Friday, December 23, 2011

Computational Economics Lecture 5

Posted by Unknown at 2:06 AM


Expressions and Conditional Execution

Controlling the flow of your program according to conditions

Epressions, True and False

Let's take a quick look at the if keyword:
>>> if 1 < 2: print 'foo'
... 
foo
>>> if 2 < 1: print 'bar'
... 
>>>
Please ignore the ..., as we'll see what it means soon
What's important now is that
  • the first line prints foo, while
  • the second line prints nothing
Here the code phrases 1 < 2 and 2 < 1 are called expressions
The expression 1 < 2 evaluates to True
>>> 1 < 2
True
While 2 < 1 evaluates to False
>>> 2 < 1
False
The code following the expression (here print 'foo' or print 'bar') is executed or ignored, depending on whether the expression evaluates to True or False

Testing Equality and Inequalty

Here are some more examples testing inequalty
>>> x, y = 1, 2
>>> x < y
True
>>> x > y
False
>>> 1 < 2 < 3   # We can 'chain' inequalities
True
>>> 1 <= 2 <= 3
True
How about testing for equality?
>>> 1 == 1
True
>>> x = y = 1
>>> x == y
True
>>> x == 2
False
Note that we use == to test for equality!
  • The statement x = y is assignment (identifier x is bound to the value of y)
For "not equal" use !=
>>> 1 != 1
False
>>> 1 != 2
True

Combining Expressions

We can combine expressions using and, or, not
These are the standard logical operations from mathematics/set theory
Here's some quick examples
>>> a, b, c = 1, 2, 3
>>> a < b and b < c
True
>>> a < b and c < a
False
>>> a < b or c < a
True
>>> not a < b
False
Rules for the and operation: True if both are True, else False
>>> True and True   
True
>>> True and False
False
>>> False and True
False
>>> False and False
False
Rules for the or operation: False if both are False, else True
>>> True or True    
True
>>> True or False   
True
>>> False or True   
True
>>> False or False   
False
Finally, the not operation:
>>> not True       
False
>>> not False
True
>>> not not True
True

Boolean Operations on Sequences

Operations on sequences which return True or False
We have already met some of these
>>> s = 'foo'
>>> s.startswith('f')
True
Some other string methods which return boolean values:
>>> 'foo'.isdigit()
False
>>> '100'.isdigit()
True
>>> 'FOO'.isupper()
True
There are more, which we wont go into now
A general boolean operation on sequences using the in keyword:
>>> X = ['foo', 'bar']
>>> 'foo' in X
True
>>> 'fool' in X
False
>>> 'fool' not in X
True
What does this return?
>>> 'foo' in X or 'fool' in X
The in construct also works with strings:
>>> 'y' in 'yes'
True

If/Else

The following code is typed into a text file, not interpreter

Basics

Let's return to the if construct
if True:
    print 'foo'
Not surprisingly, this prints 'foo'
Note the syntax:
  • The expression 'True' must be followed by a colon
  • The print statement must be indented at least one space
    • Try without: Error message says 'expected an indented block'
if False:
    print 'foo'
Of course this prints nothing
We can replace True or False with an expression that evaluates to True or False
if 1 < 2:
    print 'foo' 
if 2 < 1:
    print 'foo'

Code Blocks and Indentation

The general syntax for the if construct is
if <expression>:
    <code block>
The code block can contain as many lines as we like
if True:
    print 'foo'
    print 'bar'
    print 'foobar'
All lines in the code block must have the same indentation
This gives an error
if True:
    print 'foo'
   print 'bar'
    print 'foobar'
As does this
if True:
    print 'foo'
    print 'bar'
     print 'foobar'

The Rules of Indentation

Code blocks are denoted by indentation
  • All lines must be indentated by the same number of spaces
  • The convention in Python is 4 spaces, please use it
Note that internally, tabs and spaces are different things!
  • You can use the Tab key to insert spaces...but
  • You should configure your text editor to convert tabs to spaces
    • In IDLE this is preconfigured: Tab key inserts 4 spaces
    • For other editors, configure to replace tabs with 4 spaces
For example, in Gedit:


And in Geany


Correct Python settings for other text editors can be found by googling.

