how to initialize and define pointer to pointer 2D array?

Question

I am very new to C program. I have a function like this :

void lfit( float x[], float y[], float sig[], int ndat, float a[], int ia[], int ma,float **covar, float *chisq, float (*funcs)(float, float [], int))

//Given a set of data points x[1..ndat], y[1..ndat] with individual standard deviations
//sig[1..ndat], use χ2 minimization to fit for some or all of the coefficients a[1..ma] of
//a function that depends linearly on a, y =i ai × afunci(x). The input array ia[1..ma]
//indicates by nonzero entries those components of a that should be fitted for, and by zero entries
//those components that should be held fixed at their input values. The program returns values
//for a[1..ma], χ2 = chisq, and the covariance matrix covar[1..ma][1..ma]. (Parameters
//held fixed will return zero covariances.)Th e user supplies a routine funcs(x,afunc,ma) that
//returns the ma basis functions evaluated at x = x in the array afunc[1..ma].

{
void gaussj(float **a, int n, float **b, int m);

int i,j,k,l,m,mfit=0;
float ym,wt,sum,sig2i,**beta, *afunc;


afunc=vector(1,ma);
beta=matrix(1,ma,1,1);

for (j=1;j<=ma;j++)
    if (ia[j]) 
        mfit++;

if (mfit == 0) 
    nrerror("lfit: no parameters to be fitted");
for (j=1;j<=mfit;j++) {                       //Initialize the (symmetric)matrix.
    for (k=1;k<=mfit;k++) 
        covar[j][k]=0.0;
    beta[j][1]=0.0;
}

for (i=0;i<ndat;i++) {                       //Loop over data to accumulate coefficients of the normal equations.

(*funcs)(x[i], afunc ,ma);
ym=y[i];
if (mfit < ma) {                              //Subtract off dependences on known pieces of the fitting function.
    for (j=1;j<=ma;j++)                
        if (!ia[j]) 
            ym -= a[j]*afunc[j];
}


sig2i=1.0/SQR(sig[i]);
for (j=0,k=0,l=1;l<=ma;l++) {

    if (ia[l]) {
        wt=afunc[l]*sig2i;
        for (j++,k=0,m=1;m<=ma;m++)
            if (ia[m]) 
                covar[j][++k] += wt*afunc[m];

        beta[j][1] += ym*wt;
    }

}


for (j=2;j<=mfit;j++)                    //Fill in above the diagonal from symmetry.
    for (k=1;k<j;k++)
        covar[k][j]=covar[j][k];

gaussj(covar,mfit,beta,1);              //Matrix solution.

for (j=0,l=1;l<=ma;l++) 
    if (ia[l]) 
        a[l]=beta[++j][1];              //Partition solution to appropriate coefficients a Evaluate χ2 of the fit.

*chisq=0.0;

for (i=1;i<=ndat;i++) { 
        (*funcs)(x[i], afunc,ma);
        for (sum=0.0,j=1;j<=ma;j++) 
            sum += a[j]*afunc[j];
*chisq += SQR((y[i]-sum)/sig[i]);

}
//covsrt(covar,ma,ia,mfit);              //Sort covariance matrix to true order of fitting coefficients.
free_vector(afunc,1,ma); 
free_matrix(beta,1,ma,1,1);
}
}

in int main() part when i want to call this function i have some problems. the main part that i am writing is like this for example:

int main()
{

float x1[]={100.000000f,88.00000f,76.199997f,68.599998f,54.500000f,37.599998f,26.500000f,17.000000f,8.300000f,0.900000f,-7.200000f,-17.000000f,-24.900000f,-33.799999f,-42.500000f,-51.000000f,-60.500000f,-69.500000f,-75.300003f,-83.099998f,-94.099998f,-103.000000f,-110.099998f};
float y1[]={-2.876821f,-2.788704f,-2.596228f,-2.468143f,-1.898085f,-1.296223f,-0.664981f,-0.245603f,-0.280993f,-0.094657f,-0.184912f,-0.263328f,-0.181819f,-0.132037f,-0.029368f,0.134307f,0.257734f,0.305223f,0.091159f,0.063768f,-0.163334f,-0.136314f,-0.372106f};
float sig1[]={1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1};
float a1[]={1.8f,1.6f,1.7f,1.3f};
int ia1[]={2,3,4,2}; 
int ma=4, ndat=23 ;
float chisq1, **covar;



lfit(x1,y1,sig1,ndat,a1,ia1,ma,covar,&chisq1, &funcs);


return 0;
}

the error that i always get is this: Run-Time Check Failure #3 - The variable 'covar' is being used without being initialized. i tried different ways to initialize it but non of them worked.

what is your idea? how can i solve this problem ?

