np.savez() for multiple arrays in NumPy - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does np.savez() do in NumPy?

np.savez() saves multiple NumPy arrays into a single file with a .npz extension. It helps keep related arrays together for easy loading later.

beginner

How do you save multiple arrays with names using np.savez()?

You pass arrays as keyword arguments like np.savez('file.npz', arr1=array1, arr2=array2). This saves arrays with the names arr1 and arr2 inside the file.

beginner

How do you load arrays saved with np.savez()?

Use np.load('file.npz') to open the file. It returns a dictionary-like object where you can access arrays by their saved names, e.g., data['arr1'].

intermediate

What is the difference between np.savez() and np.savez_compressed()?

np.savez() saves arrays without compression, which is faster. np.savez_compressed() compresses the data to save disk space but takes more time to save and load.

intermediate

Can you save arrays without naming them in np.savez()? What happens?

Yes, you can pass arrays as positional arguments like np.savez('file.npz', array1, array2). They get saved with default names like arr_0, arr_1, etc.

What file extension does np.savez() create?

A .txt

B .npz

C .csv

D .npy

np.savez() creates an archive file with the .npz extension to store multiple arrays.

How do you access an array named 'data' after loading a file saved with np.savez()?

A data.get('data')

B data.data

C data['data']

D data.load('data')

Loading with np.load() returns a dictionary-like object. You access arrays by keys like data['data'].

Which function compresses the saved arrays to save disk space?

A np.savez_compressed()

B np.savez()

C np.save()

D np.load()

np.savez_compressed() compresses arrays when saving, reducing file size.

If you save arrays without names in np.savez(), what names do they get?

A arr_0, arr_1, ...

B array1, array2, ...

C default1, default2, ...

D No names assigned

Unnamed arrays get default names like arr_0, arr_1, etc.

Which of these is NOT true about np.savez()?

A It can save multiple arrays in one file

B It creates a file with .npz extension

C It allows naming arrays when saving

D It saves arrays as plain text

np.savez() saves arrays in a binary format, not plain text.

Explain how to save and load multiple arrays using np.savez().

Think about how you keep related things together in one box and then open it later.

Describe the difference between np.savez() and np.savez_compressed().

Consider saving files quickly versus saving space on your disk.

Practice

1. What is the main purpose of np.savez() in NumPy?

easy

A. To save multiple arrays into a single file for easy storage

B. To load arrays from a file

C. To create a new array from existing ones

D. To delete arrays from memory

Solution

1. Step 1: Understand the function purpose

   np.savez() is used to save multiple arrays into one file, making storage and sharing easier.

2. Step 2: Compare options with function use

   Loading arrays is done by np.load(), not np.savez(). Creating or deleting arrays is unrelated.

3. Final Answer:

   To save multiple arrays into a single file for easy storage -> Option A

4. Quick Check:

   np.savez() saves arrays [OK]

Hint: Remember: savez saves multiple arrays in one file [OK]

Common Mistakes:

• Confusing np.savez() with np.load()
• Thinking it creates new arrays
• Assuming it deletes arrays

2. Which of the following is the correct syntax to save two arrays a and b using np.savez() with names?

easy

A. np.savez('data.npz', a, b)

B. np.savez(a='data.npz', b='data.npz')

C. np.savez('data.npz', a=b)

D. np.savez('data.npz', first=a, second=b)

Solution

1. Step 1: Recall named saving syntax

   To save arrays with names, use np.savez(filename, name1=array1, name2=array2).

2. Step 2: Check each option

   np.savez('data.npz', first=a, second=b) uses named arguments correctly. np.savez('data.npz', a, b) saves unnamed arrays. Options B and D misuse argument positions and names.

3. Final Answer:

   np.savez('data.npz', first=a, second=b) -> Option D

4. Quick Check:

   Named arrays use name=array [OK]

Hint: Use name=value pairs to save arrays with names [OK]

Common Mistakes:

• Not using names for arrays
• Passing arrays as positional but expecting names
• Mixing argument order

3. What will be the output keys when loading a file saved by np.savez('file.npz', x, y) where x and y are arrays without names?

medium

A. ['arr_0', 'arr_1']

B. ['x', 'y']

C. ['0', '1']

D. ['array1', 'array2']

Solution

1. Step 1: Understand default naming in np.savez()

   If arrays are saved without names, NumPy assigns default keys like 'arr_0', 'arr_1', etc.

