1. Python Loops and Iteration

2. Repeated Steps No Yes Program: n = 5 while n > 0 : print n
n = n – 1
print 'Blastoff!'
Output: 5 4 3 2 1
Blastoff!
print n
n = n -1
print 'Blastoff'
Loops (repeated steps) have iteration variables that change each time through a loop. Often these iteration variables go through a sequence of numbers.

3. What is wrong with this loop?
An Infinite Loop
n = 5 No
Yes
n > 0 ?
print 'Lather'
n = 5
while n > 0 :
print 'Lather'
print 'Rinse'
print 'Dry off!'
print 'Rinse'
print 'Dry off!'
What is wrong with this loop?

4. Breaking Out of a Loop
The break statement ends the current loop and jumps to the statement immediately following the loop
while True:
line = raw_input('> ')
if line == 'done' :
break
print line
print 'Done!'
> hello there
hello there
> finished
finished
> done
Done!

5. line = raw_input('> ') if line == 'done' : break print line No
Yes
while True:
line = raw_input('> ')
if line == 'done' :
break
print line
print 'Done!'
True ?
....
break
...
print 'Done'

6. Definite Loops
Quite often we have a list of items of the lines in a file - effectively a finite set of things
We can write a loop to run the loop once for each of the items in a set using the Python for construct
These loops are called "definite loops" because they execute an exact number of times
We say that "definite loops iterate through the members of a set"

7. A Simple Definite Loop for i in [5, 4, 3, 2, 1] : 5 print i 4
print 'Blastoff!' 5 4 3 2 1
Blastoff!

8. A Simple Definite Loop friends = ['Joseph', 'Glenn', 'Sally']
for friend in friends :
print 'Happy New Year: ', friend
print 'Done!'
Happy New Year: Joseph
Happy New Year: Glenn
Happy New Year: Sally
Done!

9. The range() function (revisited)
x = range(5)
print x
[0, 1, 2, 3, 4]
x = range(3, 7)
[3, 4, 5, 6]
x = range(10, 1, -2)
[10, 8, 6, 4, 2]
range() is a built-in function that allows you to create a sequence of numbers in a range
Very useful in “for” loops which are discussed later in the Iteration chapter
Takes as an input 1, 2, or 3 arguments. See examples.

10. A Simple Definite Loop iterating over a range7 6 5 4 3 2 1
Blastoff!
for i in range(7, 0, -1) :
print i
print 'Blastoff!'

11. Looping through a Set print 'Before'9 41 12 3 74 15
After
print 'Before'
for thing in [9, 41, 12, 3, 74, 15] :
print thing
print 'After'

12. Counting in a Loop Before 0 1 9 2 41 3 12 4 3 5 74 6 15 After 6
zork = 0
print 'Before', zork
for thing in [9, 41, 12, 3, 74, 15] :
zork = zork + 1
print zork, thing
print 'After', zork
To count how many times we execute a loop we introduce a counter variable that starts at 0 and we add one to it each time through the loop.

13. Summing in a Loop zork = 0 print 'Before', zork
for thing in [9, 41, 12, 3, 74, 15] :
zork = zork + thing
print zork, thing
print 'After', zork
Before 0
9 9
50 41
62 12
65 3
139 74
154 15
After 154
To add up a value we encounter in a loop, we introduce a sum variable that starts at 0 and we add the value to the sum each time through the loop.

14. Finding the Average in a Loop
Before 0 0
1 9 9
4 65 3
After
count = 0
sum = 0
print 'Before', count, sum
for value in [9, 41, 12, 3, 74, 15] :
count = count + 1
sum = sum + value
print count, sum, value
print 'After', count, sum, sum / count
An average just combines the counting and sum patterns and divides when the loop is done.

15. Filtering in a Loop print 'Before'
for value in [9, 41, 12, 3, 74, 15] :
if value > 20:
print 'Large number',value
print 'After'
Before
Large number 41
Large number 74
After
We use an if statement in the loop to catch / filter the values we are looking for.

16. Control flow tools
for letter in "aeiou":
print "vowel: ", letter
for i in [1,2,3]:
print i
for i in range(0,3):
The for loop has the following general form. If a sequence contains an expression list, it is evaluated first. Then, the first item in the sequence is assigned to the iterating variable var. Next, the statements are executed. Each item in the sequence is assigned to var, and the statements are executed until the entire sequence is exhausted. For loops may be nested with other control flow tools such as while loops and if statements.
for var in sequence:
statements
vowel: a
vowel: e
vowel: i
vowel: o
vowel: u 1 2 3

17. Example
write a program that reads in the grades of 50 students in a course (out of 100 points each ) and then count the number of A students ( grade > 85 ) and the number of B students (grade > 75 ).

18. Count-Controlled Loop Example
Use a while loop to read the 100 blood pressures and find their total

19. Example
Write a program to display all the numbers divisible by 5 in the range 0 to 5000.

20. Example
Assume you put 1000 pounds in a projects that returns a profit of about 5% per year. How long will it take for your money to double ?
Assume you put 5000 pounds in a projects that returns a profit of about 10% per year. How much money will you have in 5 years ?