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NumPy | asmatrix method

schedule Aug 12, 2023

Last updated

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Python●NumPy

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Parameters Return value Examples Basic usage Case when array is not copied

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NumPy's asmatrix(~) method constructs a matrix from a sequence of data (e.g. arrays and tuples).

Parameters

1. a | array-like

The sequence of data used to construct a matrix.

NOTE

If a is a NumPy array or matrix, no copying would be done - modifying the result of asmatrix(~) would also automatically modify a.

2. dtype | string or type | optional

Type of data stored in the NumPy array. By default, the type will be inferred.

Return value

A NumPy matrix.

Examples

Basic usage

To create a 2 by 2 matrix:


        
        
            
                
                
                    np.asmatrix([[4,5],[6,7]])
                
            
            matrix([[4, 5],
        [6, 7]])

Case when array is not copied

Suppose we create a NumPy matrix out of a NumPy array, like so:


        
        
            
                
                
                    a = np.array([[4,5],[6,7]])
b = np.asmatrix(a)

When we modify the contents of b:


        
        
            
                
                
                    b[0,0] = 9
b
                
            
            matrix([[9, 5],
        [6, 7]])

This would automatically modify a as well:


        
        
            
                
                
                    a
                
            
            array([[9, 5],
       [6, 7]])

What's happening here is that the NumPy objects a and b actually share the same memory space, so modifying one would involve modifying the other. Note that this happens only if we try to call asmatrix(~) on a NumPy array or matrix.

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Published by Isshin Inada

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