functions and structured...

UNIT 6Functions and Structured Programming

LESSON 1What is a Function?

What is Structured Programming?

I can…..

• Divide a large program into small problems.

• Write a Python program as a function

Planning a Wedding

• What are some of the steps we will take when planning a

wedding?

Planning a Wedding

• Make Guest List

• Take Engagement Photos and Send out Save the Dates

• Decide on Wedding Party

• Find Wedding Gown, Bridesmaid Dresses, and Tuxes

• Pre-Marital Meeting with Officiant (Pastor, Priest, etc)

• Food Tasting

• Purchase Decorations

• Plan Rehearsal Dinner

• Do Final Preps• Decorate the Hall

• Prepare Food

• Dance and Enjoy!

What is Structured Programming?

• Organizing programs by breaking them up into smaller, easier to manage programs (called functions)

• Allows you to reuse code that is repeated!

• “Divide and Conquer” Method

• Top – Down Design• Break a large programming task (Top) into smaller and smaller

subtasks (Bottom)

**ALSO CALLED MODULAR PROGRAMMING**

Why is Structured Programming Needed?

• Past programming:

• Programming in older languages vs. modern style of programming

• Older: “Spaghetti Code”

• Hopelessly intertwined statements caused by numerous “jumps” between sections of code

• Nearly impossible to follow – NO FLOW

• Modern:

• Use functions – 1 main function where main code runs logically in order

• Will run other functions but always return to main.

Steps to Writing a Structured Program

1. Start with an Outline of the main program

• Write out pseudocode to describe major tasks to be

performed

• Avoid thinking about the details of each major task right now

2. Write the main - mostly calls to functions

• Translate each English phrase into one or two function calls

• Use documentations to describe the actions and make programs

readable

Steps to Writing a Structured Program

3. Write each function

• Define the function (name and parameters)

• Type the code for that function (indented)

4. Test and Debug

• Use a complete set of test data to be sure it works

• Make changes or refinements that are necessary or desireable

Example of the planning process

Functions Already Used

Turtle graphics….

• forward()

• color()

• left()

• goto(x, y)

Math functions…

• sqrt(n)

• pi

• pow(b, n)

• floor(n)

General Functions…

• print()

• input()

• eval()

String Functions…

• len(x)

• upper(x)

• chr(x)

• ord(x)

• split(x)

Random Functions…

• random()

• randrange(x,y,z)

• randrange(x, y)

**We’ve actually

used MORE than

this**

What is a function?

• A sub-program or block of code that accomplishes a

specific task

• Often returns a value, text, or object

• A “Mini Program”

• Also called a method or subroutine

New Approach to the Main Function

• We can (and will starting today) write our “main function”

like a function!

Functions Syntax:

def <name of function> ():

<statements>

Let’s Write an Example Together

• Let’s write a program that prints out the lyrics to the song

“Happy Birthday”

• Main program must now be written as a function so we

can add functions in later!

Program #1

# Happy Birthday Program

# create main function

def main ():

# get name from user

name = input(“What is your name?: “)

# print happy birthday to user

print(“Happy Birthday to you”)

print(“Happy Birthday to you”)

print(“Happy Birthday dear” , name)

print(“Happy Birthday to you”)

How do we make our program run?

The main function is an executable function – but we need

to tell it to execute the main!

def main () :

<statements>

…..

main() This line of code at the end of a python program will “run the main function”

Let’s Write our First Function

• In the happy birthday program – do you notice any code

repeating???

• This is a perfect time to use a function!

Let’s Look Back!

# Happy Birthday Program

# create main function

def main ():

# get name from user

name = input(“What is your name?: “)

# print happy birthday to user

print(“Happy Birthday to you”) repeated 3 times

print(“Happy Birthday to you”) repeated 3 times

print(“Happy Birthday dear” , name)

print(“Happy Birthday to you”) repeated 3 times

main()

Let’s Write a Function

**Add this code beneath the “main()” statements**

# function prints 1 line of Happy Birthday

def happy ():

print(“Happy Birthday to you”)

Let’s Update our Main!

# Happy Birthday Program

# create main function

def main ():

# get name from user

name = input(“What is your name?: “)

# print happy birthday to user

happy() calling and running the happy() function

happy() calling and running the happy() function

print(“Happy Birthday dear” , name)

happy() calling and running the happy() function

main()

LESSON 2Passing parameters to a function

I can…..

