CODEWITHSUNDEEP
pythonnumpy
cryptofish
cryptofish

Reputation: 89

How to subtract only the first number of a row in a numpy array

For example, say I have:

a = np.array([[1, 2, 3, 6], [2, 45, 34, 56],[3, 8, 56, 45]])

I want to subtract 1 from the first number in all the rows. So it prints:

array([[0, 2, 3, 6], [1, 45, 34, 56],[2, 8, 56, 45]])

I have tried doing 

a = np.array([[1, 2, 3, 6], [2, 45, 34, 56],[3, 8, 56, 45]]) -1

but, it subtracts from all the numbers rather than just the first one.

Upvotes: 2

Views: 2795

Answers (2)

Rayhane Mama
Rayhane Mama

Reputation: 2424

Just for the sake of completeness:

your numpy 2D array looks like this:

[[ 1  2  3  6]
 [ 2 45 34 56]
 [ 3  8 56 45]]

what you want to do is subtract 1 from the first column.
This can be done by slicing the entire first column and subtracting 1 from its items.

in numpy you can slice columns like array[:,col_num] or rows like array[row_num,:] where the : means the all the rows or all the columns respectively.

so your solution is:

a[:,0] -=1

where you select all items of row with index 0 and subtract 1 from them.

I highly recommend you follow the basic and intermediate python tutorials of this link as they will make you familiar with these concepts and many others.
Hope this was helpful.

Upvotes: 1

M.T
M.T

Reputation: 5231

I believe what you are looking for is:

a[:,0]-=1

[:,0] will access all values along the first axis, with the zeroth index along the second axis.

Upvotes: 6

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