How to create a pointer, two dimensional array in c++?
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Hi, with c++, I need to make a two dimensional array of a certain size to be defined, during run time, so I am going to use pointers(and not vectors).
eg float **num;
basically my problem is when I do:
num = new float[2];
I get an error bout not be able to convert a float* or something like that.
(then I would do something like: for(int i=0;i<2;i++) num = new float[3];)
I'm pretty sure I've done this before, but for some reason I can't remember what to do.
THanks
oh and btw, when deleting the array do i have to loop through each of the first array 'num[]' deleting their memory, and then just one delete after that on the 'num' thx
You're almost there. What you need to do is initialize first an array of pointers and then each element seperately. You're right about delete. Here's a snipet:
Hope I've cleared it up!
float **num = NULL;const int array_size = 12;num = new float*[array_size];for (int i = 0; i < array_size; i++) { num = new float;}// do your thingfor (int i = 0; i < array_size; i++) { delete num;}delete [] numHope I've cleared it up!
The first allocation allocates an array with pointers. So you need to do this:
num = new float *[2];
[edit]And I replied too late. =)[/edit]
num = new float *[2];
[edit]And I replied too late. =)[/edit]
Quote:Original post by johnnyBravoVectors can be resized during run time. (std::vector that is)
Hi, with c++, I need to make a two dimensional array of a certain size to be defined, during run time, so I am going to use pointers(and not vectors).
Quote:num = new float[2];You shouldn't allocate float array, you should allocate an array of pointers* to floats. num = new float*[2];
Quote:oh and btw, when deleting the array do i have to loop through each of the first array 'num[]' deleting their memory, and then just one delete after that on the 'num' thxYes.
If the number of elements of one of the dimensions is known at compile time then use a pointer to an array instead, you'll have more contiguous of elements in other words its more efficient:
#include <cstddef> // std::size_t#include <iostream>int main() { const std::size_t N = 30; // constant, is known at compile-time int (*foo)[N]; // pointer to an array of integers std::size_t M = 0; std::cout << "enter size: "; std::cin >> M; foo = new int[M][N]; delete[] foo;}
Boost.MultiDimensional Array Library
Example:
Example:
#include "boost/multi_array.hpp"#include <cassert>int main () { // Create a 3D array that is 3 x 4 x 2 typedef boost::multi_array<double, 3> array_type; typedef array_type::index index; array_type A(boost::extents[3][4][2]); // Assign values to the elements int values = 0; for(index i = 0; i != 3; ++i) for(index j = 0; j != 4; ++j) for(index k = 0; k != 2; ++k) A[j][k] = values++; // Verify values int verify = 0; for(index i = 0; i != 3; ++i) for(index j = 0; j != 4; ++j) for(index k = 0; k != 2; ++k) assert(A[j][k] == verify++); return 0;}This topic is closed to new replies.
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