Numpy Issue About Autoupdate Array When Subtracting Float Number To Int Array

Hi I'm trying to subtract a number for a entire column on a Numpy 2D array, and put the resultant values into the same column, but when the subtracted number is float and the array

Solution 1:

NumPy arrays have a fixed datatype (dtype) which is inferred from the initialization data if you don't specify it yourself. It won't change unless you tell it to, so in your first case:

a[:,1] - 0.5

you're OK because you create a new array with a new, floatdtype inferred as necessary from the calculation(the original a is not changed.) In your second, you are actually trying to change the values in a, which is an integer array, so the result of the calculation is cast to an integer: int(2-0.5) is 1, for example.

To do float arithmetic on your array, upcast it to float explicitly with astype:

In [32]: a.astype('float') - 0.5
Out[32]: 
array([[ 0.5,  1.5],
       [ 1.5,  2.5],
       [ 2.5,  3.5]])