User Input¶
To begin this chapter, Python’s input() function is discussed.
Python can be used to ask users for input. The input entered by a user can be saved to a variable and used in subsequent parts of the program. The syntax of Python’s input() function is below:
var = input('message')
Where var is the variable that stores the user’s input and 'message' is the message the user sees at the prompt. A string enclosed in quotes, like 'message', needs to be passed as an input argument to the input() function. Let’s ask a user for their age:
age = input('how old are you? ')
Since the user’s input is assigned to a variable, further operations can be run on it. Now, let’s print the user’s age back to them. This can be accomplished with an f-string. Note the f' ' inserted before the string. A set of curly braces { } surrounds the variable’s name and the variable’s value is printed back to the user.
age = input('how old are you? ')
print(f'you are {age} years old')
Let’s try another example. We will we ask the user for the base and height of a triangle and print out the area of the triangle.
But, there is a problem with the approach below. The code block does not run because a common error is present.
b = input('base of triangle: ')
h = input('height of triangle: ')
A = (1/2)*b*h
print(f'The area of the triangle is: {A}')
The previous section of code returns an error because of the data type of the variables b and h. We can investigate b and h’s data type with Python’s type() function.
b = input('base of triangle: ')
h = input('height of triangle: ')
print(f'b and h are of type: {type(b)}, {type(h)}')
Notice both b and h are strings, even though the numbers 5 and 2 were entered as input. The output of the input() function is always a string, even if the user enters a number.
To complete the area calculation, b and h first need to be converted to floats using Python’s float() function, then the mathematical operation will run without error:
b = input('base of triangle: ')
h = input('height of triangle: ')
A = (1/2)*float(b)*float(h)
print(f'The area of the triangle is: {A}')
Now that you are familiar with Python’s input() function, let’s utilize a user’s input to decide which lines of code will run. The concept of an selection statement is introduced the next section.