PYTHON CONDITIONALS CHAPTER 5 FROM THINK PYTHON HOW

PYTHON CONDITIONALS CHAPTER 5 FROM THINK PYTHON HOW TO THINK LIKE A COMPUTER SCIENTIST

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 All of these return a True or False Note that True and False are NOT strings

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

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

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 Execute these instruction only if <condition> Is True. Four space Indention is required! # an example if x!=0: y = 20. 0/x print y #makes sure the we are not dividing by 0

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’ else: print ‘x is less than or equal to zero’

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

DETERMINE A LETTER GRADE FROM A NUMBER grade= ? #Assign some value here if grade>=90: print ‘I made an A’ elif grade>=80: #Note: As soon as a true is found # the remainder is skipped!! print ‘I made a B’ elif grade>=70: print ‘I made a C’ else: print ‘I better get to studying’

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: Note: This is a contrived example. There is a much easier way to do this! if a>c: print a, ‘is the largest’ else: print a, ‘is not the largest’ else: print a, ‘ is not the largest’ Think about using the and operator!!

KEYBOARD INPUT HTTP: //DOCS. PYTHON. ORG/2/LIBRARY/FUNCTIONS. HTM L 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

OK, LOOK AT THIS >>> 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 Type. Error: cannot concatenate 'str' and 'int' objects WHY? So what can we do to fix it?

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

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) Value. Error: invalid literal for int() with base 10: '3. 45' Note: float(x) will work!

SOME PROBLEMS TO CONTEMPLATE 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 Enter a: 2 Enter b: 3 Enter c: 4 Enter x: 5 The answer is 69. 0 >>>

FIND THE LARGEST INTEGER ( USE MAX SUPPORT VARIABLE ) x = raw_input('Enter an integer : ') max=int(x) x = raw_input('Enter an integer : ') x = int(x) if x>max: max = x x = raw_input('Enter an integer : ') x=int(x) if x>max: max = x print 'The largest is', max

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"

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'