Transcript Day 2 – Lesson 7 Conditionals and Loops Python Mini-Course University of Oklahoma Department of Psychology Python Mini-Course: Day 2 - Lesson 7 4/18/09

Day 2 – Lesson 7
Conditionals and Loops
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Day 2 - Lesson 7
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Lesson objectives
1. Use conditional statements in
Python, including the “if”, “if…else”,
and “if…elif” statements
2. Use Boolean logic to create complex
conditionals
3. Use the “for” statement to create
loops
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Python Mini-Course: Day 2 - Lesson 7
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Conditional execution (if…)
 Syntax:
if condition:
do_something
 Condition must be statement that
evaluates to a boolean value (True or
False)
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Example:
Checking user input
x = raw_input("X? ")
if x.isdigit():
print "You input a number"
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Python Mini-Course: Day 2 - Lesson 7
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Alternative execution
 Syntax:
if condition:
do_something
else:
do_alternative
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Example:
Checking user input
x = raw_input("x? ")
if x.isdigit():
print "You input a number"
else:
print "Please input a number\
next time"
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Example:
Avoiding division by zero
x = int(raw_input("x? "))
y = int(raw_input("y? "))
if y <> 0:
print x / y
else:
print "Attempted division by\
zero"
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Chained conditionals
 Syntax:
if condition:
do_something
elif condition:
do_alternative1
else:
do_alternative2
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Example:
x = int(raw_input("x? "))
y = int(raw_input("y? "))
if x < y:
print 'x is less than y'
elif x > y:
print 'x is greater than y'
else:
print 'x and y are equal'
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Notes on conditionals
 You have to have at least one
statement inside each branch but
you can use the pass command
while stubbing
if x < 0:
pass #Handle neg values
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Notes on conditionals
 You can have as many elif
branches as you need
 This replaces the select and switch
commands in other languages
 Conditionals can be nested
 Example: nested.py
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Complex condition statements
 Use Boolean logic operators and,
or, and not to chain together
simple conditions
 Example: boolean.py
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The for loop
 Used to repeat a section of code
a set number of times
 Syntax:
for target in sequence:
do_statements
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Example: (power.py)
def power(base, exponent):
val = base
for x in range(1,exponent):
val = val * base
print val
power(3,4)
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Example: (power.py)
def power(base, exponent):
val = base
for x in range(1,exponent):
print x, val
val = val * base
print val
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Example: (power.py)
def power(base, exponent):
val = base
for x in range(1,exponent):
print x, val
val = val * base
print val
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Exercises
1. Find a major bug in the power
function and correct it
2. Add a docstring to the power
function
3. Add type checking and value
checking to the power function
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