Day 3 – Lesson 10
Iteration: Loops
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Lesson objectives
1. Use arithmetic and binary operators
2. Use the increment and decrement
operators
3. Create loops using the for and
while statements
4. Control loop execution using the
break and continue statements
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Arithmetic operators
add
subtract
x+y
exponent
3
divide
x*y
x/y
x–y
floor divide
modulus
x//y
x%y
x**y
negation
-x
multiply
absolute value
abs(x)
Python Mini-Course: Day 3 - Lesson 10
05/02/09
Binary (bitwise) operators
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AND
OR
XOR
a&b
a|b
a^b
Left shift
Right shift
a<<x
a>>x
Python Mini-Course: Day 3 - Lesson 10
Complement
~a
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Incrementing
 Updating the value of a variable
by adding some value to the
current value
 Example:
x = 4
x = x + 3
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Decrementing
 Updating the value of a variable
by subtracting some value from
the current value
 Example:
x = 4
x = x - 3
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Updating operators
 Python uses the augmented
assignment operators:
+=
Increment
-=
Decrement
*=
Update by multiplication
/=
Update by division
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Examples
x = 5
print x
x += 4
print x
x -= 3
print x
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Python Mini-Course: Day 3 - Lesson 10
x *= 15
print x
x /= 5
print x
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Implicit type conversion
 Just like the arithmetic operators,
the update operators perform
automatic type conversion
x = 3
print x, type(x)
x += 1.5
print x, type(x)
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
The while statement
 Syntax
while conditional:
do_something
 A general loop that executes code
as long as the conditional
statement is true
 Exits loop when conditional is false
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Example 1: blastoff2.py
def countdown(n):
while n > 0:
print n
n = n-1
print 'Blastoff!'
countdown(10)
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Example 2: sequence.py
def sequence(n):
while n != 1:
print n,
if n%2 == 0: # n is even
n = n/2
else: # n is odd
n = n*3+1
sequence(15)
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Important notes
 The while statement uses negative
logic to express the stop condition:
 "keep going until that happens"
 Also, the loop can only terminate at the
beginning of each iteration
 Often we want positive logic:
 "stop when this happens"
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
The break statement
 Used to break out of a loop (for
or while loop) early
 Loop stops immediately and
execution picks up at the next line
outside the loop
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Example: break.py
while True:
line = raw_input('> ')
if line == 'done':
break
print line
print 'Done!'
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
The continue statement
 Used to restart a loop early
 Execution immediately goes back to
the loop header
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Example: print_odd.py
def print_odd(start=1, stop=1):
for x in range(start, stop):
if x % 2 == 0: continue
print x
print_odd(1,7)
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Python Mini-Course: Day 3 - Lesson 10
05/02/09
Example: print_odd2.py
def print_odd(start=1, stop=1):
if start % 2 == 0: start += 1
for x in range(start, stop, 2):
print x
print_odd(4,10)
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Python Mini-Course: Day 3 - Lesson 10
05/02/09