np.savez() for multiple arrays in NumPy - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to save two arrays into a single .npz file using np.savez.

NumPy

import numpy as np
arr1 = np.array([1, 2, 3])
arr2 = np.array([4, 5, 6])
np.savez('data.npz', [1]=arr1, arr2=arr2)

Drag options to blanks, or click blank then click option'

Aarray1

Bdata1

Carr1

Dfirst

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that does not match the variable name.

Forgetting to name the first array.

You must provide a name for the first array when saving with np.savez. Here, arr1 is the correct name matching the variable.

2fill in blank

medium

Complete the code to load the arrays saved in 'data.npz' and print the first array.

NumPy

import numpy as np
loaded = np.load('data.npz')
print(loaded[[1]])

Drag options to blanks, or click blank then click option'

A'arr1'

B'arr2'

C'array1'

D'data1'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong key name to access the array.

Forgetting to use quotes around the key.

When loading arrays from a .npz file, you access them by the names used when saving. Here, the first array was saved as arr1.

3fill in blank

hard

Fix the error in the code to save three arrays named 'a', 'b', and 'c' into 'arrays.npz'.

NumPy

import numpy as np
a = np.array([1, 2])
b = np.array([3, 4])
c = np.array([5, 6])
np.savez('arrays.npz', a, [1], c)

Drag options to blanks, or click blank then click option'

Aarr_b

Bb

Carray_b

Db_array

Attempts:

3 left

💡 Hint

Common Mistakes

Using a name that does not match the variable.

Forgetting to include the second array.

When saving multiple arrays without naming them, you must pass the variables in order. Here, b is the variable to include as the second array.

4fill in blank

hard

Fill both blanks to create a dictionary of array lengths for words longer than 3 characters.

NumPy

words = ['apple', 'bat', 'carrot', 'dog']
lengths = {word: [1] for word in words if len(word) [2] 3}

Drag options to blanks, or click blank then click option'

Alen(word)

B>

C<

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using the word itself instead of its length.

Using the wrong comparison operator.

The dictionary comprehension maps each word to its length using len(word). The condition filters words with length greater than 3 using >.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values greater than 0.

NumPy

data = {'a': 1, 'b': -2, 'c': 3}
result = [1]: [2] for k, v in data.items() if v [3] 0}

Drag options to blanks, or click blank then click option'

Ak.upper()

Bv

C>

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Not converting keys to uppercase.

Using the wrong comparison operator.

Using keys instead of values for the dictionary values.

The dictionary comprehension creates keys as uppercase letters using k.upper(), keeps values as v, and filters for values greater than 0 using >.

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