# what is the meaning of the following c code

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 May 5, 2007, 09:08 what is the meaning of the following c code #1 ztdep Guest   Posts: n/a Hi The code are as following: " double **matrix; matrix = (double **) malloc (N * sizeof (double)); for (row = 0; row < N; row++) matrix[row] = (double *) malloc (N * sizeof (double));" i do not understand these four lines , would you please detail it. Regards

 May 5, 2007, 09:49 Re: what is the meaning of the following c code #2 Luca Guest   Posts: n/a it is the usual way in C to create a 2D matrix A. you can access to its element like matlab simply requiring: A[i][j]. Luca

 May 5, 2007, 10:41 Re: what is the meaning of the following c code #3 Dominic Guest   Posts: n/a Suppose you say double *A. This means A will hold the address of a variable. You will allocate memory like: A=(double *)malloc(sizeof(double)*N); and *A will give the value at the first address. Then subsequent values can be obtained by *(A+1), *(A+2), etc.... Now lets say double **A. This tells *A will hold the address of a variable. You will allocate memory like: A = (double **)malloc(size(double)*N); Actually u are allocating memory for N rows only. The column part hasnt been allocated yet. So For each Row of A, that loop you mentioned will allocate for all N columns: for (row = 0; row < N; row++) A[row] = (double *) malloc (N * sizeof (double)); The above will allocate space for N columns when row=0,1,2...N etc. So ultimately at the end you wud have allocated memory for a NxN matrix. To find the value at the ith row and jth column of the matrix, just issue, *(*(A+i) + j). -Dominic

 May 5, 2007, 11:17 Re: what is the meaning of the following c code #4 ztdep Guest   Posts: n/a Thank you all for your excellent explaination.

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