CODEWITHSUNDEEP
c++arraysdynamic-allocationstatic-allocation
Stefan Octavian
Stefan Octavian

Reputation: 620

Replace dynamic allocation with a static one

I have a project and I have to define an array of arrays of different dimensions (like a triangle) because I am not allowed to use std::vector or other container class. For this, I am using an array of pointers. Normally, I would do this:

int* triangle[n]; 
for(int i = 0; i < n; i++) {
    triangle[i] = new int[i + 1];
    for(int j = 0; j <= i; j++) cin >> triangle[i][j];
}

But I must not use dynamic memory! I thought that doing

int* triangle[n];
for(int i = 0; i < n; i++) {
    int row[i + 1];
    triangle[i] = row;
    for(int j = 0; j <= i; j++) cin >> triangle[i][j];
}

would do the trick. But it didn't. Instead, when I iterate over the arrays and print the contents, I get garbage. So, how can I replace a dynamic allocation with a static one?

Upvotes: 0

Views: 448

Answers (2)

Stefan Octavian
Stefan Octavian

Reputation: 620

As @BeyelerStudios pointed out in the comments, at the end of the for loop, the memory allocated for the row array gets freed. When I am trying to print out the contents, I am dereferencing pointers to this freed up memory, so I am getting garbage. Thank you!

Upvotes: 0

Yury Schkatula
Yury Schkatula

Reputation: 5369

So, if you want "array of arrays" simplified, your choice is array linearization. It means that instead of 7 arrays of 10 items each you should declare single array of 70 elements. Then, just change nested indexes with a function that calculates resulting shift in linearized array, and viola!

Here you can find one of such examples: How to map the indexes of a matrix to a 1-dimensional array (C++)?

In case you don't know in advance how long is your array, it may be a tough choice to determine preliminary reservation size (for example, STL containers like vector etc. do the same trick: they allocate a chunk of memory then grow the container until free capacity left, then re-allocate bigger chunk moving the data from old to new buffer, and again, and again, and again...)

Upvotes: 3

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