np.concatenate() for joining arrays in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does np.concatenate() do in NumPy?

np.concatenate() joins two or more arrays into one bigger array along a specified axis.

beginner

What is the default axis for np.concatenate()?

The default axis is axis=0, which means arrays are joined row-wise (stacked vertically).

beginner

Can np.concatenate() join arrays of different shapes?

No, arrays must have the same shape except in the dimension along the joining axis.

beginner

How do you join two 1D arrays a = [1, 2] and b = [3, 4] using np.concatenate()?

Use np.concatenate([a, b]) to get [1, 2, 3, 4].

intermediate

What happens if you set axis=1 when concatenating 2D arrays?

Arrays are joined column-wise (side by side), increasing the number of columns.

What is the result of np.concatenate([np.array([1,2]), np.array([3,4])])?

A [[1 2] [3 4]]

B [[1 3] [2 4]]

C [1 2 3 4]

D Error

Arrays are joined end-to-end along axis 0 by default, resulting in a 1D array [1 2 3 4].

Which axis joins arrays side by side (columns) in 2D with np.concatenate()?

A axis=1

B axis=-1

C axis=0

D axis=2

axis=1 joins arrays column-wise (side by side) in 2D arrays.

If two arrays have shapes (2,3) and (2,4), can they be concatenated along axis=0?

A Yes

B No

C Only if axis=1

D Only if reshaped

Along axis=0, arrays must have the same number of columns. Here, 3 != 4, so no.

What type of object must be passed to np.concatenate()?

A A scalar value

B A single array

C A dictionary

D A list or tuple of arrays

np.concatenate() requires a list or tuple of arrays to join.

What happens if arrays have different shapes on non-concatenation axes?

A An error is raised

B They concatenate successfully

C Arrays are padded automatically

D Arrays are truncated

Arrays must match in all dimensions except the concatenation axis; otherwise, an error occurs.

Explain how np.concatenate() works and what rules arrays must follow to be joined.

Think about stacking arrays like stacking blocks either vertically or horizontally.

Describe a real-life example where you might use np.concatenate() to combine data.

Imagine putting together pieces of a puzzle to see the full picture.