4. Input and Changing Types

  • We know how to get information out of the program

    • print

  • And there is a similarly simple way to get information into the program

    • input

1my_inputted_value = input("Enter a value: ")

  • If you run the above code, you will see Enter a value:

  • Python will wait for the user to enter the value they want

  • Once the user enters the value, it will be stored in the my_inputted_value variable

  • The string between the parentheses is what will be displayed to the user, but it is entirely optional

    • my_inputted_value = input()

    • If you do leave it blank, nothing will be displayed to the user (this is what we want when using Kattis in lab)

Activity

  1. Read in some value from the user.

  2. Print out the value you entered.

  3. What is the type of the value? How can I test this?

    • Hint: type

4.1. The Type Of The Entered Value

  • Whenever data is input like this, Python will always treat the data as a string

    • If you enter Hello world, the value of my_inputted_value would be the string "Hello world"

    • If you enter 1, the value of my_inputted_value would be the string "1"

  • This may be fine in some cases, but if we want to do some math with the entered values, we do not have numbers

1first = input("Enter a number: ")           # Enter 1
2second = input("Enter another number: ")    # Enter 2
3print(first + second)                       # Results in 12 (not 3)

  • If you enter 1 and 2, you might expect 3, but you will get 12

    • Since both values are strings, + concatenates the strings

    • Remember, the + operator is context sensitive

  • Fortunately, there is a simple way to try to change the type of the value

  • For example, if we want to enter the integer 1

1my_value_as_string = input("Enter a value: ")
2my_value_as_int = int(my_value_as_string)

  • In the above example, on the first line of code, if we enter 1 as the input, the value of my_value_as_string will be the string "1"

  • On the next line of code we are then taking the value of my_value_as_string ("1") and converting it to an integer with int(my_value_as_string)

  • After everything, the value of my_value_as_int will be the integer 1

  • Writing my_value_as_int = int(input("Enter a value: ")) would achieve the same thing, but on one line of code

    • Removing the middleman (my_value_as_string)

    • However, this does have one line of code doing multiple things

    • There is something nice about having one line of code do one thing

4.2. Changing Types

  • It is possible to use the same idea to convert the types of values, not just when reading input

  • For example, if I had the integer 1 stored in some_integer, but wanted it to be a string, I could write str(some_integer)

  • int is used to convert something to an integer

  • str is used to convert something to a string

  • float is used to convert something to a float

  • However, this assumes that the value whose type is being changed can actually be changed to that type

  • Python is happy to change the type of the integer 1 to a float or a string

  • But, if I try to change the type of the string "Hello world" to an integer, we run into a problem

    • int("Hello world") will cause an error

    • Python will say ValueError: invalid literal for int() with base 10: 'Hello world'

Activity

Write a program to:

  1. Ask the user to input their weight in pounds and save the value to a variable

  2. Convert the inputted value to a float and save the result to a variable

  3. Calculate the mass of the individual in kilograms based on the inputted weight and save the result to a variable

  4. Print out the mass in kilograms

Hint: \(1 lbs = 0.453592 kg\)

4.3. For Next Topic