Conditional Execution, Multi-way Decision

//Program
x = 5
if x < 10:
	print('Smaller')
if x > 20:
	print('Bigger')

print('Finis')

//Outputs
Smaller
Finis

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 or Yes / No
• Comparison operators look at variables but do not change the variables

Python	Meaning
<	Less than
<=	Less than or Equal to
==	Equal to
>=	Greater than or Equal to
>	Greater than
!=	Not equal

Indentation

• Increase indent after an if statement or for 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 blcok
• 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(4 spaces)!

• Atom automatically uses spaces for files with “.py” extension (nice!)
• Most text editors can turn tabs into spaces - make sure to enable this feature
  • NotePad++ : Settings → Preferences → Language Menu/Tab Settings
  • TextWragler : TextWrangler → Preferences → Editor Defaults
• Python cares a lot about how far a line is intended. If you mix tabs and spaces, you may get “indentation errors” even if everything looks fine

Two-way Decisions

• Sometimes we want to do one thing if a logical expression is true and something else if the expression is false
• It is like a fork in the road - we must choose one or the other path but not both

---

Multi-way Decision

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 = 'Hello Bob'
try:
		istr = int(astr)
except:
		istr = -1

print('First', istr)

astr = '123'
try:
		istr = int(astr)
except:
		istr = -1

print('Second', istr)

Sample try/except

rawstr = input('Enter a number:')
try:
		ival = int(rawstr)
except:
		ival = -1

if ival > 0 :
	print('Nice work')
else:
	print('Not a number')

Summary

• Comparison operator
  • ==, <=, >=, >, <, !, =
• Indentation
• One-way Decisions
• Two-way decisions: if: and else:
• Nested Decisions
• Multi-way decisions using elif
• try/except to compensate for errors