How do you enter dimensions of matrix in c plus plus?

Answer 1

Place each dimension in a separate array subscript. E.g., a table of 5 rows of 6 integers could be declared as follows:

int table[5][6];

A cuboid of 3x4x5 integers could be declared as follows:

int cube[3][4][5];

Of course, with static arrays, the dimensions are known in advance, and they are fixed for each dimension. In other words, in an array of 5x6 elements, each of the 5 elements in the first dimension has exactly 6 elements in the second dimension. This is also true of dynamic arrays (the only difference being the dimensions are not known in advance). However, sometimes you want an array where the number of elements in each dimension can vary. That is, the first element in the first dimension might only have 1 element, but the second might have 2 or more elements. In this case you are no longer dealing with a traditional multi-dimensional array, you are dealing with a 1 dimensional array of 1 dimensional arrays: or an array of arrays.

The following code demonstrates this concept, but rather than using dynamic C-style arrays, it uses an array of vectors instead. Vectors are always the preferred method of instantiating dynamic arrays in C++ because, unlike an array, a vector knows exactly how many elements it contains, and is therefore much easier to work with, especially when passing dynamic arrays into external functions.

The following code increases the size of the second dimension by one for each first dimension, thus creating a triangular table of values, rather than the more traditional rectangular table. The values are taken from the standard multiplication tables, thus the first row contains 1x1, the second contains 2x1 and 2x2, the third contains 3x1, 3x2 and 3x3, and so on. The final row contains the complete 12 times table, from 12x1 to 12x12.

#include<iostream>

#include<iomanip>

#include<vector>

int main()

{

using namespace std;

typedef vector<int> array1d;

// Build a 1 dimensional array of vectors of increasing length:

array1d A[12];

for( size_t x=0; x<12; ++x )

{

array1d& B = A[x];

for( size_t y=0; y<x+1; ++y )

B.push_back(( x+1 )*( y+1 ));

}

// Print array:

for( size_t x=0; x<12; ++x )

{

for( size_t y=0; y<A[x].size(); ++y )

cout<<setw(3)<<A[x][y]<<" ";

cout<<endl;

}

return(0);

}