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
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