Transcript CSReview Keith O’Hara [email protected] Sep 3 2007 CS1301 -http://wiki.roboteducation.org O'Hara Programming Jargon  Value - fundamental programming quantity with a type     Float - 3.0 Integer - 3 String -

CS
1301
Review
Keith O’Hara
[email protected]
Sep 3 2007
CS1301 -http://wiki.roboteducation.org
O'Hara
1
Programming Jargon
 Value - fundamental programming quantity with a type
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Float - 3.0
Integer - 3
String - “3”, “Three”
Boolean - True, False
 Expression - Evaluates to a value.
 3 + 2 - 6 *8
 Statement - segment of code python executes and does something
 print 3 + 2
 Variable - name that refers to a value
 x = 3 + 2
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Expressions
 Code snippets that evaluate to some value.
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3**2 #evaluates to 9
(3+2)*(4+2)
3.0/2.0
“hello” + “world”
(3 == 4)
#evals
(3 != 4)
#evals
(3 < 4)
#evals
“abc” < “bcd” #evals
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to
to
to
to
False
True
True
True
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Types of Values
 Integers (like integers in math)
 -1, -2, 300000, 0
 Floating Points (like “decimals”)
 -1.5, 1.5, 3.1415, 1.0
 Character (like symbol of an alphabet)
 ‘a’, ‘b’, ‘c’, ‘z’
 Strings (a string of characters)
 “CS”, “1301”, “rocks”
 Booleans (a truth value)
 True or False
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Have Type-Sense
 Your expressions should make sense in terms of
what type they are
 Some make perfect sense
 3 + 4 = 7 [int + int = int]
 Some expressions make no sense
 “hello” + 4 [ string + int]
 Some expressions make (uncommon) sense
 4.0 + 3 [float + int]
 4.0 + 3 = 7.0 [float + int = float]
 “CS” * 3 = “CSCSCS” [string * int = string]
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Order of Operations
 Order an expression is
evaluated
 PEMDAS
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Parentheses
Exponentiation
Multiplication, Division
Addition, Subtraction
(3-2)*(4+2)**2
(1)*(4+2)**2
(1) * (6)**2
(1) * (36)
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 Left-to-Right
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Variables
 Variables refer to values
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b = 3 + 2
a = b * 2
myName = “Keith”
inCS1301 = True
# b = 5
# a = 10
 “=“ means assignment not equality
 b = 3 + 2 # store 5 in the variable b
 b = 3 * 2 # store 6 in the variable b
x = x +1
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Statements
 Code snippets that do stuff!
 Driving the robot
 forward(1, 0.5)
 stop
 beep(1, 440)
 Assignment
 classname = “cs1301”
 Displaying to the screen
 print classname
 print “We love”, classname, “it’s great”
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Useful Functions
 A function is a piece of code you can
use over and over again
input
 Treat it like a black box
 You pass it values, it does some
work, and it returns values
 You “call it”,”invoke it”, or “use it” by
using its name and parentheses
 The things you pass it go inside the
parentheses
 output = function(input)
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function
output
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Using Simple Functions
 Functions that interact
with the robot
 forward (speed)
 beep(time, frequency)
 Pass them arguments
 Execute in sequential
order
forward(1)
stop()
beep(1, 440)
 flow of execution
 Top-level
 not in any function
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Writing Simple Functions
 Defining
functions
 Creates function
 Does not
execute/run
them
 Indenting implies
“scope” or code
ownership
 Call functions
from top-level or
other functions
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Inden
t
def nudge():
print “going forward”
forward(1)
print “about to stop”
stop()
nudge()
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No Indention
“Top Level”
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Writing Simple Functions
def function-name():
statement
statement
…
statement
name()
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Writing Functions with Parameters
def nudge(speed):
print “Going forward with speed”, speed
forward(speed)
print “About to stop”
stop()
nudge(.2)
nudge(.9)
nudge(1)
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Octaves of A
Do I need the
parentheses
around 2**octave?
