Slicing in Python stands for retrieving elements from a certain index to another index in a sequence. In this chapter, however, we will focus on slicing in NumPy arrays.

Slicing in 1D Arrays

General syntax for slicing in 1D arrays is the following: array[start:end:step]. Let’s have a more detailed look at it:

  • start is the index from which to start slicing;
  • end is the index at which the slicing ends (the index itself is not included);
  • step specifies the increments between the indices (1 by default).

Here is an example to clarify everything:

Omitting start, end and step

As you can see, we can often omit the start, end, step or even all of them at the same time. step, for example, can be omitted when we want it to be equal to 1. start and end can be omitted in the following scenarios:

  1. Omitting start:
    • slicing from the first element (step is positive);
    • slicing from the last element (step is negative).
  2. Omitting end:
    • slicing to the last element inclusive (step is positive);
    • slicing to the first element inclusive (step is negative).

Slicing in 2D Arrays

Slicing in 2D and higher dimensional arrays works in a similar way as it does in 1D arrays. However, in 2D arrays there are already two axes.

If we want to perform slicing only on axis 0 to retrieve 1D arrays, then the syntax remains the same: array[start:end:step]. In case we want to perform slicing on the elements of these 1D arrays (axis 1), then the syntax is the following: array[start:end:step, start:end:step]. Basically, in this case the number of slices is equal to the number of dimensions of an array.

Moreover, we can use slicing for one axis and basic indexing for the other axis. Let’s have a look at an example with 2D slicing:

2D Slicing example

The purple squares are the elements retrieved from slicing. Here is the code for this example:


  1. Create a slice of array_1d with every second element starting from the second element to the end and store it in slice_1d (use positive index for start and don't specify end).
  2. Create a slice of array_2d with the two last elements of the first row (first 1D array) using basic indexing (positive indexing) and slicing (specify only positive start).

Everything was clear?

Section 2. Chapter 3
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