13.1. Using Decisions with Strings

Learning Objectives:

• Give examples of programs that use conditions with strings.

• Use the input function to get input from a user

• Introduce using elif to handle 3 or more options.

• Introduce using str to convert a number to a string.

Here’s an example of conditional execution using if statements with strings. We can print different strings based on the value of a number. For example, if the user only orders 1 item we can print that out differently than if the user orders more than one item. Notice the use of str(numItems) in the code below. The function str turns a number into a string so that it can be appended to a string.

numItems = 1 if numItems == 1: message = "You ordered 1 item" if numItems > 1: message = "You ordered " + str(numItems) + " items" print(message)

Change the value of numItems to see how that changes the output.

What will be printed if numItems = -1?

• You ordered 1 item
• This line will only print when the numer of items is 1.
• Your ordered -1 items
• This line will print whenever the number of items is greater than 1.
• Nothing will be printed.
• If numItems is negative neither of the if's will be true so the variable message will not be created.
• You will get an error message that message is not defined.
• The variable message won't be created if the number of items is less than 1, so trying to print the value of message will cause an error.

What will be printed if you change the second if to an else and set numItems = -1?

• You ordered 1 item
• This line will still only print when the numer of items is 1.
• Your ordered -1 items
• This line will print whenever the number of items was not equal to 1 when you change the second if to an else.
• Nothing will be printed.
• If numItems is negative neither of the if's will be true so the variable message will not be created.
• You will get an error message that message is not defined.
• The variable message won't be created if the number of items is less than 1, so trying to print the value of message will cause an error.

What if you want different messages to be printed based on the user’s score in a game? The code below should print “You can do better!” if the score is less than 10, “Good job” if the score is between 10 and 19 inclusive, and “Amazing” if the score is 20 or more, but it needs to be fixed. First run it with different values to see what happens and then answer the multiple choice questions below.

score = 8 if score < 10: print("You can do better.") if score > 10: print("Good job!") if score > 20: print("Amazing")

What is printed when the score is 10?

• You can do better.
• This line will only print when score is less than 10.
• Good job!
• This line will only print whenever the score is more than 10.
• Amazing!
• This line will only print whenever the score is more then 20.
• Nothing is printed
• When score equals 10 none of the current if statements will be true, so nothing is printed.

What is the first thing printed when the score is 25?

• You can do better.
• This line will only print when score is less than 10.
• Good job!
• This line will print whenever the score is more than 10, so if the value is 25 it will print first. And then it will also print "Amazing!".
• Amazing!
• This line will print whenever the score is more than 20, but another line will print first.
• Nothing is printed
• The value 25 is more than 10.

Now go back and change the last active code (csp_sd_score) to work correctly. Remember that you can use and to join two logical expressions. This is especially useful if you want to test if a number is in a range of numbers like 10 to 19 inclusive. So change the example to print the first thing if less than 10, the second thing if it is between 10 and 19 and the third thing if it is 20 or more.

Note

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