Download part 4 notes - Newcastle University

 Introduction to Programming with Python – Session 4 Notes Nick Cook, School of Computing Science, Newcastle University Contents 1. Challenge 6 and battleships .......................................................................................... 1 2. Recap functions ............................................................................................................ 1 3. Draw shape function ..................................................................................................... 2 4. Solutions to Section 3 exercises .................................................................................... 4 5. For loops ....................................................................................................................... 5 6. Resources ...................................................................................................................... 5 1. Challenge 16 and battleships Work through Challenge 16 from Python Programming challenges: Write a program where the computer thinks of a number between 1 and 100 (i.e. picks a number at random). The program then asks the user to guess what number it is thinking of. If the user guesses correctly, the program should say well done and say how many guesses it took the user. If the user guesses incorrectly, it should say whether the number the computer is thinking of is higher or lower than the user's guess, and ask the user to guess again. Then play the first two battleships from the CS-­‐Unplugged searching algorithms activity < http://csunplugged.org/searching-­‐algorithms>. 2. Recap functions Reusing code, putting a unit of work into a separate function (or procedure) and using it again wherever necessary. Also like sub-­‐routines, breaking up a program into smaller component parts. Do 3.3a from book (p. 19) with class, save to file firstfunction.py. Function with more than one parameter Do following nsquared_plus_n function with class, save to file nsquaredplusn.py # function definition
def nsquared_plus_n(nsquared, n):
print(nsquared, '+',n,'=',nsquared + n)
# main program
counter = 0
while counter < 21:
nsquared_plus_n(counter ** 2, counter)
counter = counter + 1
© Newcastle University, 2013 1 Turtle draw a square function Open myshapes.py from Part 3 and do draw_square function with class: Delete all code but version 3, so left with: import turtle
turtle.color('green')
sidesToDraw = 4
angle = 360 / sidesToDraw
while sidesToDraw > 0:
turtle.forward(100)
turtle.right(angle)
sidesToDraw = sidesToDraw – 1
window = turtle.Screen()
window.exitonclick()
Modify as follows: import turtle
# function definition
def draw_square():
sidesToDraw = 4
angle = 360 / sidesToDraw
while sidesToDraw > 0:
turtle.forward(100)
turtle.right(angle)
sidesToDraw = sidesToDraw - 1
# main program
turtle.color('green')
draw_square()
window = turtle.Screen()
window.exitonclick()
Class exercises •
•
create a draw_triangle function and draw a red triangle create a draw_hexagon function and draw a blue hexagon 3. Draw shape function The myshapes.py program from Section 2 should be: import turtle
# function definitions
def draw_square():
sidesToDraw = 4
angle = 360 / sidesToDraw
while (sidesToDraw > 0):
turtle.forward(100)
© Newcastle University, 2013 2 turtle.right(angle)
sidesToDraw = sidesToDraw – 1
def draw_triangle():
sidesToDraw = 3
angle = 360 / sidesToDraw
while (sidesToDraw > 0):
turtle.forward(100)
turtle.right(angle)
sidesToDraw = sidesToDraw - 1
def draw_hexagon():
sidesToDraw = 6
angle = 360 / sidesToDraw
while (sidesToDraw > 0):
turtle.forward(100)
turtle.right(angle)
sidesToDraw = sidesToDraw - 1
# main program
turtle.color('green')
draw_square()
turtle.color('red')
draw_triangle()
turtle.color('blue')
draw_hexagon()
window = turtle.Screen()
window.exitonclick()
Question: what is the problem with the above code? Answer: code duplication – the only difference between draw_square, draw_triangle and draw_hexagon is the value of sidesToDraw, which is fixed/hard-­‐coded in each function. Together with class develop a draw_shape function that can draw any equilateral shape. For draw_shape version 1 (parameterised with sidesToDraw), take the draw_hexagon function, rename it to draw_shape, parameterise with sidesToDraw, and delete the line that gives sidesToDraw the value 6. def draw_shape(sidesToDraw):
angle = 360 / sidesToDraw
while (sidesToDraw > 0):
turtle.forward(100)
turtle.right(angle)
sidesToDraw = sidesToDraw – 1
© Newcastle University, 2013 3 draw_shape is a function that will draw any regular polygon (equilateral and equiangular) with sides of length 100. To draw different shapes just call the function with different values for the sidesToDraw, e.g: # green square
turtle.color('green')
draw_shape(4)
# red triangle
turtle.color('red')
draw_shape(3)
# blue pentagon
turtle.color('blue')
draw_shape(5)
Question: what happens when the number of sides to draw is less than 3? Question: what other fixed values in the function could be passed as parameters? Class exercises •
•
Change the draw_shape function to print a message to say that sidesToDraw is too small for a suitable value of sidesToDraw and only draw a shape if sidesToDraw is greater than the value. Consider which other fixed value in the function could be parameterised to allow us to control the size of the shape. Change the function accordingly. 4. Solutions to Section 3 exercises Work through corrections with class. Demonstrate original draw_shape with values of 2, 1, 0 for sidesToDraw
Question: how do we correct the function? def draw_shape(sidesToDraw):
if sidesToDraw > 2:
angle = 360 / sidesToDraw
while sidesToDraw > 0:
turtle.forward(100)
turtle.right(angle)
sidesToDraw = sidesToDraw – 1
Question: which other fixed value in the function can be parameterised? •
The length of the line: def draw_shape(lineLength, sidesToDraw):
if sidesToDraw > 2 and lineLength > 0:
angle = 360 / sidesToDraw
while sidesToDraw > 0:
turtle.forward(lineLength)
turtle.right(angle)
sidesToDraw = sidesToDraw – 1
© Newcastle University, 2013 4 # blue square
draw_shape(100, 4)
# red triangle
draw_shape(100, 3)
# green pentagon
draw_shape(100, 5)
Demonstrate changing lineLength. Question: what are the differences between draw_square and draw_shape functions? Class exercises •
Modify myshapes.py to provide functions to draw a square, triangle and a pentagon using the draw_shape function functions. 5. For loops We often know how many times we want to iterate (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 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 with given input. 2. Do 3.2f loop challenge on p.18 of Mark Clarkson's book, writing a function to calculate the factorial 6. Resources • Other material from Newcastle University Introduction to Python CPD: http://www.ncl.ac.uk/computing/outreach/resources/programming/python/intro2p
ython • Mark Clarkson's Introduction to Python resources including textbook, workbooks, example code and GCSE controlled assessment: http://www.ncl.ac.uk/computing/outreach/resources/protected/mwclarkson-­‐
resources.zip • Other Python resources: http://www.ncl.ac.uk/computing/outreach/resources/programming/python/ © Newcastle University, 2013 5