Foundations

Functions

Reusable Code

Your calculator app needs to convert temperatures in three different places. Instead of copying the formula three times, you create a function to_celsius(f) and call it wherever needed. Fix a bug once, it's fixed everywhere.

Sum of list function

Wrap the sum logic in a reusable function.

sum.py 
def sum(arr):
    total = 0
    for i in range(len(arr)):
        total = total + arr[i]

    return total

nums1 = [10, 20, 30]
nums2 = [1, 2, 3, 4, 5]

print("nums1=" + str(nums1))
result1 = sum(nums1)

print("nums2=" + str(nums2))
result2 = sum(nums2)

The function takes a list, returns the sum.

def `def name(parameters):` defines a function in Python.
parameter Input values passed to a function when calling it.
return The value a function sends back to the caller.

Temperature conversion

Convert between Fahrenheit and Celsius. Two functions, opposite directions.

temperature.py 
def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

def fahrenheit_to_celsius(f):
    return (f - 32) * 5 / 9

def celsius_to_fahrenheit(c):
    return c * 9 / 5 + 32

f1 = 
c1 = fahrenheit_to_celsius(f1)

c2 = 
f2 = celsius_to_fahrenheit(c2)

Functions can take one value and return another. Each function does one specific job.

Fibonacci function

Get the first n Fibonacci numbers.

fibonacci.py 
def fibonacci(n):
    fib = [0] * n
    fib[0] = 0
    fib[1] = 1
    
    for i in range(2, n):
        fib[i] = fib[i - 1] + fib[i - 2]
    
    return fib

n = 
result = fibonacci(n)
print("result=" + str(result))

def fibonacci(n):
    fib = [0] * n
    fib[0] = 0
    fib[1] = 1
    
    for i in range(2, n):
        fib[i] = fib[i - 1] + fib[i - 2]
    
    return fib

n = 
result = fibonacci(n)
print("result=" + str(result))

def fibonacci(n):
    fib = [0] * n
    fib[0] = 0
    fib[1] = 1
    
    for i in range(2, n):
        fib[i] = fib[i - 1] + fib[i - 2]
    
    return fib

n = 
result = fibonacci(n)
print("result=" + str(result))

The function handles all the list setup and calculation internally. The caller just says "give me 10 Fibonacci numbers."

Find maximum function

Wrap the "find max" logic in a reusable function.

max.py 
def max(arr):
    result = arr[0]
    for i in range(1, len(arr)):
        if arr[i] > result:
            result = arr[i]
    return result

nums = [23, 45, 12, 67, 34, 89, 41]

print("nums=" + str(nums))
m = max(nums)

Now any code can find the maximum of a list with a single call.

Multiple function calls

Use functions together to solve a problem.

combined.py 
def sum(arr):
    total = 0
    for i in range(len(arr)):
        total = total + arr[i]
    return total

def average(arr):
    return sum(arr) / len(arr)

scores = [85, 92, 78, 95, 88]

print("scores=" + str(scores))
total = sum(scores)
avg = average(scores)

Functions can call other functions. Build complex logic from simple pieces - this is called composition.