In the last two weeks, we have used and created many functions.

Here we are going to cover new types of functions. These functions are capable of taking in information, and give back information. We will be introducing some new functions – functions that will allow us to work with variables and operators in interesting ways.

Passing Parameters

Remember the print and range functions? We introduced them as “special” functions, capable of taking in information and using the information to accomplish something. The print function allowed us to write messages using the string we put inside the parentheses. The range function allowed us to specify the number of times we wanted to execute the code inside some for loop.

We sometimes pass (or give) parameters, to functions.

Definition Parameter - An object given to a function for it to use when executing the code inside of it.

We pass parameters to functions in order to specify what we want them to do. All we have to do in order to pass a parameter to a function is put a value in between its parentheses when we call the function. In this week’s exercises, we are going to be introducing a new function called random_int. We need to pass two parameters called min_number and max_number. These two parameters specify what we want from the function. Here, we establish that we want a random number between min_number and max_number.

# Gives us one of the following numbers: 5, 6, 7, or 8. 
x = random_int(5, 8)

# Gives us a number between 10 and 100.
y = random_int(10, 100)

Parameter passing allow us to write functions that adapt to our needs. Above, we see that we store the values of the random_int function calls in variables x and y. In computer science, we say that the random_int function returns a value to us for us to use.

Returning Values

Do you remember the functions has_fruit, is_front_blocked, and front_is_blocked? We taught in class that these functions could ask PyBot questions. We said that these functions could give us True or False responses to questions. In actuality, these functions returned objects in order to communicate results.

Definition Return - To create an object and give it back to the user. The object can be any variable class (Integers, Floats, Strings, Booleans, etc.). We can store this object in a variable.

Some functions are capable of creating the objects that we need. These functions sometimes take parameters, sometimes they don’t. Examples of each type include the capitalize_string function, random_bool function, and the input_int, input_float, and input_string functions.

The capitalize_string function is a function that requires a single string object (parameter). If we call this function, it will create a new string object that is based on the parameter we gave the function. Here is an example of capitalize_string in action:

message = "cs106r"
capitalized_message = capitalize_string(message)
print(message)             # prints "cs106r"
print(capitalized_message) # prints "cs106r"

The object created by capitalize_string is completely separate from the message variable. Because it is a new object, we have label the object with a variable. That is why we have the variable capitalized_message.

In summary,capitalize_string executes code that creates a new object. We catch and label this new object with the variable capitalized_message. We can then print out both message and capitalized_message, because they are two separate objects.

The random_bool function works just like the random_int function. It will return to the programmer True or False randomly. However, it does not take any parameters. It doesn’t have to, because there are only two values to choose from, True and False! The returned value takes the place of the function call when the program is executed. Let’s take a look at an example:

Example Here we are going to create a program that randomly selects a person. Here is what our code looks like right now:

is_selected = random_bool()
print("Welcome to the selection process!")
print("Loading your selection result...")
print(is_selected)

The random_bool function will return to us either True or False, and that value will be stored in the variable is_selected. Here, the program will simply print True if you have been selected and False if you haven’t been selected. What if we wanted to make our program easier to understand? We could use the boolean value returned by the random_bool function in an if statement, just like we did with PyBot! Here’s what that code would look like:

is_selected = random_bool()
print("Welcome to the selection process!")
print("Loading your selection result...")
if is_selected:
    print("Congratulations! You have been selected!")
else:
    print("Sorry, you haven't been selected. Better luck next time.")

Now, what if we wanted to randomly select from 10 people? We could use a for loop. However, we have to be careful. Returning functions execute again every time they are called. This means that in the case of random_bool, the function will output a different randomly selected bool value every time it is called. This is what we want! It is important that we put random_bool inside the for loop. Otherwise, we will only get one value for all 10 people. Here is what the correct code looks like:

print("Welcome to the selection process!")
for i in range(10):
    is_selected = random_bool()
    print("Loading your selection result...")
    if is_selected:
        print("Congratulations! You have been selected!")
    else:
        print("Sorry, you haven't been selected. Better luck next time.")  

Finally, to put all these ideas together, we have the input_string, input_int, and input_float functions. These functions both take parameters and return values. All of the functions do basically the same thing – they take one message parameter (a string), which acts as a prompt to the user. When the program is run, the value given by the user is returned. The value class depends on the function used. Here is an example of each one, starting with input_string:

response = input_string("Do you like Beyoncé?")
if response == "Yes":
    print("Great, me too!")
else:
    print("You should listen to her album, titled `"4`".")

Here is an example with input_int:

num_exponent = input_int("Enter the value you want to multiply with itself:")
print("Here is your result:")
print(num_exponent * num_exponent)

Finally, here is an example with input_float:

temp_celsius = input_float("Enter the temperature you want to convert to Fahrenheit:")
temp_fahrenheit = temp_celsius * 1.8 + 32
print("Here is your result:")
print(temp_fahrenheit)

The ability to pass parameters and return values make functions much more powerful than they ever were before. Get some practice using the functions described above; next week, we will be defining functions capable of doing the same thing.