在C ++中是否可以有一个行大小不同的矩阵？

Question

I need to read from a file some ints. The first int indicates how many ints I will have in that line.

Example:

5 1 4 7 10 2
4 5 6 7 1
3 1 0 8
3 4 5 6
4 1 2 3 4

Is there a way to create a matrix like this?

I saw that there's a way of doing something similar but it isn't with matrix: Is it possible to make a matrix with rows that have different #rows on Matlab?

Answer 1

You want a jagged edged matrix.

In mathematical modelling this is normally a most undesirable thing indeed, but in your particular case, a

std::vector<std::vector<int>>

will model this perfectly. Be mindful that the properties of an int vary from platform to platform. The minimum range an int can take is -32767 to + 32767. Consider using a long &c. if necessary.

Answer 2

It is not possible to have 2-d arrays with different row sizes in C++ (If this is what you mean by matrix). You can use some C++ std library container or some workaround to overcome this limitation. Some options are:

• Array of pointers where each pointer points to an array (usually dynamically allocated) of required size.
• Array of std::vector or other container. Or std::vector , std::array or other container of std::vector . (For ex: std::vector< std::vector< T > > )
• 2-d array with max row size.

In all of the above approaches data of rows is not contiguous. To make data contiguous (if required), you may create a class of your own which creates a big 1-d array and emulate it as a 2-d array.

Answer 3

Matrices are by definition rectangular. So, what you are describing is not a matrix. It is a jagged array.

You cannot have an array of arrays where the subarrays are different sized. All elements of an array must have the same type. But you can have an array of pointers, which can point to arrays of any size. A simplest solution is to use std::vector :

std::vector<std::vector<int>> vec_of_vecs;

This is actually the simplest way to represent a 2 dimensional array structure (jagged or not), although the memory representation isn't contiguous like you would expect from a non jagged 2D array.

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For performance, it is possible to store the arrays in a single memory block as well, but that is going to be much more complicated. What you can do, is to allocate a single block of memory that contains all of the rows, and another array to store pointers to beginnings of each row.

Answer 4

It is possible to create a std::vector<std::vector<int> > ie a std::vector with elements of type std::vector<int> .

The individual std::vector<int> s can be of different sizes.