1. Hossain Shahriar shahriar@cs.queensu.ca
CISC 101: Fall 2011
Hossain Shahriar

2. Announcement and reminder!
Tentative date for final exam need to be fixed!
Topics to be covered in this lecture(s)
More on range function
While loop
Break

3. Range function >>> range(5)
[0, 1, 2, 3, 4] # we are getting one less element that expected
>>> range(5, 10)
[5, 6, 7, 8, 9] # we are getting one less element that expected
>>> range(4, 12, 3)
[4, 7, 10]
>>> range(-10, -100, -30)
[-10, -40, -70]
Write a program that sum the series:
sum =0;
For i in range (2, 12, 2):
sum+= i

4. While loop
while expr1 :
statement
Two things to keep in mind for using while loop
expr1 must include an iterating variable
You are responsible for incrementing/decrementing the iterating variable to terminate the loop
Let us sum the series: … 10 [watch the red lines!!]
sum =0
i=2
while i < 10:
sum+= i
i+= 2

5. Loop (cont.) – Break statement
Useful in certain situations and often used in an if-else structure within a loop
Imagine, we want to iterate a list to find at least one negative number
We do not need to continue the loop once we find a case
for x in len (list)
If x < 0:
Print ‘found a negative number, goodbye!’
break
Caution: if you have multiple loops, then break will only affect the immediate loop where it is located
For …. : #loop1
For … : #loop2
If … break # break loop2 only and loop1 will continue as usual

6. Loop (cont.) – Break statement
Take home assignment: (3 marks)
Write a program that checks if a given number N is prime or not.
Tips: N is a prime number when it not divisible by any number other than 1 and N
You need to use a for/while loop, if-else, and a break statement
Iterate from 1 to N/2 and check the remainder is always non zero to conclude that N is a prime number

7. Exception A generic way to handle incompatible input
E.g., a user provides string instead of a number
def main():
try:
x = int (input ("enter a number : ")) …
except Exception:
print "The supplied input is not a number"
main()

8. Tomorrow’s class
File handling