I really appreciate any help!

I would suggest std::array and std::vector instead of C-arrays - easier and less error prone — Karthik T
– Karthik T, Commented Dec 3, 2012 at 10:41
What exactly didn't work? What you wan't that double ponter to point to? — user1773602
– user1773602, Commented Dec 3, 2012 at 10:42
You are not actually allocating any memory for it as far as I can tell. You need to do this or point it to another 2D array if you want this code to do what you want. — Karthik T
– Karthik T, Commented Dec 3, 2012 at 10:43
@Marrow Gnower i tried this before, but i get another error which is Access violation reading location and after checking the program i found out this error is related to covar, then i thought may be it is not a good way to define covar ! Do you think it is the best way to define it? — user1654988
– user1654988, Commented Dec 3, 2012 at 10:46

noelicus · Accepted Answer · 2012-12-03 11:50:56Z

2

First thing: your array indexes must be zero-based, so for loops are for(i=0; i < max; i++)

The easy way to get a 2D array is:

float covar[4][4]; // Or whatever size is correct for you

If you need to malloc it you can just do this:

float ** Malloc2DFloat(int x, int y)
{
    float **MainArray = (float**)malloc(sizeof(float*) * x);
    int i;
    for (i=0; i < x; i++)
        MainArray[i] = (float*)malloc(sizeof(float) * y);

    return MainArray;
}
float **covar = Malloc2DFloat(4,4);

You need to do this in main, or pass in a pointer to that to your function and malloc it in there:

func(float ***covar)
{
    *covar = Malloc2DFloat(mfit, mfit); // Allocate like this
    (*covar)[0][0] = 3.41f; // Reference it like this
}

edited Dec 3, 2012 at 11:50

answered Dec 3, 2012 at 10:42

noelicus

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2 Comments

Bart van Ingen Schenau Over a year ago

Unfortunately, that is not the correct way to get a multi-dimensional array with malloc

noelicus Over a year ago

Thanks @BartvanIngenSchenau. How's that?

MOHAMED · Accepted Answer · 2012-12-03 13:36:21Z

0

You have to allocate memory for covar before you filling it. you can allocate memory into the lfit function. To do:

you have to update your input covar in the function lfit

void lfit( float x[], float y[], float sig[], int ndat, float a[], int ia[], int ma,float ***covar, float *chisq, float (*funcs)(float, float [], int))

and then when you initialize matrix you have to do it in this way

*covar = calloc(mfit,sizeof(float *));   
for (j=1;j<=mfit;j++) {   //Initialize the (symmetric)matrix.
    (*covar)[j] = calloc(mfit,sizeof(float)); //calloc will initialize the matrix to 0
    beta[j][1]=0.0;
}

instead of the old way

for (j=1;j<=mfit;j++) {                       //Initialize the (symmetric)matrix.
    for (k=1;k<=mfit;k++) 
        covar[j][k]=0.0;
    beta[j][1]=0.0;
}

and to access to covar data in the lfit function:

float x=(*covar)[j][k];
(*covar)[j][k]=5.2;

so for example this line

covar[j][++k] += wt*afunc[m];

should be updated to

(*covar)[j][++k] += wt*afunc[m];

and when you call the function lfit

lfit(x1,y1,sig1,ndat,a1,ia1,ma,&covar,&chisq1, &funcs);

edited Dec 3, 2012 at 13:36

answered Dec 3, 2012 at 10:43

MOHAMED

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6 Comments

user1654988 Over a year ago

thanks for your answer , but when i use 'float covar[][]` in my lfit function i get this eeror error C2087: 'covar' : missing subscript

MOHAMED Over a year ago

try to change it by covar[100][] or covar[][100]

user1654988 Over a year ago

Thanks Mohamed, can you please tell where should i write this way of initialization that you have suggested me? inside main() or where?

MOHAMED Over a year ago

in the place where you initialize your old covar matrix

user1654988 Over a year ago

thanks @Mohamed , i could use your information and run the program finally :) but i have one more question, can you please tell me how can i print covar matrix ? should i write like printf("%f c \n", covar[i][j]); or printf("%f c \n", &covar[i][j]);? the difference is only in &. Thanks alot

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