Conditionals – miro.borodziuk.eu

1. The comparison (otherwise known as relational) operators are used to compare values. The table below illustrates how the comparison operators work, assuming that x = 0, y = 1, and z = 0:

Operator	Description	Example
`==`	returns `True` if operands’ values are equal, and `False` otherwise
`!=`	returns `True` if operands’ values are not equal, and `False` otherwise	`x != y # True` `x != z # False`
`>`	`True` if the left operand’s value is greater than the right operand’s value, and `False` otherwise	`x > y # False` `y > z # True`
`<`	`True` if the left operand’s value is less than the right operand’s value, and `False` otherwise	`x < y # True` `y < z # False`
`≥`	`True` if the left operand’s value is greater than or equal to the right operand’s value, and `False` otherwise	`x >= y # False` `x >= z # True` `y >= z # True`
`≤`	`True` if the left operand’s value is less than or equal to the right operand’s value, and `False` otherwise	`x <= y # True` `x <= z # True` `y <= z # False`

2. When you want to execute some code only if a certain condition is met, you can use a conditional statement:

- a single if statement, e.g.:

x = 10

if x == 10: # condition
   print("x is equal to 10") # Executed if the condition is True.

x = 10

if x == 10: # condition

print("x is equal to 10") # Executed if the condition is True.

- a series of if statements, e.g.:

x = 10

if x > 5: # condition one
   print("x is greater than 5") # Executed if condition one is True.

if x < 10: # condition two
   print("x is less than 10") # Executed if condition two is True.

if x == 10: # condition three
   print("x is equal to 10") # Executed if condition three is True.

x = 10

if x > 5: # condition one

print("x is greater than 5") # Executed if condition one is True.

if x < 10: # condition two

print("x is less than 10") # Executed if condition two is True.

if x == 10: # condition three

print("x is equal to 10") # Executed if condition three is True.

Each if statement is tested separately.

- an if-else statement, e.g.:

x = 10

if x < 10: # Condition
  print("x is less than 10") # Executed if the condition is True.

else:
  print("x is greater than or equal to 10") # Executed if the condition is False.

x = 10

if x < 10: # Condition

print("x is less than 10") # Executed if the condition is True.

else:

print("x is greater than or equal to 10") # Executed if the condition is False.

- a series of if statements followed by an else, e.g.:

x = 10

if x > 5: # True
print("x > 5")

if x > 8: # True
  print("x > 8")

if x > 10: # False
  print("x > 10")

else:
  print("else will be executed")

x = 10

if x > 5: # True

print("x > 5")

if x > 8: # True

print("x > 8")

if x > 10: # False

print("x > 10")

else:

print("else will be executed")

Each if is tested separately. The body of else is executed if the last if is False.

- The if-elif-else statement, e.g.:

x = 10

if x == 10: # True
   print("x == 10")

if x > 15: # False
   print("x > 15")

elif x > 10: # False
   print("x > 10")

elif x > 5: # True
   print("x > 5")

else:
print("else will not be executed")

x = 10

if x == 10: # True

print("x == 10")

if x > 15: # False

print("x > 15")

elif x > 10: # False

print("x > 10")

elif x > 5: # True

print("x > 5")

else:

print("else will not be executed")

If the condition for if is False, the program checks the conditions of the subsequent elif blocks – the first elif block that is True is executed. If all the conditions are False, the else block will be executed.

- Nested conditional statements, e.g.:

x = 10

if x > 5: # True
   if x == 6: # False
      print("nested: x == 6")
   elif x == 10: # True
      print("nested: x == 10")
   else:
      print("nested: else")
else:
   print("else")

x = 10

if x > 5: # True

if x == 6: # False

print("nested: x == 6")

elif x == 10: # True

print("nested: x == 10")

else:

print("nested: else")

else:

print("else")

Exercise 1

What is the output of the following snippet?

x = 5
y = 10
z = 8

print(x > y)
print(y > z)

x = 5

y = 10

z = 8

print(x > y)

print(y > z)

False

True

Exercise 2

What is the output of the following snippet?

x, y, z = 5, 10, 8

print(x > z)
print((y - 5) == x)

x, y, z = 5, 10, 8

print(x > z)

print((y - 5) == x)

False

True

Exercise 3

What is the output of the following snippet?

x, y, z = 5, 10, 8
x, y, z = z, y, x

print(x > z)
print((y - 5) == x)

x, y, z = 5, 10, 8

x, y, z = z, y, x

print(x > z)

print((y - 5) == x)

True

False

Exercise 4

What is the output of the following snippet?

x = 10

if x == 10:
  print(x == 10)
if x > 5:
  print(x > 5)

if x < 10:
  print(x < 10)
else:
  print("else")

x = 10

if x == 10:

print(x == 10)

if x > 5:

print(x > 5)

if x < 10:

print(x < 10)

else:

print("else")

True

True

else

Exercise 5

What is the output of the following snippet?

x = "1"

if x == 1:
  print("one")
elif x == "1":
  if int(x) > 1:
    print("two")
  elif int(x) < 1:
    print("three")
  else:
    print("four")
if int(x) == 1:
  print("five")
else:
  print("six")

x = "1"

if x == 1:

print("one")

elif x == "1":

if int(x) > 1:

print("two")

elif int(x) < 1:

print("three")

else:

print("four")

if int(x) == 1:

print("five")

else:

print("six")

four

five

Exercise 6

What is the output of the following snippet?

x = 1
y = 1.0
z = "1"

if x == y:
  print("one")
if y == int(z):
  print("two")
elif x == y:
  print("three")
else:
  print("four")

x = 1

y = 1.0

z = "1"

if x == y:

print("one")

if y == int(z):

print("two")

elif x == y:

print("three")

else:

print("four")

one

two

Ex. 7

What is the output of the following code snippet?

incipit = "It was a dark and stormy night"

if 'it' in incipit:
  print('one')
elif 'ht"' in incipit:
  print('two')
else:
  print('three')

incipit = "It was a dark and stormy night"

if 'it' in incipit:

print('one')

elif 'ht"' in incipit:

print('two')

else:

print('three')

three

The program executes the else statement and outputs three to the console.
The if condition is False because the lowercase substring 'it' is not present in incipit.
The elif condition is also False, because the substring 'ht"'includes a quotation mark that is not part of the string contained in the incipit variable.

Ex. 8

What is the output of the following code snippet?

a = 3
b = '3'
c = 3.0

if int(b) == a:
  print('A')

if a == b:
  print('B')

if c == a:
  print('C')

a = 3

b = '3'

c = 3.0

if int(b) == a:

print('A')

if a == b:

print('B')

if c == a:

print('C')

Correct answer