Array indexing and slicing in Data Analysis Python - Step-by-Step Execution

Concept Flow - Array indexing and slicing

Start with array

↓

Choose index or slice

↓

If index: return single element

↓

If slice: return sub-array

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Use result for further analysis or display

You start with an array, pick an index or a slice, then get either one element or a sub-array to use.

Execution Sample

Data Analysis Python

import numpy as np
arr = np.array([10, 20, 30, 40, 50])
print(arr[2])
print(arr[1:4])

This code shows how to get one element by index and a slice (sub-array) from a numpy array.

Execution Table

Step	Action	Index/Slice	Result	Explanation
1	Create array	-	[10 20 30 40 50]	Array with 5 elements created
2	Access element	2	30	Index 2 returns the third element (30)
3	Access slice	1:4	[20 30 40]	Slice from index 1 up to but not including 4 returns sub-array
4	End	-	-	No more operations

💡 All requested indexing and slicing operations completed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
arr	undefined	[10 20 30 40 50]	[10 20 30 40 50]	[10 20 30 40 50]	[10 20 30 40 50]
result_element	undefined	undefined	30	30	30
result_slice	undefined	undefined	undefined	[20 30 40]	[20 30 40]

Key Moments - 3 Insights

Why does arr[2] return 30 and not 20?

Array indexing starts at 0, so arr[0] is 10, arr[1] is 20, and arr[2] is 30 as shown in execution_table row 2.

Why does arr[1:4] return three elements, not four?

Slicing includes the start index but excludes the end index, so it returns elements at indices 1, 2, and 3, as shown in execution_table row 3.

What happens if I use a slice like arr[3:10]?

Python returns elements from index 3 to the end without error, even if 10 is beyond array length. This is safe slicing behavior.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of arr[2] at step 2?

A 20

B 30

C 40

D 50

💡 Hint

Check the 'Result' column at step 2 in the execution_table.

At which step does the slice [20 30 40] appear in the execution?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Result' column for the slice in the execution_table.

If we change arr[1:4] to arr[1:3], what would the slice return?

A [30 40]

B [20 30 40]

C [20 30]

D [10 20]

💡 Hint

Remember slicing excludes the end index, so it returns elements at indices 1 and 2.

Concept Snapshot

Array indexing and slicing:
- Indexing: arr[i] returns element at position i (0-based)
- Slicing: arr[start:end] returns elements from start up to but not including end
- Negative indices count from the end
- Slicing handles out-of-range end safely
- Useful to extract parts of data easily

Full Transcript

This lesson shows how to get elements from an array using indexing and slicing. Indexing uses a single number to get one element, starting at zero. Slicing uses two numbers separated by a colon to get a sub-array from the start index up to but not including the end index. The example array has five numbers. Accessing arr[2] returns the third element, 30. Slicing arr[1:4] returns elements at indices 1, 2, and 3, which are 20, 30, and 40. Slicing is safe even if the end index is beyond the array length. These operations help you pick and use parts of your data easily.