CPSC120A - Day 10 Notes

Robot Food Service

Reading Questions

None

All Python functions return something, whether you specify it or not

If you don’t return something, the function returns the special value None

def square(number):
    number_squared = number * number

print(square(number)) # this prints 'None'

This is useful to know because sometimes your program will print None, when it isn’t supposed to
Also, there are questions on the quiz where the options are None and Nothing
None, means ‘None’ is actually printed and Nothing means no characters are printed

Quiz

Fruitful Functions

Functions are named code
Parameters are the inputs to the named code

Return values are the output of the named code

def square(x):
    x_squared = x * x
    return x_squared;

Note, return is not a function, so there should not be parentheses around the value that is returned
Call the function just like other functions we have learned
```
square(3)
```
What is different about this compared to functions we learned in the last class with the turtle and graphics module, is that this function has no side effects, so calling it like this does nothing
Need to do something with the returned value
```
print(square(3))
```
Note, this is function composition, we’ve done this bunches of times already, it uses the output of one function as the input to another function
Could also do it with a variable
```
result = square(3)
print(result)
```

Fruitful Function Gotchas

Forgetting the return:

def square(x):
    y = x * x

print(square(3))

All functions return something, if you don’t explicitly specify the return value, it will return None.

Dead code:

def square(x):
    return x
    y = x * x

print(square(3))

The function ends execution when the return statement is executed, all code after the return is never executed, it is dead code

Order matters:

print(square(3))

def square(x):
    return x
    y = x * x

Just like with variables the order matters
Can’t call a function before it is defined
Typically we put all function definitions at the top of the file, after the import statements

Printing in the function:

def square(x):
    y = x * x
    print(y)

var value = 3
var value_squared = square(value)
print(value, 'squared is', squareed_value)

Printing in the function makes the function less flexible

Test Code

Functions are a great tool for organizing code
They allow you to break large programs up into small pieces
They also make it easy to test the small pieces individually instead of the program as a whole

We test a function by calling it with multiple inputs and printing the result next to the expeced value

print('Input:', 3, Actual:', square(3), 'Expected:', 9)
print('Input:', 2, Actual:', square(2), 'Expected:', 4)
print('Input:', 1, Actual:', square(1), 'Expected:', 1)
print('Input:', 0, Actual:', square(0), 'Expected:', 0)
print('Input:', -1, Actual:', square(-1), 'Expected:', 1)
print('Input:', -2, Actual:', square(-2), 'Expected:', 4)

Fruitful Vs Fruitless

You could write a function using return statements or not

def fruitless_square(number):
    number_squared = number * number
    print(number_squared)

def fruitful_square(number):
    number_squared = number * number
    return number_squared

The way the functions are called is different

fruitless_square(3)
print(fruitful_square(3))

But more importantly, the fruitful function is more useful
The fruitless function always prints
The fruitful function could be used to print or it could be used as the imput to another function
In general prefer fruitful functions
Write functions without side effects as much as possible
It makes them more useful and less error prone

CPSC120A
Fundamentals of Computer Science

Day 10 Notes