User Input

User Input#

Getting Data from the User#

Sometimes your program needs information from the person running it: a temperature, a number of iterations, or a filename. Python’s input() function lets you ask for this.

In numerical methods, interactive input is useful for:

  • Testing your code with different parameter values (mass, drag coefficient, step size)

  • Running “what-if” scenarios without editing the code

  • Building simple tools for lab partners who don’t want to modify Python files

The input() Function#

name = input("Enter your name: ")
print(f"Hello, {name}!")

When this runs, the program pauses and waits for you to type something. After you press Enter, whatever you typed is stored in the variable.

Enter your name: Sumit
Hello, Sumit!

Important: input() Always Returns a String#

Even if the user types a number, Python treats it as text:

value = input("Enter a number: ")
print(type(value))

Enter a number: 42
<class 'str'>

This matters because you can’t do math with strings:

value = input("Enter a temperature: ")
new_value = value + 10  # ERROR! Can't add str and int

Converting Input to Numbers#

Use int() or float() to convert the text to a number:

temp_str = input("Enter temperature (K): ")
temp = float(temp_str)  # Convert to a decimal number

# Now you can do math
temp_celsius = temp - 273.15
print(f"That's {temp_celsius:.2f} °C")

You can do this in one line:

temp = float(input("Enter temperature (K): "))

int() vs float()#

  • Use int() for whole numbers (iterations, counts, indices)

  • Use float() for decimal numbers (temperatures, concentrations, pressures)

n_iterations = int(input("How many iterations? "))
pressure = float(input("Enter pressure (kPa): "))

Handling Bad Input#

What if the user types “hello” when you asked for a number?

value = float(input("Enter a number: "))

Enter a number: hello
ValueError: could not convert string to float: 'hello'

Your program crashes! To handle this gracefully, use try/except:

user_input = input("Enter a number: ")

try:
    value = float(user_input)
    print(f"You entered: {value}")
except ValueError:
    print("That's not a valid number!")

Asking Again Until Valid Input#

while True:
    user_input = input("Enter a positive number: ")
    try:
        value = float(user_input)
        if value > 0:
            break  # Valid input, exit the loop
        else:
            print("Number must be positive!")
    except ValueError:
        print("That's not a valid number!")

print(f"Using value: {value}")

Practical Example: Simple Calculator#

print("Temperature Converter")
print("---------------------")

temp_str = input("Enter temperature: ")
unit = input("Is this (C)elsius or (K)elvin? ")

temp = float(temp_str)

if unit.upper() == "C":
    kelvin = temp + 273.15
    print(f"{temp} °C = {kelvin:.2f} K")
elif unit.upper() == "K":
    celsius = temp - 273.15
    print(f"{temp} K = {celsius:.2f} °C")
else:
    print("Unknown unit. Please enter C or K.")

The .upper() method converts the input to uppercase, so both “c” and “C” work.

Getting Multiple Values#

One at a time:#

x1 = float(input("Enter x1: "))
x2 = float(input("Enter x2: "))

On one line (space-separated):#

line = input("Enter two numbers (space-separated): ")
parts = line.split()  # Split by whitespace
x1 = float(parts[0])
x2 = float(parts[1])

Or more compactly:

x1, x2 = map(float, input("Enter two numbers: ").split())

This uses split() to break the string into parts and map() to convert each part to a float.

Quick Reference#

Task

Code

Get text input

name = input("Prompt: ")

Get integer

n = int(input("Prompt: "))

Get decimal

x = float(input("Prompt: "))

Handle errors

try: ... except ValueError: ...

Convert to uppercase

text.upper()

Split by spaces

text.split()

When to Use input()#

input() is great for:

  • Quick scripts you run from the terminal

  • Interactive tools

  • Testing your functions with different values

For more complex programs, you’ll often use:

  • Command-line arguments

  • Reading from files

  • Configuration files

But input() is a good starting point!

Next Steps#

Continue to Common Functions to learn about built-in functions you’ll use frequently.

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