Thread: Error from random matrix programme

Evening all,

I've written a working code to create a matrix (the size of which is defined by the user) and fill it with random values and print out the result.

Why then am I posting it here? Well whenever I run it, as I press the key to close the programme I get an error message saying my programme has stopped working. Can any one help me with this?
Theres two places I think its occuring but I'm not entirely sure. One place it could be is in my use of pointers (farily new to them) and maybe also in the malloc/free functions

Any help is help
Cheers

Code:

// A programme to create a user defined matrix and fill it with random values
#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#include <malloc.h>
#include <time.h>
//FUNCTIONS
//Function to return a random double in the range [0.0,100.0]
double random()
{
       double n,m;
       n=rand();
       m=100*n/RAND_MAX;
       return(m);
       }
//Function to populate matrix with random values:
double *rand_matrix(double *mat, int rows, int cols)
{
       for (int i=0 ; i<rows ; i++)
       {   for (int j=0 ; j<cols ; j++)
           {   *(mat+i*rows+j)=random();
           }
        }
       return(mat);
}           
//Function to print out a matrix:
void print_matrix(double *mat,int rows,int cols)
{
     for (int i=0 ; i<rows ; i++)
     {   for (int j=0 ; j<cols ; j++)
         {   printf("%5.2lf    ",*(mat+i*rows+j));
         }
         printf("\n");
     }
} 
                    
 
 
 
 
 

int main ()
{
//Before programme starts I must seed the random generator.
srand(time(0));
    
    
//initialise variables;
int m,n,err,yesno=0;
double **ptr_rows=0,**user_ptr=0,*elem_ptr=0;
    
//Take input from the user for the size of the matrix
do
{
    printf("Enter the number of Rows and Collumns in the form m n\n>> ");
    err=scanf("%d %d",&m,&n);
    _flushall();
                  if ((err!=2)||(m<=0)||(n<=0))
                     {do
                       { 
                       printf("\nERROR you must have an integer number of rows and collumns\nEnter the number of Rows and Collumns in the form m n\n>> ");
                       err=scanf("%d %d",&m,&n);
                       _flushall();
                                     if ((err==2)&&(m>0)&&(n>0))
                                     break;
                       }while ((err!=2)||(m<=0)||(n<=0));
                       }
    printf("\nThe size of the matrix you have decided to use is %dx%d\npress 'N' to enter a different size or press any key to continue.\n",m,n);
    scanf("%c",&yesno);
    _flushall();
}while ((yesno=='N')||(yesno=='n'));

//Allocate memory for this matrix
ptr_rows =(double**)  malloc(m*sizeof(double*));
user_ptr=ptr_rows;
for( int i=0 ; i<m ; i++)
{    *user_ptr=(double*)malloc(n*sizeof(double));
     elem_ptr=*user_ptr;
     for(int j=0 ; j<n ;j++)
     *elem_ptr++=1.0;
     user_ptr++;
}
user_ptr=ptr_rows;
//Use rand_matrix function to create a ransom Matrix
  
print_matrix((double*) rand_matrix(*user_ptr,m,n),m,n);    
    
    
    
    
    getch();
//free the memory from the matrix
for( int i=0; i<n ; i++);
free(*user_ptr++);
free(ptr_rows);
    return (0);
}

There's an issue in free().
You're pointers are like this. ptr_rows <-- user_ptr <-- elem_ptr
So esentially the latter 2 are all pointing to the memory of ptr_rows.
This means you only need to do free(ptr_rows)

In your code if you free user_ptr, then it will result in an error when freeing ptr_rows, because that memory is already freed. You can also see that from the run error, in mine it said "double free or corruption"

Originally Posted by laserlight

No. free should also be used for each inner dynamic array. This is a consequence of allocating each inner dynamic array in a loop.

Oh I see! Thanks Hm weird, what did the error about the double free mean then?
I thought it meant the first free, because when I removed it, the program worked fine.

Thanks for the replies, I would have gotten back sooner but my internet was down. Yes I've gotten rid of all the user_pointer stuff as it was pointless, in fact I rewrote the whole code taking into account what you said. Fixing the errors and now when i run it, I don't get any errors.

Thanks for the help