Modify values in numpy array based on index and value

Question

How do I modify a np array based on the current value and the index? When I just want to modify certain values, I use e.g. arr[arr>target_value]=0.5 but how do I only modify the values of arr > target_value where also the index is greater than a certain value?

Answer 1

You have to combine two conditions:

 arr[(arr>target_value) & (np.arange(len(arr)) > certain_value)]

Answer 2

For that very specific example you would just use indexing I believe

eg arr[100:][arr[100:] > target_value]=0.5

in general it could be conceptually easier to do these two things separately. First figure out which indices you want, then check whether they satisfy whatever condition you want.