Conditionals - flow-control structures¶

conditional statements control the flow of execution of codes
- makes some block of code to skip or execute based on some conditions
- loosely speaking, it helps computer think and make decision
boolean values - True or False
boolean expressions - comparison operators are used to compare values that result in True or False value.

Comparison operators¶

allow us to compare data resulting in True or False outcome
==, !=, >, >=, <, <=

In [7]:

# Examples of various comparison operators
# x == y => True if x is equal to y
# x != y => True if x is not equal to y
# x > y => True if x is greater than y
# x < y => True if x is less than y
# x >= y => True if x is greater than or equal to y
# x <= y => True if x is less than or equal to y
x = 5
y = 10
print(x==y)
print(x!=y)
print(x>y)
print(x<y)
print(x<=y)
print(x>=y)

False
True
False
True
True
False

Conditional execution¶

Three types of conditional selectors
One-way, Two-way and Multi-way selectors

One-way selector¶

just one if statement by itself
syntax:

if <boolean expression> == True:
    # execute this block of code

In [8]:

if True:
    print('hello')

hello

In [9]:

if 'a' == 'a':
    print('a equals to a')

a equals a

Two-way selector¶

if statement followed by else statement
syntax:

if <boolean expression> == True:
    # execute this block of code
else:
    # do this

In [10]:

if 10 >= 20:
    print ('print 10 is greater than or equal to 20')
else:
    print ('print 10 is NO greater than or equal to 20')

print 10 is NO greater than or equal to 20

In [11]:

if 100.5 >= 100.5:
    print('True')
else:
    print('False')

True

Multi-way selector¶

similar to multiple-choice questions with only one valid answer
start from first if statement and as soon as a if expression is evaluated True all the rest elif, else are ignored

if <condition1>: 
    # block1
    # execute this...
elif <condition2>:
    # block2 
    # execute this
... 
... 
else: 
    # else block
    # execute code in this block

In [12]:

# Guess the number entered by user
ans = int(input('Enter a number between 1-5:'))
if ans == 1:
    print('You entered 1')
elif ans == 2:
    print('You entered 2')
elif ans == 3:
    print('You entered 3')
elif ans == 4:
    print('You entered 4')
elif ans == 5:
    print('You entered 5')
elif ans > 5:
    print('You entered a number larger than 5')
else:
    print('I do not know what you entered! Hmm....')

Enter a number between 1-5:10
You entered a number larger than 5

Visualize using pythontutor.com: https://goo.gl/ZSm9KS ¶

Logical Operators¶

and, or, and not -- allow to build more complex boolean expressions

Truth table for and

a	b	a and b
T	T	T
T	F	F
F	T	F
F	F	F

Truth table for or

a	b	a or b
T	T	T
T	F	T
F	T	T
F	F	F

Truth table for not

a	not a
T	F
F	T

Order of Evaluations: Highest to Lowest:

http://www.informit.com/articles/article.aspx?p=459269&seqNum=11

In [13]:

num = 10
if (num %2 ==0 and num > 0):
    print(num, "is positive and even")
    

10 is positive and even

In [15]:

num = -14
if (num %2 ==0 and num < 0):
    print(num, "is negative and even")

In [14]:

if not False:
    print('True')

True

Exercises¶

Exercise 1: Write a program to test whether a given whole number is even, odd or zero

In [ ]:

x = input('Enter a whole number:' )
x = int(x)
if x == 0:
    print(x, 'is zero')
elif x%2 == 0:
    print(x, 'is even')
else:
    print(x, 'is odd')

Exercise 2. Improve the above program to tell whether the given whole number is positive or negative.

Exercise 3. Write a program that helps someone decide where to go eat lunch depending on amount of money one has in their pocket.

Exercise 4. Given a day of week as integer (say 1 for Sunday) write a program that tells whether that day is weekend, or weekday and the actual name of the day.

Nested conditionals¶

if condition:
    if condition1:
        # do something
    else:
        # do something else
        if condition2:
            # do something...
else:
    # do this...

Exercise 5: Write a program that determines whether someone is eligible to vote in the US federal election.

Exercise 6:

Write a function day_name that converts an integer number 0 to 6 into the name of a day. Assume day 0 is “Sunday”. Once again, return None if the arguments to the function are not valid. Here are some tests that should pass:


assert day_name(3) == "Wednesday"
assert day_name(6) == "Saturday"
assert day_name(42) == None

Exercise 7:

Write a function that helps answer questions like ‘“Today is Wednesday. I leave on holiday in 19 days time. What day will that be?”’ So the function must take a day name and a delta argument — the number of days to add — and should return the resulting day name.

Here are some tests that should pass:
assert day_add("Monday", 4) ==  "Friday"
assert day_add("Tuesday", 0) == "Tuesday"
assert day_add("Tuesday", 14) == "Tuesday"
assert day_add("Sunday", 100) == "Tuesday"
assert day_add("Sunday", -1) == "Saturday"
assert day_add("Sunday", -7) == "Sunday"
assert day_add("Tuesday", -100) == "Sunday"