• Write a function in Python.

• Write a program that uses functions

Benefits of Structured Programs1. Easier to read

• Each function will have documentations explaining its purpose

2. Easier to write

• Complicated programs broken down into smaller, easier to manage

functions

3. Easier to use, change, adapt, modify, debug

• Only need to change in one location (not every time appears in

program)

• Easily determine where an error is occuring

4. Easier to reuse

• Can be copied into another program

• Can be called in the same program multiple times

5. Other, More powerful programming languages

require this kind of structure.

Writing Python Programs with Functions!

def main():

<statements>

function1()

function2()

def function1():

<statements>

def function2():

<statements>

main()

Question to Ponder

1. If each function is its own “mini program”, how is

information shared between different functions?

2. How do I get data that is held within one subroutine into

another one for processing??

Parameter

• Variable values passed into the function

• The function needs these values in order to “do its job”

• May include as many parameters as the function needs

• If no parameters are needed – the parentheses are empty

• Example: happy() function in lesson 1’s program

Parameter Passing

A function definition with parameters:

def <name> (<formal parameters>) :

<statements>

Example:

def hello (person) :

print(“hello” , person)

Parameter Passing

A function call with parameters:

<name>(<actual parameters>)

Parameters

• The parameter provides a method of transferring data

from the main function to be the function being called

• Ex:

ord(“A”) takes in a CHARACTER

chr(45) takes in an INTEGER

lower(message) takes in a STRING

Let’s update our Happy Birthday Program

Version 1:

• Create a function called “sing” that will take in a

parameter “name” and sing happy birthday to the

person

• The Sing function should use the “happy()”

function we create yesterday

• Program should sing “Happy Birthday” to Joe,

Callie, and Alex

Let’s update our Happy Birthday Program

# Happy Birthday Program

# create main function

def main ():

# get name from user

name = input(“What is your name?: “)

# print happy birthday to user

happy()

happy()

print(“Happy Birthday dear” , name)

happy()

main()

Sing Function

# Sings happy birthday to a person with a given name

def sing (personname):

happy()

happy()

print(“Happy Birthday dear, personname)

happy()

How do we call a function with parameters?

• The same way we call any other function – but we need to

ensure we “send it” that parameter requirements

Example:

sing(personname)

Let’s update our Happy Birthday Program

# Happy Birthday Program

# create main function

def main ():

# get name from user

name = input(“What is your name?: “)

#sings happy birthday to user

sing(name) # sends in name of user

main()

Let’s Sing to More People!

# Happy Birthday Program

# create main function

def main ():

# get name from user

name = input(“What is your name?: “)

#sings happy birthday to user and others!

sing(name) # sends in name of user

sing(“Joe”)

sing(“Callie”)

sing(“Alex”)

main()

LESSON 3Returning a Value

I can…..

• Write a function in Python that returns a value.

• Write a program that uses functions

Review of Terminology:

• What is a function?

• What is “structured programming”?

• What is “structured programming” also called?

• What are the benefits of structured programming?

How do we get a value from a function?

• Most functions we have used so far in this class have

provided us with a value.

• Examples:

len() returns the number of characters in a string

lower() returns a copy of a string in all lower case letters

randrange(4,9) returns a random integer between 4 and 8

ord() returns a character at the ASCII number

chr() returns an integer for the ASCII character

Returning a Value

• Called Function “returns” a value to the program calling

the function.

• If a value is returned, it must either be saved inside a

variable in the program calling the function or used

immediately when called

• If the returned value isn’t saved or used, it is LOST

• Function are NOT required to return a value

Returning a Value

Returning a Value

def <name> (<formal parameters>) :

<statements>

return <variable name>

Returning a Value

Example in Code

def addition(num1, num2):

sum = num1 + num2

return sum

Write a Program that

• Takes in value from a user a converts it from pounds to

kilograms.

Program: Convert Pounds to Kilograms

# main program

def main():

#welcome statement

print(“Converts pounds to kilograms \n \n”)

# gets weight from user

lbs = eval(input(“Enter your weight in lbs: “)

# converts pounds to kilograms, returns kgs, and

# SAVES the value inside a new variable named kgs

kgs = compute(lbs)

Program continued

# prints kilograms to screen

print(Weight in kgs: “ ,kgs)

# function that converts pounds to kilograms

def compute(lbs):

kilograms = lbs/2.2

return kilograms

#runs main program

main()

2nd Way to Use returned values

# main program

def main():

#welcome statement

print(“Converts pounds to kilograms \n \n”)

# gets weight from user

lbs = eval(input(“Enter your weight in lbs: “)

#prints kgs to screen

print(Weight in kgs: “ , compute(lbs) )

LESSON 4Passing more than one Parameter

I can…..

