Introduction to Programming with Python Session 5 Notes

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1 Introduction to Programming with Python Session 5 Notes Nick Cook, School of Computing Science, Newcastle University Contents 1. Modifications to the draw_shape function Positional and named parameters For loops A function that returns a result Square spirograph challenges Resources... 5 Appendix 1: myshape.py... 6 Appendix 2: a note on functions and abstraction... 7 Appendix 3: draw_shape using a for loop Modifications to the draw_shape function Start with: import turtle def draw_shape(sidestodraw): if sidestodraw > 2: while (sidestodraw > 0): turtle.forward(100) sidestodraw = sidestodraw 1 # main program turtle.color('green') draw_shape(3) # draw other shapes window = turtle.screen() window.exitonclick() Note, line length is fixed in the function body. Modification 1 parameterise with line length. def draw_shape(linelength, sidestodraw): if sidestodraw > 2: while (sidestodraw > 0): Newcastle University,

2 turtle.forward(linelength) sidestodraw = sidestodraw 1 # main program turtle.color('green') draw_shape(50, 3) window = turtle.screen() window.exitonclick() Modification 2 what is an appropriate limit on the value of linelength? Should be greater than 0 def draw_shape(linelength, sidestodraw): if linelength > 0 and sidestodraw > 2: while (sidestodraw > 0): turtle.forward(linelength) sidestodraw = sidestodraw 1 Test the result with linelength less than 1 and sidestodraw less than 3 2. Positional and named parameters To call a function use its name and provide the expected parameters in order, e.g.: 01 do_something() 02 do_something(myvalue) 03 do_something(myvalue1, myvalue2) Line 1 calls the version of do_something that takes no parameters. Line 2 calls the version of do_something that takes one parameter (param) and there is an implicit assignment of myvalue to param. Line 3 calls the version of do_something that takes two parameters (param1 and param2) and there is an implicit assignment of myvalue1 to param1 and myvalue2 to param2 (in the order they are passed to the function). This is called positional parameter passing the order of the parameters determines the assignment of the values that the function uses. Demonstrate using the draw_shape function: Positional parameter function call is as before: import turtle # function definition here # main program turtle.color('green') draw_shape(50, 3) win = turtle.screen() Newcastle University,

3 win.exitonclick() In Python, we can also explicitly name the values to be passed to a function the assignments are made explicit in the function call by using the actual parameter names as keywords: import turtle # function definition here # main program turtle.color('green') draw_shape(linelength = 100, sidestodraw = 4) win = turtle.screen() win.exitonclick() In this case, ordering does not matter, provided all parameters are named, e.g.: draw_shape(sidestodraw = 4, linelength = 100) 3. For loops Discuss difference between the following two while loops. # while loop from draw_shape: while sidestodraw > 0: turtle.forward(linelength) sidestodraw = sidestodraw - 1 # while loop from user_guesses: number = random.randint(1, 100) guess = 0 while guess!= number: guess = int(input('guess my number: ')) if guess == number: print('well done! My number is:', number) elif guess < number: print('sorry, my number is higher') else: print('sorry, my number is lower') We often know how many times we want to iterate through a loop (as in the draw shape example). We can use a for loop for this. Work through Section 3.2b on p.16 of Mark Clarkson's book. On whiteboard: general form of for loop for i in range([start,] stop [,step]): # do something Starting with a value for variable i of start, iterate through the loop (executing any code in the block on each iteration) until ireaches the value stop. At the end Newcastle University,

4 of each iteration, increment the value of i by step. The default value for start is 0. The default value for step is 1. Class exercises 1. In a program file, do 3.2c loop challenges on p.16 of Mark Clarkson's book. o For challenge 3 of 3.2c, write a function to calculate the times table of a positive number that is given as a parameter to the function. 2. Do 3.2f loop challenge on p.18 of Mark Clarkson's book, writing a function to calculate and print the factorial of a number given as a parameter to the function. Recap: in a loop the "loopcounter" is just a variable that is given each value in turn from the specified range. Demonstrate: for loopcounter in range(10): print(loopcounter) Range: 0 to 9 inclusive (10 iterations) Output: 0 # iteration 1: loopcounter = 0 1 # iteration 2: loopcounter = 1 2 # iteration 3: loopcounter = 2 3 # iteration 4: loopcounter = 3 4 # iteration 5: loopcounter = 4 5 # iteration 6: loopcounter = 5 6 # iteration 7: loopcounter = 6 7 # iteration 8: loopcounter = 7 8 # iteration 9: loopcounter = 8 9 # iteration 10: loopcounter = 9 Demonstrate solution to 3.2c challenge 3 (timestable.py) and challenge 3.2f (factorial.py). Question which invalid values should we check for? For times table any value < 1 is invalid For factorials any value < 0 is invalid 4. A function that returns a result It is often useful to return a value from a function. For example, the factorial function would be more useful if it returned the calculated factorial rather than just printing it. Then we could use the factorial in another calculation. Demonstrate modification of the factorial function to return the value, and using it in a calculation in the main program. Newcastle University,

5 5. Square spirograph challenges Class exercises 1. Rewrite the draw_shape function in myshape.py to use a for loop instead of a while loop (demonstrate my solution see Appendix 3). 2. Do square spirograph challenges. 6. Resources Other material from Newcastle University Introduction to Python CPD: Mark Clarkson's Introduction to Python resources including textbook, workbooks, example code and GCSE controlled assessment: resources.zip Other Python resources: Python Web site: Newcastle University,

6 Appendix 1: myshape.py """ myshape.py containing a function to draw an equilateral shape download from: programming/python/intro2python/myshapes.py Nick Cook, Newcastle University, 2013 """ import turtle # equilateral shape function def draw_shape(linelength, sidestodraw): if linelength > 0 and sidestodraw > 2: while sidestodraw >0: turtle.forward(linelength) sidestodraw = sidestodraw 1 Newcastle University,

7 Appendix 2: a note on functions and abstraction Functions allow us to organise code into reusable units of work. They also provide a higher level of abstraction. Instead of having to think about the individual instructions to draw a shape using a turtle in python, we can use a function draw_shape function to provide the abstraction of drawing a shape. Functions have a name and parameters, and can also have a return value (or result). The name and parameters are called the signature of the function. The function signature allows us to distinguish between functions that may have the same name the combination of name and parameters distinguishes one function from another. For example, here are three different functions with the same name (do_something) but with a different numbers of parameters: def do_something(): # function body does something def do_something(param): # function body does something with param # - takes a single parameter def do_something(param1, param2): # function body does something # with param1 and param2 # - takes two parameters Note: in Python a function definition begins with the def keyword. The (meaningful) name of a function gives the user an idea of what the function does but not how it does it. The user of a function does not need to know how the function performs a task. The user only needs to know what the function does (and the information they have to provide, and the information they get back if there is a result). This is abstraction. It is a good idea to document the purpose of a function (what it does but not how it does it). Newcastle University,

8 Appendix 3: draw_shape using a for loop def draw_shape(linelength, sidestodraw): if linelength > 0 and sidestodraw > 2: for i in range(sidestodraw): turtle.forward(linelength) Newcastle University,