Thread: Double Array of Pointers (unknown length)

Hi there,

this is my first post as I like to try and work things out wherever possible without bothering other people, but I'm completely at a wit's end with this one.

I am trying to read in a 2D array of unknown length from a file into "myarray" using malloc and realloc as I saw this method recommended elsewhere. The data to be read in is known to be 4 columns but the number of rows is not known.

So far I manage to allocate 5000 pointers (I think), each pointing to 4 doubles.

Code:

    int datasize=0;
    int nrows=5000;
    float** myarray=(double**)malloc(nrows*sizeof(double*));
        for(i=0;i<=nrows;i++)
        {
           myarray[i]=(double*)malloc(4*sizeof(double));
        }

When this runs out, I attempt to add another 1000 rows with realloc.

Code:

if(datasize==nrows)
                {
                    nrows+=1000;
                    myarray=(double**)realloc(myarray,nrows*sizeof(double*));
                    for(j=0;j<nrows;j++)
                        {
                        myarray[j]=(double*)realloc(myarray[j],4*sizeof(double));
                        printf("%d\n",j); //this is just to see how far through the programme it gets
                        }
                }

When I run this, I get the pop-up "Data Fitting.exe has stopped working" at one of three points. Either at the first attempt of memory allocation i.e. j=0, or when j=5074 or j=5110. There is never any variation from this. Running a debugger shows there is a segmentation fault (SIGSEGV). I understand this has something to do with memory but I do not really know exactly what it means.

I am new to C so sorry if the answer is very simple. I would be extremely grateful for any insight as I have spent a long long time on this.

Here is my whole code for reference.

Code:

/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////dynamic version////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int main()
{
    FILE *myfile;
    int i=0;
    int j=0;
    int c=0;
    int datasize=0;
    int nrows=5000;
    int elements=0;
    float x,y,sx,sy;
    double a, b, SigA, SigB;
    char fname[100]={0};
    double** myarray=(double**)malloc(nrows*sizeof(double*));
        for(i=0;i<=nrows;i++)
        {
           myarray[i]=(double*)malloc(4*sizeof(double));
        }
//1) Ask user to input filename from keyboard
    printf("Enter a file path:\n");
    scanf("%s",fname);
    if (fname[0]=='\"')         //removes quotations automatically inserted when copying a file path from windows explorer
    {
        for ( c = 0 ; c < 99 ; c++ )
        {
            fname[c] = fname[c+1];
        }
        for ( c = 0 ; c < 100 ; c++ )
        {
            if (fname[c]=='\"')
            {
                fname[c]=0;
            }
        }
    }
//2) Call function to read in data from file (check the file was actually found and read in correctly)

    myfile=fopen(fname,"r");
    if (myfile!=NULL)
        {
            for (i=0; !feof(myfile); i++)
            {
                fscanf(myfile,"%f %f %f %f",&x,&y,&sx,&sy);
                myarray[i][0]=x;
                myarray[i][1]=y;
                myarray[i][2]=sx;
                myarray[i][3]=sy;
                datasize++;
                if(datasize==nrows)
                {
                    nrows+=1000;
                    myarray=(double**)realloc(myarray,nrows*sizeof(double*));
                    for(j=0;j<nrows;j++)
                        {
                        myarray[j]=(double*)realloc(myarray[j],4*sizeof(double));
                        printf("%d\n",j); //this is just to see how far through the programme it gets
                        }
                }


            }
            elements = i-1;
            fclose(myfile);
        }
    else
        {
        printf("File does not exist");
        }
//3) Call the function to calculate the coefficients
    double sumyoversqsig=0;
    double sumsqxoversqsig=0;
    double sumxyoversqsig=0;
    double sumoneoversqsig=0;
    double sumxoversqsig=0;
    for (i=0; i<(elements); i++)
    {
    sumyoversqsig+=((myarray[i][1])/pow(myarray[i][3],2));
    sumsqxoversqsig+=((pow(myarray[i][0],2))/pow(myarray[i][3],2));
    sumxoversqsig+= ((myarray[i][0])/pow(myarray[i][3],2));
    sumxyoversqsig+=(((myarray[i][0])*(myarray[i][1]))/pow(myarray[i][3],2));
    sumoneoversqsig+=(1/pow(myarray[i][3],2));
    }
printf("datasize = %d \nnrows = %d",datasize,nrows);
printf("%f\n%f\n%f\n%f\n%f\n",sumyoversqsig, sumsqxoversqsig, sumxoversqsig, sumxyoversqsig, sumoneoversqsig);
    a=((sumyoversqsig*sumsqxoversqsig)-(sumxoversqsig*sumxyoversqsig))/((sumoneoversqsig*sumsqxoversqsig)-pow(sumxoversqsig,2));
    b=((sumoneoversqsig*sumxyoversqsig)-(sumyoversqsig*sumxoversqsig))/((sumoneoversqsig*sumsqxoversqsig)-pow(sumxoversqsig,2));
    SigA=sqrt((sumsqxoversqsig)/((sumoneoversqsig*sumsqxoversqsig)-pow(sumxoversqsig,2)));
    SigB=sqrt((sumoneoversqsig)/((sumoneoversqsig*sumsqxoversqsig)-pow(sumxoversqsig,2)));
//4) Display coefficients on the screen. ;
    printf("Coefficient a =%le\nCoefficient b =%le\nSigma a       =%le\nSigma b       =%le\n",a,b,SigA,SigB);
    free(myarray);
    return 0;
}

Thank you so much for your help. I made the first two corrections but the problem persists. I will perhaps re-write the array section improvements tomorrow and hope that works

I don't know if it was mentioned yet but your code would be very simple if you just used a 1d array but indexed it like a 2d one, via :

Code:

double *data = malloc( ( initial_rows * initial_cols ) * sizeof( *data ) );

for ( int i = 0; i < num_rows; ++i )
{
    for ( int j = 0; j < num_cols; ++j )
    {
        data[ i * num_rows + j ] = /* ... */;
    }
}

This way, reallocation would be very simple and so would free()'ing the data as all it would require is a single call to free().