Writing interactive programs

Table of Contents

What Makes a Program “Interactive”?

A program is interactive when it reacts to the user:

It asks questions.
It waits for input.
It decides what to do based on that input.
It gives feedback and often asks again.

In this chapter, you’ll combine input and output with simple logic to create small, conversational programs.

You already know how to:

Get input with input()
Convert input to numbers
Print results with print()

Now you’ll learn how to structure these pieces into a smooth interaction.

Basic Structure of an Interactive Program

Most simple interactive programs follow this pattern:

Greet the user.
Ask for some information.
Process that information.
Show a result.
Optionally repeat or ask something else.

In pseudocode, that looks like:

Say hello
Ask a question
Wait for the answer
Do something with the answer
Show a message

In Python:

print("Welcome to the program!")
name = input("What is your name? ")
age_text = input("How old are you? ")
age = int(age_text)  # process/convert
print("Nice to meet you,", name)
print("You are", age, "years old.")

The important idea: input → processing → output.

Asking Multiple Questions

Interactive programs rarely ask just one question. You can ask one after another and reuse previous answers.

print("Welcome to the trip planner!")
name = input("What is your name? ")
destination = input("Where would you like to travel? ")
days_text = input("How many days will you stay there? ")
days = int(days_text)
print()
print("Trip summary:")
print("Traveler:", name)
print("Destination:", destination)
print("Duration:", days, "days")

Notes:

Use blank print() to add a blank line for readability.
Store each answer in a variable with a clear name.
Convert to numbers only when needed.

Making Decisions Based on Input

Interactivity becomes more interesting when the program reacts differently to different answers. Here’s a simple example of using if with input:

color = input("What is your favorite color? ")
if color == "blue":
    print("Blue is calming!")
elif color == "red":
    print("Red is energetic!")
else:
    print("Nice choice! I like", color, "too.")

Here, the user’s answer changes the program’s response. You’ll learn if, elif and else in more detail in the conditions chapter; here the key idea is:

Ask a question
Check the answer
Respond differently depending on what they typed

Building a Simple Question–Answer Flow

You can chain several questions and decisions to create a mini “conversation”.

print("Welcome to the coffee machine!")
name = input("What is your name? ")
print("Hello,", name)
drink = input("Do you want coffee or tea? ")
if drink == "coffee":
    size = input("Small, medium, or large? ")
    print("Preparing a", size, "coffee for you...")
elif drink == "tea":
    kind = input("Green or black tea? ")
    print("Brewing some", kind, "tea for you...")
else:
    print("Sorry, I only have coffee or tea.")

This example shows:

Step-by-step interaction: each question depends on earlier answers.
Branching: different paths depending on drink.

Handling Simple Input Errors

Real users don’t always type what you expect. For interactive programs, it’s good practice to:

Warn the user when something is unexpected.
Ask again, or choose a default behavior.

Here is a basic approach without loops (you’ll do this better once you learn loops):

age_text = input("Enter your age: ")
if age_text.isdigit():
    age = int(age_text)
    print("In 10 years, you will be", age + 10)
else:
    print("That doesn't look like a number.")

str.isdigit() returns True if the string contains only digits like "123".

This is a very simple form of “defensive” interaction: check the input before using it.

Simple Menus

Many interactive programs present a menu of options. For now, you can implement a menu with input and conditions.

print("Calculator")
print("1) Add two numbers")
print("2) Multiply two numbers")
choice = input("Choose an option (1 or 2): ")
if choice == "1":
    a = float(input("First number: "))
    b = float(input("Second number: "))
    print("Result:", a + b)
elif choice == "2":
    a = float(input("First number: "))
    b = float(input("Second number: "))
    print("Result:", a * b)
else:
    print("Invalid choice.")

Key ideas:

Show clear numbered options.
Ask for the choice.
Use if/elif/else to run the correct section of code.
Use the same pattern for simple “text menus” in many programs.

Keeping the Conversation Clear

Good interactive programs are easy to understand. Some practical tips:

1. Use Clear Prompts

Bad:

x = input("?> ")

Better:

age_text = input("How old are you? ")

Prompts should tell the user exactly what to type.

2. Show Examples in the Prompt

This helps the user know the expected format:

color = input("Choose a color (red/green/blue): ")

3. Echo Important Information Back

Confirm what the user entered:

name = input("What is your name? ")
print("You entered:", name)

Useful when inputs are long or important.

4. Use Blank Lines to Group Messages

It’s easier to read:

print("Welcome to the registration form!")
print()
name = input("Name: ")
email = input("Email: ")
print()
print("Thank you for registering,", name)

Combining Input, Processing, and Output: Mini Examples

Here are a few complete interactive snippets you can type and run.

Age in Months Calculator

print("Age in months calculator")
age_years_text = input("How old are you (in years)? ")
if age_years_text.isdigit():
    age_years = int(age_years_text)
    age_months = age_years * 12
    print("You are about", age_months, "months old.")
else:
    print("Please enter your age as a number, like 20.")

Simple Greeting Customizer

print("Custom greeting")
name = input("What is your name? ")
mood = input("How are you today? ")
print()
print("Hello", name + "!")
print("I'm glad to hear you are", mood + ".")

Temperature Converter (Celsius to Fahrenheit)

Using the formula
$$
F = C \times \frac{9}{5} + 32
$$

print("Celsius to Fahrenheit converter")
celsius_text = input("Enter temperature in Celsius: ")
celsius = float(celsius_text)
fahrenheit = celsius * 9 / 5 + 32
print("That is", fahrenheit, "degrees Fahrenheit.")

Simple Pattern for Interactive Programs

You can use this general pattern when you start a new interactive script:

Intro message: what the program does.
Ask inputs: one or more input() calls.
Convert/process: turn text into numbers or make decisions.
Output result: print() answers, summaries, or next steps.

Template:

print("Short description of what the program does")
# 1. Ask for input
user_input1 = input("Question 1: ")
user_input2 = input("Question 2: ")
# 2. Convert/process
# (convert to numbers, do calculations, make decisions, etc.)
# 3. Output results
print("Some result using", user_input1, "and", user_input2)