1. Python Conditionals chapter 5
From
Think Python
How to Think Like a Computer Scientist

2. Boolean Expression
A Boolean expression is an expression that results in either a True or False. We can test these in easily. For example >>> 5 == 2 #note the double == is used to test for equality False >>> 5 == 5 # Note that 5=5(or x=5) is really bad!! True # Other relational operators include x != y x < y x <= y x>y x>=y All of these return a True or False Note that True and False are NOT strings

3. Logical Operators
There are three logical operators, and, or and not These are used as we do in English >>> True and True True >>> True and False if both true return true else false # False >>> True or False # if either is true return true >>> not True >>> True and not False

4. Further examples
>>> x=10 >>> y = 0 >>> x>5 and y == 0 True >>> x != 10 or not y == 3 >>> x>5 and x<=20 # returns True if x is between 5 and 20 >>> x%2==0 or x%3==0 # divisible by 2 or by 3 return True

5. Conditional execution
The first instruction that we will look at that uses conditions is the if statement. Here is the syntax if <condition>: statement more_statements # an example if x!=0: y = 20.0/x #makes sure the we are not dividing by 0 print y
Execute these instruction only
if <condition> Is True. Four space
Indention is required!

6. Alternative example
if <condition>: Instruction to execute if <condition> is True else: Instructions to execute if <condition> is False # Example if (x>0): print ‘x is positive’ print ‘x is less than or equal to zero’

7. Chained Conditionals
If x<y: print ‘x is less than y’ elif x>y: print ‘x is greater than y’ else: print ‘x is equal to y’ #You can chain as many of these elif’s as you want # Lets do an example with grades

8. Determine a Letter Grade from a number
grade= ? #Assign some value here if grade>=90: print ‘I made an A’ #Note: As soon as a true is found elif grade>=80: # the remainder is skipped!! print ‘I made a B’ elif grade>=70: print ‘I made a C’ else: print ‘I better get to studying’

9. We can also put an if within an if (nested)
Suppose we want to see if a is greater than b and c if a>b : if b>c: print a, ‘is the largest’ else: if a>c: print a, ‘is not the largest’ print a, ‘ is not the largest’
Note: This is a contrived example. There is a much easier way to do this!
Think about using the
and operator!!

10. Keyboard INPUT http://docs.python.org/2/library/functions.html
Before we look at more if-else examples we need to see how to get input (keyboard) from the person running the python program.
raw_input([prompt]) If the prompt argument is present, it is written to screen without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that.
When EOF is read, EOFError is raised.
Example:
x = raw_input(‘Enter a number:’)
Prompt
A string

11. Ok, Look at this So what can we do to fix it?
>>> x = raw_input('Enter a number:') Enter a number:23 >>> x+3 #What will be printed? Traceback (most recent call last): File "<pyshell#3>", line 1, in <module> x+3 TypeError: cannot concatenate 'str' and 'int' objects WHY?
So what can we do to fix it?

12. Convert it to an integer or float
#method 1 >>> x = raw_input('Enter a number:') Enter a number:12 >>> x=int(x) #convert x to an integer >>> x+4 16 #method 2 >>> x = int(raw_input('Enter a number:'))+4 Enter a number:23 27
What if I typed in 3.45
instead of 12

13. Here is what happens
>>> x = raw_input('Enter a number:') Enter a number:3.45 >>> int(x) Traceback (most recent call last): File "<pyshell#12>", line 1, in <module> int(x) ValueError: invalid literal for int() with base 10: '3.45' Note: float(x) will work!

14. Some problems to contemplate
Evaluate a polynomial 𝒚=𝒂 𝒙 𝟐 +𝒃𝒙+𝒄 where a,b and c are entered from the keyboard. Here is a script
Enter a:2
Enter b:3
Enter c:4
Enter x:5
The answer is 69.0
>>>
a=float(raw_input(‘Enter a:’))
b=float(raw_input(‘Enter b:’))
c=float(raw_input(‘Enter c:’))
x=float(raw_input(‘Enter x:’))
y = a*x**2+b*x+c
print ‘The answer is’,y

15. Find the largest integer ( use max support variable )
x = raw_input('Enter an integer :') max=int(x) x = int(x) if x>max: max = x x=int(x) print 'The largest is',max

16. Divisibility tests A sequence of if’s
x = raw_input('Enter an integer :') x=int(x) # don’t forget this one. if x%2==0: print "divisible by 2" if x%3==0: print "divisible by 3" if x%5==0: print "divisible by 5" if x%7==0: print "divisible by 7"

17. Using strings
x = raw_input('Enter your first name :') # note x is a string already so no conversion is required! if x=='Richard': print x,'is an awesome teacher' else: print x, 'must be an awesome student'