Dynamic memory allocation with malloc not working.

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2.00/5 (1 vote)

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#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
#include<stdio.h>
#include<io.h>


void main()
{
    FILE *in;
    in = fopen("A.txt","r");
    double f;
    int i,j;
    n = 430;
    
    double **arr;
    arr = (double**)malloc(sizeof(double*)*430);
    for(i=0;i< 430; i++)
        arr[i] = (double*)malloc(sizeof(double)*430);

    for(i=0;i<n;i++)
    {
        for(j=0;j<n;j++)
        {   
            fscanf(in, "%lf",&f);
            fprintf(stdout, "%.15lf",f);

            arr[i][j] = f;
            printf("A[%d][%d] = : %f",i,j,f);
        }
        getch();
    }

    cout<<endl<<" Matrix A Fed ";
    getch();

    fclose(in);
}

What I have tried:

Input is a matrix containing 430x430 double type values.
Output iterates till i=17 and stops.
Why so?
please help.
this arises from a bigger problem presented here.
c++ - LU decomposition using dynamic memory allocation for large Matrices. - Stack Overflow[^]

Posted 6-Apr-16 5:46am

Member 12440837

Updated 6-Apr-16 9:52am

Patrice T

v4

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Member 12440837 6-Apr-16 11:47am

the #include didnt show up.. no idea why
#include<iostream.h>
#include<conio.h>
#include<stdlib.h>
#include<stdio.h>
#include<io.h>

Richard MacCutchan 6-Apr-16 11:53am

Because you forgot to surround your code with <pre> tags. I think Jochen may be doing it as I write this.

Jochen Arndt 6-Apr-16 11:55am

That is because the angle brackets are treated as HTML tags.
I fixed it for you.

Member 12440837 6-Apr-16 11:58am

thanks for that
can u also fix the error?

Jochen Arndt 6-Apr-16 12:04pm

You mean declaring double g and using f (I just recognised that)?
I will do so.

Two tips:
You can edit your question yourself using the green 'Improve question' link (move the mouse to the bottom right of your question to let the link show up).

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Member 12440837 6-Apr-16 12:08pm

Yes that's an error. Double should be for f not g.
but the code still not works!

Jochen Arndt 6-Apr-16 12:31pm

I can't see any problems in the code besides that you may not read in the correct data. How is the input file organised (delimiters between values and line feeds)?

To check if that sources your problem you might comment the fscanf call and assign some value to f like i * j.

Matt T Heffron 6-Apr-16 15:36pm

Jochen said the code looks correct (and I think so, also).
You said: "but the code still not works!"
What is happening, or not happening?
Is it crashing? Infinite loop? Nonsense output?
We can't help if we don't know what is going on.

Sergey Alexandrovich Kryukov 6-Apr-16 12:47pm

One idea is: if you code is C++, use C++, not C. With "new", constructors and destructors, not with malloc...
By the way, you only allocate. Where do you deallocate?
—SA

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CPallini · Answer 1 · 2016-04-06T09:52:00

Your program actually works. For sure it is not robust and you are probably feeding it with a bad data file.
A slightly modified it (removed the useless cout call, as well the legacy conio header, etc...)

C

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

int main()
{
    FILE *in;
    in = fopen("A.txt","r");
    double f;
    int i,j;
    const int n = 430;

    double **arr;
    arr = (double**)malloc(sizeof(double*)*430);
    for(i=0;i< 430; i++)
        arr[i] = (double*)malloc(sizeof(double)*430);

    for(i=0;i<n;i++)
    {
        for(j=0;j<n;j++)
        {
            fscanf(in, "%lf",&f);
            fprintf(stdout, "%.15lf",f);

            arr[i][j] = f;
            printf("A[%d][%d] = : %f",i,j,f);
        }
    }

    printf(" MAtrix A Fed \n");

    getchar();

    fclose(in);
}

then I fed it with the random numbers generated by the following program

 #include <stdio.h>
 #include <stdlib.h>
const int N = 430;

int main()
{
  int i,j;
  for (i=0; i<N; ++i)
    for(j=0; j<N; ++j)
    {
      if (j %10)
        printf(" ");
      else
        printf("\n ");

      float f = ((float)rand()/RAND_MAX);
      printf("%12g ", f);

    }
  return 0;
}

and eventually got the Matrix A Fed message.