def beepA(length, octave):
beep(length, 27.5 * (2**octave))
beepA(1,4)
beepA(1,1)
beepA(3,6)
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# A4
# A5
# A6
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A4 : 440 Hz
A5: 880 Hz
A6: 1760 Hz
A7: 3520 Hz
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Writing Functions with Parameters
def function-name(p1, p2, …, pn):
statement
statement
…
statement
function-name(v1, v2, …, vn)
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Using Functions that Return Values
name = raw_input(“Enter your name”)
print “Hello”, name
print “Robot battery voltage”, getBattery()
p = takePicture()
show(p)
v = abs(-3)
print “Absolute value of (-3) =“, v
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Converting between types
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float(3000)
int(3.0)
int(3.99999)
str(3.9)
int(“3”)
int(“3.0”)
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#
#
#
#
#
#
returns
returns
returns
returns
returns
error
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3000.0
3
3
‘3.9’
‘3’
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Composing Functions
print abs(int(0 - 3.5))
print abs(int(-3.5))
print abs(-3)
print 3
show(takePicture())
n = int(raw_input(“Enter a number”))
n = int(“9”)
n = 9
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Writing Functions
that Return Values
def area(radius):
return 3.14 * radius**2
def circumference(diameter):
return 3.14 * diameter
print “Area of a 3 ft circle”, area(3)
print “Circumference”, circumference(2*3)
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Functions with Local Variables
def area(radius):
a = 3.14 * radius**2
return a
def circumference(diameter):
c = 3.14 * diameter
return c
print “Area of a 3 ft circle”, area(3)
print “Circumference”, circumference(2*3)
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Variables in a Function are Local
 Variables in a function are private
 Including the parameters
 Each function has its own variables
 Even when the names are the same
 Allows you to write functions independently
without worry about using the same name
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Different Variables - Same Name
def area(radius):
a = 3.14 * radius**2
return a
def circumference(radius):
a = 3.14 * 2 * radius
return a
print “Area of a 3 ft circle”, area(3)
print “Circumference”, circumference(3)
print a
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Writing Functions with Return
Values
def function-name(list-of-params):
statement
statement
…
statement
return value
output = function-name(list-of-params)
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Passing variables to functions
userinput = raw_input(“Enter a number”)
number = int(userinput)
print “Absolute value = “, abs(number)
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Calling Your Own Functions
def area(radius):
return 3.14 * radius**2
invalue = raw_input(“Enter the radius”)
r = int(invalue)
Print “Area of a”, r, “ft circle”, area(r)
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Calling Your Own Functions
def rect_area(length, width):
area = length*width
return area
l = int(raw_input(“Enter the length”))
w = int(raw_input(“Enter the width”))
print “Area of rectangle”, rect_area(l,w)
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Same Name - Different Variables
def rect_area(length, width):
area = length*width
return area
length = int(raw_input(“Enter the length”))
width = int(raw_input(“Enter the width”))
print “Area of rect”, rect_area(length, width)
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Same Name - Different Variables
def rect_area(length, width):
area = length*width
length = 0
width = 0
return area
length = int(raw_input(“Enter the length”))
width = int(raw_input(“Enter the width”))
area = rect_area(length, width)
print “The rectangle length =”, length
print “The rectangle width =”, width
print “The rectangle area =”, area
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Functions in general
# description of this function
# what it expects as input
# what is provides as output
def function (p0, p2, …, pn):
statement
…
statement
return value
z = function(a0, a2, …, an)
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Math Functions
 Math module
 Set of useful math functions
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 import math
 math.sin(math.pi)
 math.log(100)
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Where’s the Error?
def avgLight():
left = getLight(‘left’)
center = getLight(‘center’)
right = getLight(‘right’)
avg = (left + center + right) / 3.0
print “Average Light Reading:”, avgLight()
 Forgot to return the value!
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Where’s the Error?
def avgLight():
left = getLight(‘left’)
center = getLight(‘center’)
right = getLight(‘right’)
avg = (left + center + right) / 3.0
return avg
print “Average Light Reading:”, avgLight()
 No Indentation
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Where’s the Error?
def avgLight():
left = getLight(‘left’)
center = getLight(‘center’)
right = getLight(‘right’)
avg = (left + center + right) / 3.0
return avg
print “Average Light Reading:”, avgLight(1)
 Not calling function correctly
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Where’s the Error?
def avgLight():
left = getLight(‘left’)
center = getLight(‘center’)
right = getLight(‘right’)
avg = (left + center + right) / 3.0
return avg
avgLight()
print “Average Light Reading:”, avg
 avg is a local variable to the avgLight function
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Where’s the Error?
def avgLight():
left = getLight(‘left’)
center = getLight(‘center’)
right = getLight(‘right’)
avg = left + center + right / 3.0
return avg
print “Average Light Reading:”, avgLight()
 Order of Operations wrong!
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Where’s the Error?
def avgLight():
left = getLight(‘left’)
center = getLight(‘center’)
right = getLight(‘right’)
avg = (left + center + right) / 3
return avg
print “Average Light Reading:”, avgLight()
 Integer Division
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Test on Friday
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