Daidalos October 31, 2019

Examples of how to replace some elements of a matrix using numpy in python:

Let's try to replace the elements of a matrix called M strictly lower than 5 by the value -1:

`>>> import numpy as np`

`>>> M = np.arange(10)`

`>>> M`

`array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])`

`>>> M[M > 5 ] = -1`

`>>> M`

`array([ 0, 1, 2, 3, 4, 5, -1, -1, -1, -1])`

Another example using a 2D matrix

`>>> A = np.arange(16)`

`>>> A`

`array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15])`

`>>> A = A.reshape(4,4)`

`>>> A`

`array([[ 0, 1, 2, 3],`

`[ 4, 5, 6, 7],`

`[ 8, 9, 10, 11],`

`[12, 13, 14, 15]])`

`>>> A[A<=5]=0`

`>>> A`

`array([[ 0, 0, 0, 0],`

`[ 0, 0, 6, 7],`

`[ 8, 9, 10, 11],`

`[12, 13, 14, 15]])`

`>>> A[A>1]=1`

`>>> A`

`array([[0, 0, 0, 0],`

`[0, 0, 1, 1],`

`[1, 1, 1, 1],`

`[1, 1, 1, 1]])`

Exemple using multiple conditions: try to replace the elements > 3 and < 7 using the following syntax M[(M > 2) & (M < 7)] = -1, illustration:

`>>> import numpy as np`

`>>> M = np.arange(10)`

`>>> M`

`array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])`

`>>> M[(M > 2) & (M < 7)] = -1`

`>>> M`

`array([ 0, 1, 2, -1, -1, -1, -1, 7, 8, 9])`

Another solution is to use the numpy function where

`>>> A = np.array((1,7,3,8,4,9,1))`

`>>> np.where(A>4,1,A)`

`array([1, 1, 3, 1, 4, 1, 1])`

Links | Site |
---|---|

Replace all elements of Python NumPy Array that are greater than some value | stackoverflow |

Replace “zero-columns” with values from a numpy array | stackoverflow |

numpy.place | numpy doc |

Numpy where function multiple conditions | stackoverflow |

Replace NaN's in NumPy array with closest non-NaN value | stackoverflow |

numpy.put | numpy doc |

numpy.nan_to_num | numpy doc |

How to: Replace values in an array | kite.com |

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Modified: Oct. 31, 2019, 7:12 p.m.

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