• Write a function in Python that takes in multiple

parameters.

Review of Terminology:

• What is a function?

• What is structured programming?

• What is a parameter?

• What is a returned value?

What happens when we send in a parameter?

• Main function (or the function calling another function to

run) sends in the VALUE

• This means if a variable name is placed in the parameter

list, the VALUE is the only thing sent inTHE VARIABLE VALUE WILL NOT BE CHANGED IN THE MAIN PROGRAM

AS THE FUNCTION FUNS

• The variable value sent in will only be USED by the

function to complete its task

Example

def main():

name = input(“What is your name: “)

double(name)

print(name)

def double(name) :

name = name*2

print(name)

main()

Functions Using Parameters

name name

Each function stores its local variable information in different

places in memory

In this program – there are actually 2 variables called “name”

• one saved in the main program’s block of memory

• another stored in the double function’s memory

Local Variable

• All variables are LOCAL to the function they are defined

in.

• **Bubbler Explanation Example**

• To access a variable outside a function, it must be passed

to the function

Passing Parameters

Function Definition Syntax:

def <name>(<formal parameter1>, … , <formal parameterN>)

<body>

Function Call Syntax:

<name>(<actual parameter1>, … <actual parameterN>)

Passing Parameters by VALUE

• A copy of the VALUE of a variable are passed to the

function.

• The function than creates it’s own variable with that value

– this variable is LOCAL to the function

When Python Comes to a Function Call

1. The calling function suspends execution at the point of

the call

2. The formal parameters of the function get assigned the

values supplied by the actual parameters in the call.

3. The body of the function is executed.

4. Called function returns a value (if necessary) and the

program returns to the point just after where the

function was called.

Example Program #1

def main() :

print(“The value of x in main before the function call” , x)

changeX(x)

print(“The value of x in main after the function call” , x)

def changeX(x)

print(“The value of x at beginning of changeX call” , x)

x = x + 25

print(“The value of x after changeX call” , x)

main()

Example Program #2

def() main:

length = eval(input(“Enter length: “))

width = eval(input(“Enter width: “))

area = computeArea(length, width)

print(“area = “ , area)

def computeArea(l, w) :

a = l*w

return a

main()

Write a Program Together

• Write a program that ultimately asks the user to enter in

the base and height of a triangle and then calculates the

area of the triangle.

• Main

• Welcome message (function or not)

• Ask the user for the base and height (function or not)

• Area Function – need the base and height sent into it – return the

area of the triangle

• Print out the output (function or not)

LESSON 5Combining Structure Programming with

Control Structures (if and loops)

I can…..

• Use control structures (loops and conditionals) with

function calls.

Example Program 1

• Roll two dice until get a roll of 2 (snake eyes – each die is

a 1 = total of “2”)

• Track how many rolls it takes to roll snake eyes

• Print the number of rolls to the screen

Rolls (# of times I have rolled the die)

Rolltotal (total with the two die values added together)

Keep rolling until I roll a 2 (while loop)

Function – roll – roll two die (randomly 1-6), return the

“rolltotal”

Example Program 1 - Code

from random import randrange

def main():

roll = 0

rolltotal = 0

while rolltotal != 2 :

rolltotal = rolldice()

roll = roll + 1

print(“It took” , roll, “rolls to get snake eyes”)

Example Program 1 – Code (pg 2)

def rolldice():

roll1 = randrange(1, 7)

roll2 = randrange(1, 7)

rolltotal = roll1 + roll2

return rolltotal

main()

Example Program 2

• Takes in a number

• States whether the number is even or odd (using a

function)

Takes in a number

Example Program 2 - Code

def main():

number = eval(input(“Please enter in a number: “))

evenodd = evenOrOdd(number)

print(“Your number is” , evenodd)

def evenOrOdd(number):

if number%2 == 0:

return “even”

else :

return “odd”

main()

EXTRA

Program 5

• How many items did you buy

• Cycles through item prices and finds sum

• Asks for tax

• Calculates total cost

• Asks user if they want to play again

Widgets and Dodads project