03-Conditional

Comparison Operators

• Boolean expressions ask a question and produce a Yes or

No result which we use to control program flow

• Boolean expressions using comparison operators evaluate to - True / False - Yes / No

• Comparison operators look at variables but do not change the variables

Python Meaning

< Less than

<= Less than or Equal

== Equal to

>= Greater than or Equal

> Greater than

!= Not equal

x = 5

if x == 5 :

print 'Equals 5'

if x > 4 :

print 'Greater than 4'

if x >= 5 :

print 'Greater than or Equals 5'

if x < 6 : print 'Less than 6'

if x <= 5 :

print 'Less than or Equals 5'

if x != 6 :

print 'Not equal 6'

Indentation

• Increase indent indent after an if statement or for statement (after : )

• Maintain indent to indicate the scope of the block (which lines are affected by the if/for)

• Reduce indent back to the level of the if statement or for statement to indicate the end of the block

• Blank lines are ignored - they do not affect indentation

• Comments on a line by themselves are ignored with regard to indentation

Warning: Turn Off Tabs

• Most text editors can turn tabs into spaces - make sure to enable this feature

> NotePad++: Settings -> Preferences -> Language Menu/Tab Settings

> TextWrangler: TextWrangler -> Preferences -> Editor Defaults

• Python cares a *lot* about how far a line is indented. If you mix tabs and spaces, you may get “indentation errors” even if everything looks fine

Two way using else:

x = 4

if x > 2 :

print 'Bigger'

else :

print 'Smaller'

print 'All done'

Multi-way

if x < 2 :

print 'small'

elif x < 10 :

print 'Medium'

else :

print 'LARGE'

print 'All done'

The try/except Structure

• You surround a dangerous section of code with try and except

• If the code in the try works - the except is skipped

• If the code in the try fails - it jumps to the except section

astr = 'Bob'

try:

print 'Hello'

istr = int(astr)

print 'There'

except:

istr = -1

print 'Done', istr