Decision structures
In class exercises
$ update21
inclass/w03-decisions/broken.py
inclass/w03-decisions/cold.py
inclass/w03-decisions/isAlpha.py
inclass/w03-decisions/isOdd.py
inclass/w03-decisions/speeding.py
Number and letter representation
By now we know that python can save and store data in variable for future use.
Additionally, vim and linux save our python programs in files
for future use. Where does all this data go and how does the computer keep
track of it all? For a full explanation, you would need to take one or more
additional CS courses (See CS31: Introduction to Computer Systems), but the
short answer is that data is stored in the computer's memory, which to an
extremely rough approximation is like a sheet of graph paper where each cell
can hold a single symbol. While we commonly work with the decimal digits 0-9,
the letters a-z, and a few other common symbols, the computer really only
understands two symbols: 0 and 1. These symbols can be represented as an
electrical signal or bit that is either on (a 1) or off (a 0).
Switches are easy to make, we can make them really small, and we can cram about
half a billion of them into a single chip. A billion bits can store a lot of
data, but it is still a finite resource. To represent data types such as
integers, floating point numbers, and characters, we must encode these data
types as a sequence of 1's and 0's. Integers are stored in binary format.
Characters are stored using ASCII character codes. We can
convert between characters and their ASCII equivalent using the functions
ord and chr. A quick example is shown below. We will see how
this is useful later.
>>> let='a'
>>> let
'a'
>>> ord(let)
97
>>> chr(97)
'a'
>>> chr(98)
'b'
>>> chr(ord('d')+18)
'v'
>>> chr(ord('z')+2)
'|'
A word of caution: Do NOT use ASCII character codes directly. There is no need to memorize them in python. You may want to memorize them for party tricks, but it is unlikely that you will gain many friends this way. If you need to know the ASCII character of 'c', use ord('c'), not 99. If I see a spurious 65, 90, 97, or 122 in your code, I will take off points. I assign no significance to these numbers.
Boolean type
In addition to the basic types int, float, string, and list, python has support for a Boolean type. A Boolean can have one of two possible values: True or False.
x=True y=FalseNote, there are no quotes around the values True and False.
Relational Boolean operators: <, >, <=, >=, ==, !=. Relations Boolean operators compare elements of a similar type and return a Boolean value.
Logical Boolean operators: and, or, not. Logical Boolean operators compare and and or compare two Boolean expressions and return a Boolean value. The operator not negates a single Boolean expression.
Why is == a Boolean operator instead of just =? Think syntax and semantics.
Decision structures
We'll be looking at some in class examples for decision structures today that will require you to copy some files from my directory.
Change into your inclass directory.
[~]$ cd [~]$ cd cs21/inclass/ [inclass]$ ls w01-intro/ w03-decisions/If the w03-decisions directory does not yet exist, create it using update21
if, else, elif
Sometimes we need to make decisions based on various input parameters or
conditions in our programs. Python supports automated decision making using if, else, elif syntax.
if <condition>: <body> if <condition>: <body> else: <body> if <condition>: <body> elif <condition>: <body> else: <body>The condition must be an expression that evaluations to True or False. The term Boolean is used to describe a type of data that can be either true or false.
temp=float(raw_input("Enter the current temperature in Fahrenheit: "))
if temp <= 0:
print "%d is cold. Brrr!" % (temp)
else:
print "%d is at least above 0" % (temp)
Never odd or even?
The first program we will look at is isOdd.py. Modify the program so that it prints out if a number entered by a user is odd or even. Think about how the mod or remainder operator % can be used with an if statement to solve this problem. Some sample runs are shown below.
$ python isOdd.py Please enter a number: 0 0 is even $ python isOdd.py Please enter a number: 1243678 1243678 is even $ python isOdd.py Please enter a number: 2371 2371 is odd
What did you type?
Next open isAlpha.py and modify the program so that it determines if a single character (a length 1 string) entered by the user is an uppercase letter, a lowercase letter, a decimal digit, or none of the above (usually punctuation). This program relies on python's ability to compare two strings using the operators < >, ==, <=, >=, and !=. Try it! Comparisons are done using the ASCII character codes. If you want to quickly see what happens, try a few example in a separate python shell. Some example runs of a sample program are shown below.
$ python isAlpha.py Enter a single keyboard character: 3 '3' is a decimal digit $ python isAlpha.py Enter a single keyboard character: a 'a' is a lowercase letter $ python isAlpha.py Enter a single keyboard character: Q 'Q' is an uppercase letterWhat if the user enters more than one character? Can you inform them politely on how to run the program correctly?