1. COSC 1306 COMPUTER SCIENCE AND PROGRAMMING
Jehan-François Pâris
Fall 2016 1

2. THE ONLINE BOOK CHAPTER VI FUNCTIONS

3. Motivation
The for loop let us identify segments of code that are used again and again
Replace them with functions
Can also use functions to break down big programs into more manageable chunks

4. Example Program has 260 lines # START MAIN PROGRAM readinputs()
# MAIN LOOP
while eventlist != [] :
[time, event, pid] = eventlist.pop(0)
if event == 'ARRIVAL' :
arrival(pid)
elif event == 'CORE' :
corecompletion(pid)
elif event == 'DISK' :
diskcompletion(pid)
elif event == 'I/O' :
iocompletion(pid)
else :
print("PANIC" )
# PRINT SUMMARY
printsummary()

5. Functions Straight extension of mathematical functions
sin(…), cos(…), exp(…)
Also apply to strings, lists, …
Can be
Built-in
Imported from a Python module
import math y = math.sqrt(x)
User defined

6. Writing your own functions
def function_name(argument(s)) : statements
Note
The column at the end of the first line
The indenting

7. An example def mysqrt(number) : root = number/2 for i in range(10) :
inverse = number/root
root = (root + inverse)/2
return root

8. A very simple function def increment(number) : return number + 1
print("increment(5)", increment(5))
Note that the print() statement is not part of the function
Many people prefer to leave an empty line after each function declaration

9. Back to turtles (I) import turtle def draw_square(t, sz):
"""Make t draw a sz by sz square."""
for i in range(4):
t.forward(sz)
t.left(90)

10. Back to turtles (II) wn = turtle.Screen() wn.bgcolor("lightgreen")
wn.title("Alex meets a function") # New
alex = turtle.Turtle()
draw_square(alex, 50) # Call draw_square
wn.mainloop()

12. A more general function
def draw_rectangle(t, w, h):
"""Get turtle t to draw a rectangle of
width w and height h."""
for i in range(2) :
t.forward(w)
t.left(90)
t.forward(h)

13. A new draw_square function
def draw_square(tx, sz):
draw_rectangle(tx, sz, sz)
If we have to draw rectangles and squares
Write most general of the two
Make the other a special case of the first
Avoids doing the work twice

14. Flow of control (I) When Python encounters a function definition
It does not do anything with code
It just remembers that the programmer defined that function
When Python encounters a function call
It looks if the function was defined in an imported module or in the program
It then executes that function

15. Flow of control (II)
Think of function declarations as specifying something the program can do
Put them ahead of the first place where they will be used:
Included modules
Function declarations
"Main" program

16. Function parameters Most functions have parameters
Makes them more general
mysqrt(number), draw_square(t, sz), draw_rectangle(t, w, h)

17. Functions returning a value
Some functions return values
mysqrt(number)
Some do not
draw_square(t, sz),
draw_rectangle(t, w, h)

18. What happens inside a function
def mysqrt(number) :
root = number/2
for i in range(10) :
inverse = number/root
root = (root + inverse)/2
return root
i = 0
rt2 =mysqrt(2.0)

19. The question Have two variables with the same name In the main program
In the function definition
What will happen to the i variable in the main program when we call mysqrt()?
Will it be set to 9?
Will it remain equal to zero?

20. The answer What happens in a function declaration
… remains in the function declaration
All variables used in a function declaration
Are newly created variables
Exist only inside the function body

21. What happens inside a function
def mysqrt(number) :
root = number/2
for i in range(10) :
inverse = number/root
root = (root + inverse)/2
return root
i = 0
rt2 =mysqrt(2.0)
...
Use the
green i
here
Use the
red i
here

22. Main advantage Can use any variable name inside a function declaration
No need to worry whether the name is used elsewhere

23. Organizing our code (I)
def make_window(colr, ttle):
"""Set up a new window."""
w = turtle.Screen()
w.bgcolor(colr)
w.title(ttle)
return w

24. Organizing our code (II)
def make_turtle(colr, sz):
""" Set up a new turtle."""
t = turtle.Turtle()
t.color(colr)
t.pensize(sz)
return t

25. Organizing our code (III)
wn = make_window("lightgreen", "Tess and Alex dancing")
tess = make_turtle("hotpink", 5)
alex = make_turtle("black", 1)
dave = make_turtle("yellow", 2)
Our main program is shorter and easier to understand

26. The accumulator pattern
Suppose we want to count the sum of the first n numbers
def sum_them_all(n) : running_total = 0 # before the loop for i in range(1, n + 1) : running_total += i return running_total #after it

