# Resolved: numpy slicing in the for loop

I am in doubt about the slice used in this lesson, i know it is not the core of this excercise. i do not understand this [0, :] in the for loop.

What is the function of the zero + comma before the colon?

Hey Carlos,

Thank you for your question!

What this notation allows is to perform slicing of N-dimensional `numpy`

arrays. Take a look at the example below. I have defined a `numpy`

array with three rows (indices 0, 1 and 2) and four columns (indices 0, 1, 2 and 3).

Let's do the following:

The first colon within the brackets reads *'take all rows from the matrix'*, whereas the second colon reads

*'take all*. Therefore, the comma between the colons separates rows from columns in a matrix. Alright, now, typing

**columns**from the matrix'`a[0,:]`

would slice the matrix such that we take only the row with index 0 together with all the columns. This would produce the following result:Analogously, typing

`a[:,0]`

would take the numbers from all rows and only the first column:You can of course slice the matrix in many different ways. One way which you are probably familiar with is to slice only one number from the matrix. For example

`a[0,3]`

would output the number in the first row and the fourth column:I hope this answers your question! Play around with the notation yourself to get a good feeling about what is going on! If you are still in doubt, you can always refer to our 'Data Preprocessing with NumPy' course, Section 4: 'Working with Arrays', Lecture 1: 'Basic Slicing'. There, you can find a very comprehensive explanation on the matter :)

Kind regards,

365 Hristina