Else and Elif

Consider the following program
x = float(raw_input("Enter any number: "))
if x >= 0:
    print "The number is nonnegative."
else:
    print "The number is negative."
The 'else' clause only executed if condition is false
Don't forget the colon
If we need more conditions use 'elif'
x = float(raw_input("Enter any number: "))
if x > 0:
    print "The number is positive."
elif x == 0:
    print "The number is zero."
else:
    print "The number is negative."
Here's another example
username = raw_input("Enter username: ")
password = raw_input("Enter password: ")
if username == "john" and password == "loser":
    # Let john into the system
elif username == "hacker" and password == "winner":
    # Let hacker into the system
else:
    print "Invalid username or password"

Conditional Assignment

The conditional assignment
if x < 0:
    value = 'negative'
else:
    value = 'nonnegative'
can be done more succinctly as follows
value = 'negative' if x < 0 else 'nonnegative'

Problems

Recently I received the following chain email:
Don't tell me your age; your waiter may know!
  1. First of all, pick the number of times a week that you would like to go out to eat. (more than once but less than 10)
  2. Multiply this number by 2
  3. Add 5
  4. Multiply it by 50
  5. If you have already had your birthday this year add 1758...If you haven't, add 1757.
  6. Now subtract the four digit year that you were born.
You should have a three digit number
The first digit of this was your original number. (I.e., How many times you want to go out to restaurants in a week.) The next two numbers are YOUR AGE!
Write a program which asks the user to answer these questions, does the calculations and prints our their age.
Solution:
guess = int(raw_input("how many nights p/w do you eat out? (1 <= n < 10): "))
guess = (guess * 2 + 5) * 50
bday = raw_input("have you had your birthday this year? (y/n): ")

if 'y' in bday or 'Y' in bday:
    guess = guess + 1758
else:
    guess = guess + 1757

byear = int(raw_input("what year where you born? (4 digits): "))
guess = guess - byear

guess = str(guess)
print "your age is " + guess[1] + guess[2]

General Expressions

An expression is any code phrase which returns a value (object) when passed to the interpreter
Here are some examples
>>> 1 + 1
2
>>> 'foo' + 'bar'
'foobar'
>>> (1, 2) + (3, 4)
(1, 2, 3, 4)
>>> 1  # This is an expression too (a very simple one)
1
All of these are expressions
Often expressions come on the right hand side of equal signs
>>> x = 2 + 2  #  Expression evaluated, binds `x` to result (i.e., 4)
>>> x = 2      #  The right-hand side is also an expression
When testing conditions, we can use any expression
if 42:      # This is regarded as True
    print 'foo' 
if 0:       # This is regarded as False
    print 'foo' 
if 1 + 1:   # This is regarded as True
    print 'foo' 
if []:      # This is regarded as False
    print 'foo'
The rule here is as follows
  • Regarded as false
    • Expressions that evaluate to 0 or 0.0, empty sequences (strings, lists, tuples), None
  • Regarded as true
    • (Almost) all other values
If you're not sure, you can use the function bool():
>>> bool(1)            # 1 is usually associated with 'true'
True
>>> bool(0)            # And 0 is usually associated with 'false'
False
>>> bool(10 + 10)      # We can put in any expression
True
>>> bool(42 - 42)      # This is False, because value is 0
False
>>> bool('hello')      # Nonempty string regarded as true
True
>>> bool('')           # Empty string regarded as false
False
>>> bool([1, 2])       # Nonempty list regarded as true
True
>>> bool([])           # Empty list regarded as false
False
Actually, bool is a data type, like a float or an int
>>> x = True
>>> type(x)
<type 'bool'>
The two possible values are True and False
The operator not applied to an expression returns the opposite bool:
>>> not 'foo' + 'bar'
False
>>> not 0 * 10
True

Boolean Arithmetic

In algebraic expressions,
  • True treated as integer 1
  • False treated as integer 0
>>> True + True
2
>>> True + False
1
>>> False + False
0
>>> True * False
0
This works for the built in sum() function too:
>>> sum([True, False, True, True])
3

 

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