Slicing with start:stop:step in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the slicing notation start:stop:step mean in numpy arrays?

It means to select elements from the array starting at index start, up to but not including stop, taking every stepth element.

beginner

What happens if you omit start in start:stop:step slicing?

If start is omitted, slicing starts from the beginning of the array (index 0).

intermediate

How does a negative step value affect slicing in numpy?

A negative step means slicing goes backwards, selecting elements in reverse order.

beginner

Given arr = np.array([10, 20, 30, 40, 50]), what does arr[1:4:2] return?

It returns array([20, 40]) because it starts at index 1 (20), goes up to index 4 (not including 4), taking every 2nd element.

beginner

What is the result of arr[::-1] for a numpy array arr?

It returns the array reversed, selecting all elements with a step of -1.

In numpy slicing arr[start:stop:step], what does stop represent?

A The last element included in the slice

B The step size between elements

C The number of elements to select

D The index where slicing stops, not including this index

The stop index is where slicing stops, but this index is not included in the result.

What does arr[::2] do?

A Selects every second element from the start to the end

B Selects elements in reverse order

C Selects elements from index 2 to the end

D Selects elements from index 0 to 2

Omitting start and stop means the whole array is sliced, taking every 2nd element.

What is the output of arr[3:0:-1] for arr = np.array([5,10,15,20,25])?

A [20 15 10 5]

B [20 15 10]

C [25 20 15]

D [20 15]

Starts at index 3 (20), goes backwards to index 1 (not including 0), stepping -1.

If step is negative, what must be true about start and stop?

A <code>start</code> equals <code>stop</code>

B <code>start</code> must be less than <code>stop</code>

C <code>start</code> must be greater than <code>stop</code>

D No relation needed

When stepping backwards, slicing moves from a higher index to a lower index.

What does arr[:] do?

A Returns a copy of the whole array

B Returns an empty array

C Returns elements from index 1 to end

D Returns elements from index 0 to 1

Omitting start, stop, and step returns the whole array as a slice (a view or copy).

Explain how slicing with start:stop:step works in numpy arrays.

Think about how you pick items from a list skipping some.

Describe what happens when you use a negative step in numpy slicing and give an example.

Imagine reading a list from the end to the start.

Practice

1. What does the step value control in a NumPy array slice like arr[start:stop:step]?

easy

A. How many elements to skip between each selected element

B. The total number of elements to select

C. The starting index of the slice

D. The ending index of the slice

Solution

1. Step 1: Understand the slice syntax

   The slice arr[start:stop:step] selects elements from start up to but not including stop, moving by step.

2. Step 2: Identify the role of step

   The step determines how many elements to skip between each selected element. For example, step=2 picks every second element.

3. Final Answer:

   How many elements to skip between each selected element -> Option A

4. Quick Check:

   Step controls skipping elements = A [OK]

Hint: Step is the gap between elements in slicing [OK]

Common Mistakes:

• Confusing step with start or stop index
• Thinking step is total count of elements
• Ignoring that step can be negative

2. Which of the following is the correct syntax to slice a NumPy array arr to get every 3rd element from index 1 to 10?

easy

A. arr[10:1:3]

B. arr[1:3:10]

C. arr[1:10:3]

D. arr[3:1:10]

Solution

1. Step 1: Recall slice order

   The slice syntax is arr[start:stop:step]. Here, start=1, stop=10, step=3 means from index 1 up to 9, every 3rd element.

2. Step 2: Check each option

   arr[1:10:3] matches the correct order and values. Others have wrong order or values that don't match the requirement.

3. Final Answer:

   arr[1:10:3] -> Option C

4. Quick Check:

   Correct slice syntax = D [OK]

Hint: Remember slice is start:stop:step in order [OK]

Common Mistakes:

• Mixing up stop and step positions
• Using step as stop index
• Reversing start and stop incorrectly

3. Given arr = np.array([10, 20, 30, 40, 50, 60, 70]), what is the output of arr[1:6:2]?

medium

A. [10 30 50 70]

B. [20 30 40 50 60]

C. [30 50]

D. [20 40 60]

Solution

1. Step 1: Identify slice indices

   The slice arr[1:6:2] starts at index 1 (value 20), stops before index 6, stepping by 2.

2. Step 2: Select elements

   Indexes selected: 1 (20), 3 (40), 5 (60). So output is [20 40 60].

3. Final Answer:

   [20 40 60] -> Option D

4. Quick Check:

   Elements at 1,3,5 = [20 40 60] [OK]

Hint: Pick elements at start, start+step, ... until stop-1 [OK]

Common Mistakes:

• Including stop index element
• Using wrong step size
• Starting from index 0 instead of 1

4. What is wrong with this code snippet?
arr = np.array([1,2,3,4,5])
slice = arr[4:1:-1]
What will slice contain?

medium

A. [4 3 2]

B. [5 4 3]

C. Empty array

D. IndexError

Solution

1. Step 1: Understand negative step slicing

   With step=-1, slicing goes backwards from start index 4 down to stop index 1 (exclusive).

2. Step 2: Identify elements selected

   Indexes selected: 4 (5), 3 (4), 2 (3). So slice contains [5 4 3].

3. Final Answer:

   [5 4 3] -> Option B

4. Quick Check:

   Backward slice from 4 to 2 = [5 4 3] [OK]

Hint: Negative step slices backward, stop is exclusive [OK]

Common Mistakes:

• Expecting forward slice with negative step
• Including stop index element
• Thinking it causes error

5. You have a NumPy array arr = np.arange(20). How would you slice it to get every 4th element in reverse order starting from the last element?

hard

A. arr[19::-4]

B. arr[::4]

C. arr[19:0:4]

D. arr[0:19:-4]

Solution

1. Step 1: Understand array and goal

   arr = np.arange(20) creates array from 0 to 19. We want every 4th element backward starting from last element (index 19).

2. Step 2: Check slicing options

   arr[19::-4] starts at index 19, goes backward to start (index 0), stepping by 4, which matches the goal.

3. Step 3: Verify other options

   A: arr[::4] every 4th forward [0,4,8,...], wrong direction. C: arr[19:0:4] positive step with start>stop gives empty. D: arr[0:19:-4] start<stop with negative step gives empty.

4. Final Answer:

   arr[19::-4] -> Option A

5. Quick Check:

   Reverse every 4th from last = arr[19::-4] [OK]

Hint: Use start index with negative step for reverse slicing [OK]

Common Mistakes:

• Using positive step with decreasing indices
• Wrong start or stop indices
• Assuming arr[::-4] always starts at last element