Training data and models in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main purpose of training data in machine learning?

Imagine you want to teach a computer to recognize pictures of cats. What role does the training data play in this process?

A It provides examples for the model to learn patterns from.

B It tests how well the model performs on new data.

C It stores the final predictions made by the model.

D It cleans and prepares the data before training.

💡 Hint

Think about what the model needs to see to learn.

Training data is the set of examples the model uses to learn patterns. It helps the model understand what features relate to the target outcome.

🔍 Analysis

intermediate

What is the output of this Python code simulating model training?

Consider this simple code that simulates training a model by counting how many times a value appears in training data.

Intro to Computing

training_data = ['cat', 'dog', 'cat', 'bird', 'dog', 'cat']
model = {}
for item in training_data:
    model[item] = model.get(item, 0) + 1
print(model)

A SyntaxError

B {'cat': 3, 'dog': 2, 'bird': 1}

C {'cat': 3, 'dog': 3, 'bird': 1}

D {'cat': 2, 'dog': 2, 'bird': 1}

💡 Hint

Count how many times each animal appears in the list.

The code counts occurrences of each item in the training data. 'cat' appears 3 times, 'dog' 2 times, and 'bird' once.

❓ data_output

advanced

What is the shape of the training data matrix after preprocessing?

You have 1000 images, each resized to 28x28 pixels and converted to grayscale. The images are stored as a NumPy array for training.

What will be the shape of the training data array?

A (1000, 28, 28)

B (28, 28, 1000)

C (1000, 784)

D (784, 1000)

💡 Hint

Think about flattening each 28x28 image into a single row vector.

Each 28x28 image has 784 pixels. Flattening means each image becomes a 1D array of length 784. With 1000 images, the shape is (1000, 784).

❓ visualization

advanced

Which plot best shows model performance over training epochs?

You trained a model for 10 epochs and recorded accuracy after each epoch. Which plot type best visualizes this change over time?

A Line plot showing accuracy on the y-axis and epochs on the x-axis.

B Bar chart showing accuracy for each class.

C Scatter plot of accuracy vs. number of training samples.

D Pie chart showing percentage of correct predictions.

💡 Hint

Think about how to show change over a sequence of steps.

A line plot is ideal to show how accuracy changes over epochs because it connects points in order and shows trends clearly.

🔍 Analysis

expert

What error does this code raise when training a model with mismatched data?

Look at this code snippet where features and labels have different lengths.

Intro to Computing

features = [[1, 2], [3, 4], [5, 6]]
labels = [0, 1]
from sklearn.linear_model import LogisticRegression
model = LogisticRegression()
model.fit(features, labels)

A No error, model trains successfully

B TypeError: unsupported operand type(s) for +: 'int' and 'list'

C IndexError: list index out of range

D ValueError: Found input variables with inconsistent numbers of samples

💡 Hint

Check if the number of feature rows matches the number of labels.

Scikit-learn requires the number of samples in features and labels to be the same. Here, features has 3 samples but labels has 2, causing a ValueError.

Practice

1. What is the main purpose of training data in machine learning?

easy

A. To teach the model patterns and relationships in data

B. To test the model's accuracy after training

C. To store the final predictions of the model

D. To clean and prepare data for analysis

Solution

1. Step 1: Understand the role of training data

   Training data is used to show the model examples so it can learn patterns.

2. Step 2: Differentiate training from testing

   Testing data checks model accuracy, but training data teaches the model.

3. Final Answer:

   To teach the model patterns and relationships in data -> Option A

4. Quick Check:

   Training data = Teach model [OK]

Hint: Training data is what the model learns from [OK]

Common Mistakes:

• Confusing training data with testing data
• Thinking training data stores predictions
• Assuming training data is for cleaning only

2. Which of the following is the correct way to split data into training and testing sets in Python using scikit-learn?

easy

A. test_train_split(data, size=0.2)

B. train_test_split(data, test_size=0.2)

C. train_test(data, split=0.2)

D. split_train_test(data, 0.2)

Solution

1. Step 1: Recall the scikit-learn function name

   The correct function to split data is train_test_split.

2. Step 2: Check the parameter syntax

   The parameter test_size=0.2 correctly sets 20% data for testing.

3. Final Answer:

   train_test_split(data, test_size=0.2) -> Option B

4. Quick Check:

   Correct function and parameter = train_test_split(data, test_size=0.2) [OK]

Hint: Remember scikit-learn's function is train_test_split [OK]

Common Mistakes:

• Using wrong function names
• Mixing parameter names
• Incorrect order of arguments

3. Given the following Python code, what will be the output?

from sklearn.linear_model import LinearRegression
import numpy as np

X = np.array([[1], [2], [3], [4]])
y = np.array([2, 4, 6, 8])
model = LinearRegression()
model.fit(X, y)
pred = model.predict([[5]])
print(round(pred[0], 1))

medium

A. 9.0

B. 12.0

C. 8.0

D. 10.0

Solution

1. Step 1: Understand the training data relationship

   X values are 1 to 4, y values are double X (2,4,6,8), so model learns y = 2 * X.

2. Step 2: Predict for X=5 using the model

   Prediction = 2 * 5 = 10.0, rounding to 1 decimal is 10.0.

3. Final Answer:

   10.0 -> Option D

4. Quick Check:

   Prediction for 5 = 10.0 [OK]

Hint: Model learns y=2x, predict for 5 gives 10 [OK]

Common Mistakes:

• Ignoring rounding in output
• Misunderstanding model relationship
• Using wrong input shape for predict

4. Identify the error in this code snippet for training a model:

from sklearn.linear_model import LogisticRegression
X_train = [[0, 1], [1, 0]]
y_train = [0, 1]
model = LogisticRegression()
model.fit(y_train, X_train)

medium

A. The order of arguments in model.fit() is reversed

B. LogisticRegression cannot be used with this data

C. X_train should be a numpy array, not a list

D. y_train should contain strings, not integers

Solution

1. Step 1: Check the model.fit() argument order

   fit() expects features first (X_train), then labels (y_train).

2. Step 2: Identify the mistake in the code

   Code passes y_train first, then X_train, which is incorrect and causes error.

3. Final Answer:

   The order of arguments in model.fit() is reversed -> Option A

4. Quick Check:

   fit(X_train, y_train) correct order [OK]

Hint: fit() needs features first, labels second [OK]

Common Mistakes:

• Swapping X and y in fit()
• Assuming data type causes error here
• Thinking LogisticRegression needs string labels

5. You have a dataset with 1000 samples and 10 features. You want to train a model but avoid overfitting. Which approach helps most during training?

hard

A. Use all data for training without splitting

B. Train only on the first 100 samples

C. Split data into training and validation sets

D. Ignore feature scaling and train directly

Solution

1. Step 1: Understand overfitting and validation

   Overfitting happens when model learns training data too well and fails on new data.

2. Step 2: Use validation set to check model generalization

   Splitting data into training and validation helps detect and reduce overfitting.

3. Final Answer:

   Split data into training and validation sets -> Option C

4. Quick Check:

   Validation split prevents overfitting [OK]

Hint: Validation set helps check model generalization [OK]

Common Mistakes:

• Training on all data hides overfitting
• Using too few samples reduces learning
• Ignoring feature scaling can hurt model