CPSC120B - Day 12 Notes

Cool Computer Science Thing of the Day

Reading Questions

Quiz

quiz 12

Emacs Tip of the Day

M-/ auto complete

Fruitful Functions

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

Return values are the output of the nameed 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 n* 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 done this bunches of times already, it uses the output of one function as the input to anther 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)

Lab

lab12