Multidimensional array from text file

Question

Im looking for a way to fill up a multi-d array with numbers gotten from a text file. I have an array(?) dynamically created, but im not sure how to make it multidimensional.

basically the text document has a set of numbers, user input decides the amount of columns and rows of a matrix, and i need to fill that matrix with numbers from the text document. Any help is appreciated

ptrm2 = (int*)malloc(size2 *sizeof(int));

Take a look at Correctly allocating multi-dimensional arrays. If you declare the array as shown in the answer, as one adjacent chunk of memory, then you can just fread/fwrite the whole array in one go. — Lundin
– Lundin, Commented Oct 7, 2019 at 13:27
To make a multidimensional array with malloc, either you allocate an array of pointers for the rows, and then allocate each of them separately for the columns, or you allocate a single-dimensional array with as many cells as you need in total and then use arithmetic to map multi-dimensional coordinates to the correct cell (e.g., row * height + column). — Arkku
– Arkku, Commented Oct 7, 2019 at 13:29
There are probably 37,292 duplicates of this question -- do we have a canonical one to point to? — Steve Summit
– Steve Summit, Commented Oct 7, 2019 at 13:42
@Steve - depends on how you separate the dupes: (1) in order of using "multidimensional" or (2) in order of using "allocate!" — Adrian Mole
– Adrian Mole, Commented Oct 7, 2019 at 14:53

Adrian Mole · Accepted Answer · 2019-10-07 13:27:55Z

1

You can allocate a two-dimensional array in two stages, as follows (I'm assuming that the base data type is int here, but it could be almost anything):

int** my2dArray = malloc(sizeof(int*) * n_rows); // Makes one INTEGER POINTER for each of n_rows
for (int n = 0; n < n_rows; ++n) my2dArray[n] = malloc(sizeof(int) * n_cols); // Makes one INTEGER for each column

You can then access any element of the 2-D array, given its row and column with, for example:

int value = my2dArray[row][column];

Here, I've assumed the conventional (standard) approach of using "row priority" (so that the first index is the row).

answered Oct 7, 2019 at 13:27

Adrian Mole

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