Dynamic memory allocation in C++ language

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What is the difference between
-

double **a ;
    
a = new double *[M];

and

for (int i = 0; i < M; i++) {
a[i] = new double [N];}

and how can translate it to C language

What I have tried:

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Posted 2-Mar-18 8:07am

neveen neveen

Updated 2-Mar-18 15:08pm

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Rick York · Accepted Answer · 2018-03-02T15:09:00

The first allocates an array of pointers to doubles of size M and assigns it to a.
The second loads the array a with M arrays of doubles of size N.

The double asterisk indicates you have a two dimensional array of doubles. Those code fragments are actually the two steps involved in allocating a 2D array.

new translates to malloc or calloc in the C language. I prefer calloc. Also remember that in C++ you release objects with delete and in C you use free.

As a matter of habit, every time you write an allocation routine, like those, also write a corresponding deallocation routine. You have to keep them in synch in order to release memory correctly. That is one of the more common causes of errors so if you adopt this habit you will save yourself a lot of headache and wasted time.