Input From User

Nerd Cafe

1. What is User Input in Python?

Definition: User input allows the program to take data from a human using the keyboard while the program is running.

In Python, we use:

input("Prompt message")

Practical Note:

• The input is always stored as a string by default.

• You must convert it to numbers (if needed) using int(), float(), etc.

2. Basic Example

name = input("What is your name? ")
print("Hello,", name)

Output:

What is your name? Mr Nerd
Hello, Mr Nerd

3. Input with Conversion: Numbers

Integers

age = int(input("Enter your age: "))
print("You will be", age + 1, "next year.")

Practical Note:

• int() will cause an error if user enters non-integer like twenty.

• Use try-except for error handling (covered below).

Float (Decimal)

weight = float(input("Enter your weight (kg): "))
print("Your weight in grams is", weight * 1000)

4. Multiple Inputs in One Line

Example:

x, y = input("Enter two numbers separated by space: ").split()
x = int(x)
y = int(y)
print("Sum:", x + y)

Output:

Enter two numbers separated by space: 5 7
Sum: 12

Practical Note:

• .split() breaks input into list based on space.

• You can also split by comma split(',').

5. List Input for ML or Data Science

Often in ML we need to input feature vectors.

features = input("Enter feature values separated by comma: ").split(',')
features = [float(x) for x in features]
print("Feature vector:", features)

Output:

Enter feature values separated by comma: 5.1,3.5,1.4,0.2
Feature vector: [5.1, 3.5, 1.4, 0.2]

Machine Learning Insight:

This method is commonly used to input values into a trained model for prediction:

import numpy as np
from sklearn.linear_model import LogisticRegression

model = LogisticRegression()
# model.fit(X_train, y_train)  # Assume already trained

user_input = input("Enter feature values (comma-separated): ")
features = np.array([float(x) for x in user_input.split(',')]).reshape(1, -1)
# prediction = model.predict(features)
# print("Prediction:", prediction)

7. Advanced Tip: Loop Until Valid Input

while True:
    try:
        score = float(input("Enter your score (0 to 100): "))
        if 0 <= score <= 100:
            break
        else:
            print("Score must be between 0 and 100.")
    except ValueError:
        print("Please enter a valid number.")

print("Your score is:", score)

Summary Table

Purpose	Code Example	Notes
Basic text input	name = input()	Always string
Number input	num = int(input()) or float(input())	Convert types
Multiple inputs	a, b = input().split()	Use .split()
List for ML	[float(x) for x in input().split(',')]	For feature vector
Error handling	try-except	Catch invalid inputs

Keywords

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