Why does a[1:3] return only two elements in Python?

The numbers are cuts between the elements. a[1:3] takes everything between cut 1 and cut 3, i.e. the elements at positions 1 and 2. The stop bound is never included; the count is stop − start.

What do negative indices like a[-1] mean?

Negative numbers count from the end: a[-1] is the last element, a[-3:] the last three. Internally Python turns −k into n − k, so with seven elements −3 becomes 4.

How do you reverse a list with slicing?

With a negative step: a[::-1] returns the list backwards, a[5:1:-1] the elements from position 5 up to but not including 1. a[5:2] without a step does not reverse but is empty. A step of 0 raises a ValueError.

What does the Slicing Ruler app show?

A list or string as a row of cells with two rulers, draggable handles for start and stop, a step control, results as in Python 3.13, index and del modes, and six tasks with a list of solutions.

All apps

Slicing Ruler

About this app

The numbers in a[i:j] are cuts between the cells: two handles for start and stop glide along a ruler with positive numbers above and negative ones below, the expression stays in sync, and the result appears exactly as in Python 3.13. a[1:3] gives two cells, a[2:100] is cut off, a[7] is an IndexError, a[5:2] is empty, a[5:1:-1] runs backwards. Plus index and del modes, strings with a TypeError on change, six tasks and predictions.

With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.

Cuts instead of positions

In Python the numbers in a[i:j] stand for cuts between the elements: cut k lies before element k. What comes out are the elements between the two cuts, j − i of them, which is why a[1:3] is only [2, 3]. Negative numbers count from the end; a[-1] is the last element. A missing bound means the start or the end. Python cuts off bounds outside the list: a[2:100] returns the rest without an error, while a single index such as a[7] is an IndexError. The third number is the step; with −1 Python reads backwards.

What the app shows

A row of cells shows the list or the string, with a ruler of positive numbers above and one of negative numbers below. Two handles stand for start and stop; the stop handle is hollow because this bound is excluded. In the cuts view the numbers sit on the edges, and for a negative step to the right of the cells. The expression stays in sync with the handles, and the result card shows the new list in Python notation with a sentence as on the slide below it. Error messages appear exactly as in Python, with an explanation.

Try it yourself

Drag the handles, type your own expression or roll a random one and predict what comes out. In index mode a pointer marks one cell, and outside the list the IndexError appears. In del mode elements really disappear and the remaining cells close up. Six tasks give a target such as [3, 4, 5] or 'world'; after a hit the app lists further expressions with the same result. Own lists of up to 12 values and strings of up to 16 characters are possible.

Frequently asked questions

Why does a[1:3] return only two elements in Python?: The numbers are cuts between the elements. a[1:3] takes everything between cut 1 and cut 3, i.e. the elements at positions 1 and 2. The stop bound is never included; the count is stop − start.
What do negative indices like a[-1] mean?: Negative numbers count from the end: a[-1] is the last element, a[-3:] the last three. Internally Python turns −k into n − k, so with seven elements −3 becomes 4.
How do you reverse a list with slicing?: With a negative step: a[::-1] returns the list backwards, a[5:1:-1] the elements from position 5 up to but not including 1. a[5:2] without a step does not reverse but is empty. A step of 0 raises a ValueError.
What does the Slicing Ruler app show?: A list or string as a row of cells with two rulers, draggable handles for start and stop, a step control, results as in Python 3.13, index and del modes, and six tasks with a list of solutions.

Subject: Computer science

More from Computer science

A VisuApp by heyprof: interactive, free in your browser, no sign-up. What is a VisuApp? · All apps