How to multiply a scalar throughout a specific column within a NumPy array?

Question

I need to do some analysis on a large dataset from a hydrolgeology field work. I am using NumPy. I want to know how I can:

1. multiply e.g. the 2nd column of my ar

Answer 1

Sure:

import numpy as np
# Let a be some 2d array; here we just use dummy data 
# to illustrate the method
a = np.ones((10,5))
# Multiply just the 2nd column by 5.2 in-place
a[:,1] *= 5.2

# Now get the cumulative sum of just that column
csum = np.cumsum(a[:,1])

If you don't want to do this in-place you would need a slightly different strategy:

b = 5.2*a[:,1]
csum = np.cumsum(b)