Free Recall

• Get out a sheet of paper or open a text editor
• For 2 minutes write down whatever comes to mind about the last class
  • This could be topics you learned
  • Questions you had
  • Connections you made

Start Reading

• Let’s start by reading chapter 10
• Take about 10-15 minutes to do this

Defining Functions

• Up until now you have only been using functions
• Functions such as abs, len, and range
• However, now I will show you that you can define your own functions
• You can do this using the def keyword

Defining a Function

• Let’s say you want to define a function which sums the numbers from 1 to n and gives you back that sum
• How would you do this if this was an exercise?
• Probably something like this:

n = int(input())
result = 0
for x in range(1, n + 1):
    result += x
print(result)

Visualize

Exercise Version: Sum to N

n = int(input())
result = 0
for x in range(1, n + 1):
    result += x
print(result)

Visualize

• Here you get the input from the outside, from the user of the program
• You then take that input and do the calculation
• Finally, you print the result since this is how you return the result to the user
• Defining functions give you a way to take input from the inside, from the writer of the program

Function Version: Sum to N

def sumToN(n):
    result = 0
    for x in range(1, n + 1):
        result += x
    return result

• Here we take the input, n, from another piece of code
• We use this input to perform the calculation
• Then we return the result back
• How do we actually use this?

Using sumToN

def sumToN(n): # Defining our function
    result = 0
    for x in range(1, n + 1):
        result += x
    return result

print(sumToN(5)) # Calling our function
print(sumToN(sumToN(3)))

Visualize

• The definition of a function does nothing other than define the function
• In order to use it we must call it
• After we call it, it performs its calculations and then returns a value to us
• We can do whatever we want with this value, such as print it

Defining Functions: Two Parts

• The first line a function use the def keyword to declare a function name and its arguments
• The following syntax is required:
  • def <<function name>>(<<argument list>>):
• If a function needs to return a value to the caller of the function it must use the return keyword
• The following syntax is required:
  • return <<value>>

Common Mistakes: Forgetting Return

• It is very easy to forget the return keyword when you meant to use it
• What happens if you do this?
• Let’s try it and see:

def add1(n):
    n + 1 # Forgot return!

print(add1(2))

Visualize

Common Mistakes: Forgetting Return

• Let’s fix our mistake and try it again:

def add1(n):
    return n + 1

print(add1(2))

Visualize

Common Mistakes: Indentation

• The body of a function must be indented past the definition of the function
• This is the same rule that ifs, loops, and many other statements follow in Python
• For example:

def add1(n):
return n + 1 # Needs to be indented

print(add1(2))

Visualize

Common Mistakes: Indentation

• Wrong indentation can be more subtle though
• Consider the following code:

def shout():
    print("Hello!!!???")
print("Are you there!!??") # Should be indented

shout()

Visualize

What will be printed?

Are you there!!??
Hello!!!???

Common Mistakes: Wrong Number of Arguments

• Another common mistake is a mismatch in the number of arguments
• For example:

def add1(n):
    return n + 1

print(add1())     # 0 instead of 1 argument
print(add1(1, 2)) # 2 instead of 1 argument

Visualize

Common Mistakes: Typo

• The last mistake may be the easiest to make, you simply misspell the name of the function!
• For example:

def add1(n):
    return n + 1

print(addOne(2))
print(ad1(2))

Visualize

sumNToM

• What if you also wanted to be able to specify what number to start summing at instead of just specifying the upper limit?
• Just add another parameter!

def sumNToM(n, m):
    result = 0
    for x in range(n, m + 1):
        result += x
    return result

print(sumNToM(1, 5))
print(sumNToM(10, 25))

Visualize