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Conditionals

Making Choices/Flow Control/Conditionals

  • So far we have only written small programs that are a sequence of instructions. Sometimes you have to alter the sequential flow of a program to suit the needs of a particular situation.
  • When strings are compared, they are compared lexicographic, meaning strings are put into alphabetical order and uppercase comes before lowercase.
  • The conditional operator converts the conditional into a boolean, which is a basic Python data type.
  • bool datatype only has two values: True or False
  • There are only three Boolean operators: and, or, and not.

Truthiness and Falsiness

  • Things that are false on their own
    • None (basic data type)
    • False (basic data type)
    • Any empty sequence: '', [], ()
    • Any zero value: 0, 0.0
    • Anything whose len() returns 0
    • Empty objects
    • Everything else is True

Boolean/Logical Operators

Operator What it does? Example
and True if both a AND b are true (logical conjunction) if is_teacher and is_active: print('You can access')
or True if either a OR b are true (logical disjunction) if is_superuser or (is_teacher and is active): print('You can access')
not True if the opposite of a is true (logical negation) if not is_superuser: print('You cannot access')

Examples

not True
not False

True and True
False and False
True and False
False and True


True or True
False or False
True or False
False or True
cold = True
windy = False

(not cold) and windy # It is not cold and windy

not (cold and windy) #

Relational Operators

  • Assume a=1 and b=1
Relational Operators What it does? Example
== True if a has the same value as b a == b #True
!= True if a does not have the same value as b a != b #False
> True if a is greater than b a > b # False
< True if a is less than b a < b # False
>= True if a is greater than or equal to b a >= b # True
<= True if a is less than or equal to b a <= b # True
  • These operators evaluate to True or False depending on the values you give them.
  • Conditionals are used to instruct computer to make a decision.
45 > 34
45 > 79
45 < 79
45 < 34

23.1 >= 23
23.1 >= 23.1
23.1 <= 23.1
23.1 <= 23

67.3 == 87
67.3 == 67
67.0 == 67
67.0 != 67
67.0 != 23

Combining Comparisons

x = 2
y = 5
z = 7
x < y and y < z

(x < y) and (y < z) # better
x = 3
(1 < x) and (x <= 5)

x = 7
(1 < x) and (x <= 5)


x = 3 
1 < x <= 5 # You can chain comparisons

3 < 5 != True 
(3 < 5) and (5 != True)

3 < 5 != False
(3 < 5) and (5 != False)

String Comparisons

'A' < 'a'
'A' > 'Z'
'abc' < 'abd'
'abc' < 'abcd' # shorter is less
'Jan' in '01 Jan 1838'

'Feb' in '01 Jan 1838'

'a' in 'abc'

'A' in 'abc'

Choosing which statements to execute

if some condition is True:
    do something
elif some other condition is True: # else if 
    do something
elif some other condition is True: # else if 
    do something
elif some other condition is True: # else if 
    do something
else:
    do something
  • colons are important and indentation matters
  • can have many elif tests
  • do not need else
  • conditions can be nested

Example

pH Level Solution Category
0-4 Strong acid
5-6 Weak acid
7 Neutral
8-9 Weak base
10-14 Strong base 
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    print(ph, " is acidic.") #indentation important
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    print(ph, " is acidic.")
    print('You should be careful with that!') #indentation is important
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    print(ph, " is acidic.") #indentation important

if ph > 7.0:
    print(ph, " is basic.") 
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    print(ph, " is acidic.") #indentation important
elif ph > 7.0:
    print(ph, " is basic.") 
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    ph = 8.0 #indentation important

if ph > 7.0:
    print(ph, " is basic.") 
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    ph = 8.0  #indentation important
elif ph > 7.0:
    print(ph, " is basic.") 
ph = float(input('Enter the pH level: '))
if ph < 7.0:
    ph = 8.0  #indentation important
elif ph > 7.0:
    print(ph, " is basic.") 
compound = input('Enter the compound: ')
if compound == 'H20':
    print('Water')
elif compound == 'NH3':
    print('Ammonia')
elif compound == 'CH4':
    print('Methane')
else:
    print('Unknown compound')

Nested if statements

value = input('Enter the pH level: ')
if len(value) > 0:
    ph = float(value) 
    if ph < 7.0:
        print(ph, " is acidic.")
    elif ph > 7.0:
        print(ph, " is basic.")
    else:
        print(ph, " is neutral.")

else:
    print('No pH value was given!')
if age < 45:
    if bmi < 22.0:
        risk = 'low'
    else:
        risk = 'medium'
else:
    if bmi < 22.0:
        risk = 'medium'
    else:
        risk = 'high'
young = age < 45
slim = bmi < 22.0
if young:
    if slim:
        risk = 'low'
    else:
        risk = 'medium'
else:
    if slim:
        risk = 'medium'
    else:
        risk = 'high'
young = age < 45
slim = bmi < 22.0
if young and slim:
    risk = 'low'
elif young and not slim :
    risk = 'medium'
elif not young and slim:
    risk = 'medium'
elif not you and not slim:
    risk = 'high'
  • What is wrong with the following code?
ph = 2
if ph < 7.0:
    print(ph, ' is acidic')
elif ph < 3.0:
    print(ph, ' is VERY acidic! Be careful.')
# Implement the min() function for three inputs

number1 = int(input('Enter first integer: '))
number2 = int(input('Enter second integer: '))
number3 = int(input('Enter third integer: '))

minimum = number1  

if number2 < minimum:
    minimum = number2

if number3 < minimum:
    minimum = number3

print('Minimum value is', minimum)

https://www.analyzemath.com/Equations/solve-quadratic-equations-using-discriminants.html