9/4/2013
Announcements
Quiz this Thursday.
15-112 Foundations of
Programming
Lecture 4
September 3rd , 2012
What we know so far
How to get input from the user
How to print messages
Three type of values
Basic Arithmetic
Expressions
Variables
Functions




Expressions
Variables
Input/Output
Conditional Statements
Homework 1 is due Today at 11:59pm
Homework 2 will be posted tonight – Due
September 10th.
What do we need to learn?
Sequential Execution
Conditions
Conditional Execution
Repeating Execution
1
9/4/2013
Sequential Execution
Conditional Execution
All execution of instructions so far has
been sequential
Step 1
Example:
print "Hello to my sequential program“
Step 2
name = raw_input("Enter your name> ")
Sometimes we want to selectively
execute statement
 If your name is same as mine, I should say
something.
name = raw_input("Enter your name> ")
print "Oh Wow! Your name is same as mine"
Step 3
age = int(raw_input("Enter your age> "))
Step 4
 Is there anything wrong with this?
print name,", you are ", age , "years old"
Conditional Execution
We can fix this by using conditional
execution
name = raw_input("Enter your name> ")
Colon
if name == "saquib" :
print "Oh Wow! Your name is same as mine"
Indented
Conditional Execution
Conditional execution finishes when
you stop indenting
name = raw_input("Enter your name> ")
if (name == "saquib"):
print "Oh Wow! Your name is same as mine"
print "I really like our name"
print "Welcome to my program", name
>>>
Enter your name> saquib
Oh Wow! Your name is same as mine
I really like our name
Welcome to my program saquib
>>>
>>>
Enter your name> Bob
Welcome to my program Bob
>>>
2
9/4/2013
Conditional Execution
number = int( raw_input("Enter a number "))
if number > 0:
print "The number is positive"
print "Thank you for your number"
>>>
Enter a number 34
The number is positive
Thank you for your number
>>>
More on forming conditions
Conditional Operators
 and
 or
 not
>>>
Enter a number -5
Thank you for your number
>>>
Combining conditions
num1 = int(raw_input())
num2 = int(raw_input())
num3 = int(raw_input())
if num1 > num2 and num1 > num3:
print num1
if num2 > num1 and num2 > num3:
print num2
if num3 > num1 and num3 > num2:
print num3
Are you sick
You are sick if your head hurts and you
have a fever. If only your head hurts, you
are just sick of this class which is not a
real sickness
Write a conditional statement that checks
if you are really sick given two variables
 Headhurts
 Havefever
3
9/4/2013
Lets play doctor
Being a doctor is easy
 If your head hurts, you have a fever, but not
runny nose – you have the flu
 If you have a fever and a runny nose, but no
headache – you have bronchitis
 If you don’t have fever, or runny nose, but
only have a headache – you have brain tumor
Can you write conditional statements to
replace doctors?
If else
Sometimes we need to execute some
alternate statement
import math
num = int(raw_input("Enter a number "))
if num > 0:
print "Factorial is",math.factorial(num)
else:
print "You have entered an invalid number"
>>>
Enter a number 5
Factorial is 120
If-elif-else
Sometimes we need to make mutually
exclusive choices
score = int(raw_input("Enter your score "))
if score >= 90:
print "You have an A"
if score >= 80:
print "You have an B"
if score >= 70:
print "You have an C"
if score >= 60:
print "You have an D"
if score < 60:
print "You have an R"
print "Now you know your grade"
>>>
Enter a number 0
You have entered an invalid number
If-elif-else
Fixed grades
score = int(raw_input("Enter your score "))
if score >= 90:
print "You have an A"
elif score >= 80:
print "You have an B"
elif score >= 70:
print "You have an C"
elif score >= 60:
print "You have an D"
else:
print "You have an R"
print "Now you know your grade"
4
9/4/2013
Nested if condition
Where are you from?
Repetition
Austin, Detroit, Tokyo, Ann Arbor
Are your from North America?
Austin, Detroit, Ann Arbor
Yes
No
Tokyo
Are your from Michigan?
Detroit, Ann Arbor
Yes
No
Are your from a
university town?
Yes
Ann Arbor
I think of a number between 1 – 100, you
try to guess it
How would you do this in Python?
Austin
No
Detroit
While loop
While (some condition is true) :
first statement
second statement
….
Outside of Loop
Example
Find the sum of all digits of a number
entered by the user
Inside the loop
(Loop Body)
 234 = 2 + 3 + 4 = 9
 484 = 4 + 8 + 4 = 16
 193 = 1 + 9 + 3 = 13
5
9/4/2013
For Loop
Normally used when you want to execute
some code a fixed or known number of
times
Example: Print all numbers from 1 to 10
for x in range(1,10):
print x
Example
Print the sum of all numbers from 1 – 10
sum = 0
for i in range(1,11):
sum = sum + i
print sum
Another Example of for loop
Read a string from the user, print each
character of the string on a separate line
value = raw_input("Enter a string")
for i in range(0,len(value)):
print value[i:i+1]
6