)
      {
               fprintf (filepointer, "%d\n", rand ());
      }

call srand only once and you get good random numbers.

next thing, reading in from the user: use fgets to read in strings,
its probably the best function to use. see this

now IMO it would be better to write this program as just 1
function, that way you can avoid using pointers because you
dont seem to be too confident with how they work, and also
this program is not too complex either, although it may seem
challenging at the moment.

file i/o should be ok for you, just read any of the tutorials on
it on this site, like this

id like to help more but i dont want to end up writing the bulk
of the code for you because this is a good learing exercise
for file i/o. i just recommend reading up on the things i highlighted
and making sure that you understand them clearly. If you
run into specific problems with stuff you dont get then come back

i was in the middle of a reply but i've decided to work with your
last post.

the problem is that you're calling you're function get_info
and passing it parameters but the parameters have no values -
not good. also, the parameters in get_info are pointers, they cant
take actual data, only memory addresses. change to:

get_info(&filename, &n);

what is the function gfopen supposed to do?

well IMO the code for cfopen isn't that hot, but if you have to use
it then i guess you have to. so gfopen is probably supposed
to write to the file you specified. i mean, this line

Code:

if(strcmp(mode, "w") == 0 && (fp = fopen(filename, "r")) != NULL){

i think that's trying to test to see if the file exits - the problem
is that it will probably create the file if it doesn't exist.

i have to go for about an hour so leave it with me and i'll try
and figure out what it is you need.

just so we are clear - you have to generate a user defined
number of random numbers, and write these to a text file -
nothing else?

yes, sir. to a text file that the user also defines.

This is what I have so far:

Code:

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

void get_info(char *filename, int *n_ptr);

FILE *cfopen(char *filename, char *mode);

FILE *gfopen(char *filename, char *mode);

int main(void)
{
     FILE  *ofp;
     int     i,n=0;
     char  filename[120];

     get_info(filename, &n);
     ofp = cfopen(filename, "r+");

      for(i=1; i<=n; ++i){
           fprintf(ofp, "%12d", rand());
           if(i%6 == 0 || i == n)
              fprintf(ofp, "\n");
     }
}

void get_info(char *filename, int *n_ptr)
{
    prinf("\n%s\n\n%s",
	"This progam creates a file of random numbers.",
	"How many random numbers would you like?   ");
    scanf("%d", n_ptr);
    printf("\nIn what file would you like them?  ");
    scanf("%s", filename);
}




FILE *cfopen(char *filename, char *mode)
{
    char  reply[2];
    FILE  *fp;
 
    if(strcmp(mode, "w") == 0
      && (fp = fopen(filename, "r")) != NULL){
        fclose(fp);
	printf("\nFile exists.  Overwrite it? ");
        scanf("%1s", reply);
        if(*reply != 'y' && *reply != 'Y') {
        printf("\nBye!\n\n");
	exit(1);
	}
      fp = gfopen(filename, mode);
      return fp;
}



FILE *gfopen(char *filename, char *mode)
{
    FILE *fp;
    
    if((fp = fopen(filename,mode)) == NULL){
    fprintf(stderr, "Cannot open %s - bye!\n", filename);   
    exit(1);
    }
    return fp;
}

> char filename;

> get_info(&filename, &n);

You need a char array to hold the filename, "char filename" is just one char. Also you don't need the address (&) for passing filename, that's the default, so:

Code:

     char  filename[120];

     get_info(filename, &n);

Use srand() for better random numbers. See Richie T's first post.

Code:

if(strcmp(mode, "w") == 0 && (fp = fopen(filename, "r")) != NULL){

i think that's trying to test to see if the file exits - the problem
is that it will probably create the file if it doesn't exist.

If you open a file in mode "r", the file isn't created. fopen() returns 0 in that case.

Does it work now? Post your code.