2. Step 2: Match output keys

   Since x and y were saved unnamed, keys will be 'arr_0' and 'arr_1'. Named keys like 'x' or 'y' appear only if explicitly named.

3. Final Answer:

   ['arr_0', 'arr_1'] -> Option A

4. Quick Check:

   Unnamed arrays get keys arr_0, arr_1 [OK]

Hint: Unnamed arrays get keys arr_0, arr_1, ... by default [OK]

Common Mistakes:

• Assuming variable names become keys automatically
• Expecting numeric string keys
• Using arbitrary names without naming arrays

4. Identify the error in this code snippet:

import numpy as np
x = np.array([1,2])
y = np.array([3,4])
np.savez('data.npz', x, y)
loaded = np.load('data.npz')
print(loaded['x'])

medium

A. SyntaxError in np.savez() call

B. KeyError because 'x' was not saved with a name

C. FileNotFoundError when loading 'data.npz'

D. No error, prints array x

Solution

1. Step 1: Check how arrays were saved

   Arrays x and y were saved without names, so keys are 'arr_0' and 'arr_1'.

2. Step 2: Analyze the loading and key access

   Trying to access loaded['x'] causes a KeyError because 'x' is not a key in the file.

3. Final Answer:

   KeyError because 'x' was not saved with a name -> Option B

4. Quick Check:

   Unnamed arrays have keys arr_0, arr_1 [OK]

Hint: Access keys as arr_0 if arrays saved without names [OK]

Common Mistakes:

• Assuming variable names are keys automatically
• Ignoring KeyError on wrong key access
• Confusing save and load syntax

5. You want to save three arrays a, b, and c into one file with names 'first', 'second', and 'third'. Later, you want to load and print the sum of all elements from these arrays. Which code correctly does this?

hard

A. np.savez('arrays.npz', first=a, second=b, third=c)
loaded = np.load('arrays.npz')
total = sum(loaded.values())
print(total)

B. np.savez('arrays.npz', a, b, c)
loaded = np.load('arrays.npz')
total = loaded['a'].sum() + loaded['b'].sum() + loaded['c'].sum()
print(total)

C. np.savez('arrays.npz', first=a, second=b, third=c)
loaded = np.load('arrays.npz')
total = loaded['first'].sum() + loaded['second'].sum() + loaded['third'].sum()
print(total)

D. np.savez('arrays.npz', a=a, b=b, c=c)
loaded = np.load('arrays.npz')
total = loaded['first'] + loaded['second'] + loaded['third']
print(total)

Solution

1. Step 1: Save arrays with correct names

   np.savez('arrays.npz', first=a, second=b, third=c) loaded = np.load('arrays.npz') total = loaded['first'].sum() + loaded['second'].sum() + loaded['third'].sum() print(total) saves arrays with names 'first', 'second', 'third' matching the requirement.

2. Step 2: Load and sum elements correctly

   np.savez('arrays.npz', first=a, second=b, third=c) loaded = np.load('arrays.npz') total = loaded['first'].sum() + loaded['second'].sum() + loaded['third'].sum() print(total) accesses arrays by correct keys and sums elements using .sum(). Other options either use wrong keys or incorrect summing methods.

3. Final Answer:

   np.savez('arrays.npz', first=a, second=b, third=c) loaded = np.load('arrays.npz') total = loaded['first'].sum() + loaded['second'].sum() + loaded['third'].sum() print(total) -> Option C

4. Quick Check:

   Save with names, load by names, sum elements [OK]

Hint: Save with names, load by names, sum with .sum() [OK]

Common Mistakes:

• Using wrong keys when loading
• Trying to sum arrays directly without .sum()
• Saving arrays without names but accessing by names