27. The accumulator pattern
Suppose we want to count the sum of the first n numbers
def sum_them_all(n) : running_total = 0 # before the loop for i in range(1, n + 1) : running_total += i return running_total #after it

28. Python will refuse to execute
def sum_them_all(n) : for i in range(1, n + 1) : running_total += i return running_total #after it

29. Will return n
def sum_them_all(n) : for i in range(1, n + 1) : running_total = 0 # WRONG running_total += i return running_total #after it

30. Will return 1
def sum_them_all(n) : running_total = 0 # before the loop for i in range(1, n + 1) : running_total += i return running_total # WRONG

31. Functions can call functions
Recall
def draw_square(tx, sz):
draw_rectangle(tx, sz, sz)

32. The main function
Unlike C and C++, Python has no notion of a specian main function
If you try
def main() : print("Hello World")
nothing will happen
Python will define a main() function but not execute it

33. A personal comment
You will have to learn the little peculiarities of each language
In French, I would not say
My name is Jehan-François
but instead the equivalent of
*I call myself Jehan-François

34. Stepwise refinement
Suppose we are asked to draw the front of a house with a flat roof
How should we proceed?

35. Step one
draw_frame(turtle, width, height, color) draw_windows(???) position(turtle, x,y) draw_door(turtle, width, height, color, doorknob, doorknob_color, doorknob_side)

36. The easiest
draw_frame(trtl, w, h, clr) : draw_filled_rectangle(trtl, w, h, clr)
which will use
draw_filled_rectangle(trtl, w, h, clr) : trtl.fillcolor(clr) trtl.start_fill() draw_rectangle(trtl, w, h) trtl.end_fill

37. The door
draw_door(trtl, w, h, clr, dk, dk_clr, dk_side) : draw_filled_rectangle(trtl, w, h, clr) // must learn to make choices: // left or right position_doorknob(???) // Does not work
OMG !!!

38. The door revisited
draw_door(trtl, x, y, w, h, clr, dk, dk_clr, dk_side) : position(trtl, x, y) draw_filled_rectangle(trtl, w, h, clr) // assume knob on right side dk_x =x + w*9/10 // BAD dk_y= y + h/2 position turtle(trtl, dk_x, dk_y) draw_filled_circle(trtl, dk,clr)

39. The windows Replace draw_windows(???) by
position(trtl, x1, y1) draw_window(w, h, clr) position(trtl, x2, y2) draw_window(w, h, clr) position(trtl, x3, w3) draw_window(w, h, clr)
Will have problems drawing the window frames!

40. The windows
Use
draw_window(trtl, w, h, clr) position(trtl, x2, y2) draw_window(w, h, clr) position(trtl, x3, w3) draw_window(w, h, clr)
Will have problems drawing the window frames!

41. A turtle bar chart (I)

42. A turtle bar chart (II)
import turtle

43. A turtle bar chart (III)
# def drawBar(t, height):
""" Get t to draw one bar of height. """
t.begin_fill() # start filling
t.left(90) # up
t.forward(height)
t.write(str(height)) # write label
t.right(90) # back to horizontal
t.forward(40)
t.right(90) #down
t.left(90)
t.end_fill() # stop filling

44. A turtle bar chart (IV)
xs = [48, 117, 200, 240, 160, 260, 220] # here are the data to plot
maxheight = max(xs) # max entry in list
numbars = len(xs) # size of list
border = 10 # extra space top and right

45. A turtle bar chart (V) # Set up the window and its attributes
wn = turtle.Screen()
wn.setworldcoordinates(0-border, 0-border, 40*numbars+border, maxheight+border) # do not study this
wn.bgcolor("lightgreen")

46. setworldcoordinates()
setworldcoordinates(llx, lly, urx, ury)
Four numbers
llx : x-coordinate of lower left corner of canvas
lly : y-coordinate of lower left corner of canvas
urx : x-coordinate of upper right corner of canvas
ury: y-coordinate of upper right corner of canvas

47. A turtle bar chart (VI) # Setup turtle tess tess = turtle.Turtle()
tess.color("blue")
tess.fillcolor("red")
tess.pensize(3)

48. A turtle bar chart (VII)
# draw the chart
for a in xs:
drawBar(tess, a) # works because tess is always where
# it should be after having drawn
# a bar
wn.exitonclick()

49. There is more
There are many other aspects of functions that are left to be discovered later
Optional parameters
Parameters with default values
Declaring some variables to be global …
Python is like a big city that takes time to discover