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POAccessing double pointer causes segmentation fault
 

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    <p>I am programming Jacobis method for eigenvalue problems in QM and I have just started c++, I want to use double pointers to construct matrices but the physics problem involved requires a lot of code.</p> <p>I don't want to clutter my <code>main()</code> with unreadible lines (others will have to read this code..) and so wanted to divide the problem into subfunctions. I made a function that takes a double pointer and returns a matrix, but why can't I access it outside the function? My code <strong>segfaults</strong> (marked below) when I try to. How do I construct a matrix outside main() while still being able to access it in <code>main()</code>?</p> <p></p> <pre><code>enter code her enter code here int i, j, k; //== BEGIN MAIN ==// int main () { //Constants and variables double **A; double epsilon = pow((double)10, double(-8)); //The convergence limit for jacobis method int N = 10; //Dimension of matrix char test[] = "test"; cout &lt;&lt;"The inner matrix function:"&lt;&lt;endl; makematrix(N, A); cout&lt;&lt;endl&lt;&lt;"The outer matrix function:"&lt;&lt;endl; //This part segfaults for(i=0; i&lt;N; i++) { cout&lt;&lt;endl; for(j=0; j&lt;N; j++) { cout&lt;&lt;A[i][j]&lt;&lt;" "; } } return 0; } //== END MAIN ==// //==Begin function definitions==// void makematrix(int N, double **A) { //Function for initializing our tridiagonal matrices for jacobis method A = new double*[N]; for(i=0; i&lt;N; i++) { A[i] = new double[N]; } for(i=0; i&lt;N; i++) { for(j=0; j&lt;N; j++) { A[i][j] = 0; } } //Prints the matrix declared here for(i=0; i&lt;N; i++) { cout&lt;&lt;endl; for(j=0; j&lt;N; j++) { cout&lt;&lt;A[i][j]&lt;&lt;" "; } } cout &lt;&lt;endl; return; } </code></pre>
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    1. COThe value 10^-8 doesn't have be computed using `pow`, but can instead be written as constant `1e-8`
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