if, else, and elif Statements

1. If-else branches (general)

In many circumstances when we write a program, we need the ability to check conditions and change the behavior of the program accordingly. 

Selection statements or conditional statements, give us this ability.

Example: Let's look through the following code:

if my_class_average > 1:
print("I passed the class! Hooray!")
else:
print("Bummer! I will have to re-take this class!")

Consider another code fragment:

x = int(input("Enter an integer value:"))
y = int(input("Enter another integer value:"))
if x > y:
a = x
if x < y:
a = y
else:print("They are equal!")

Consider another code fragment:

x = int(input("Enter an integer value:"))
y = int(input("Enter another integer value:"))
if x > y: //conditions (evaluated to a Boolean value: True or False)
a = x
if x < y: //conditions (evaluated to a Boolean value: True or False)
a = y
else:
print("They are equal!")

If we type the following commands in the Python shell, we will get the responses in purple:

>>>2==2
True
>>>
2<3
True
>>>
3>7
False
>>>5>9or2<3
True

---

Source: Natalia Novak, https://academicworks.cuny.edu/bx_oers/34/
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 License.

2. If-else statement

Multi-branch if-else statements

test_score = float(input("Enter test score:"))
if test_score >= 90:
 print("This is an A grade!")
if 80 <= test_score < 90:
 print("This is a B grade!")
if 70 <= test_score < 80:
 print("This is a C grade!")
if 60 <= test_score < 70:
 print("This is a D grade!")
else: print("Unfortunately this is an F grade")

 

test_score = float(input("Enter test score:"))
if test_score >= 90:
 print("This is an A grade!")
if 80 <= test_score < 90:
 print("This is a B grade!")
if 70 <= test_score < 80:
 print("This is a C grade!")
if 60 <= test_score < 70:
 print("This is a D grade!")
else: print("Unfortunately this is an F grade")

 

test_score = float(input("Enter test score:"))
if test_score >= 90:
 print("This is an A grade!")
elif 80 <= test_score < 90: # only one branch will execute!
 print("This is a B grade!")
elif 70 <= test_score < 80:
 print("This is a C grade!")
elif 60 <= test_score < 70:
 print("This is a D grade!")
else: print("Unfortunately this is an F grade")

3. Equality and relational operators

Equality and relational operators

Equality operators

An equality operator checks whether two operands' values are the same (==) or different (!=). 

Note that equality is ==, not just =.

Equality operators	Description	Example (assume x is 3)
==	a == b means a is equal to b	x == 3 is true x == 4 is false
!=	a != b means a is not equal to b	x != 3 is false x != 4 is true

An expression evaluates to a Boolean value.
A Boolean is a type that has just two values: True or False

Relational operators

A relational operator checks how one operand's value relates to another, like being greater than.

Relational operators	Description	Example (assume x is 3)
<	a < b means a is less than b	x < 4 is true x < 3 is false
>	a > b means a is greater than b	x > 2 is true x >3 is false
<=	a <= b means a is less than or equal to b	x <= 4 is true x <= 3 is true x <= 2 is false
>=	a >= b means a is greater than or equal to b	x >= 2 is true x >= 3 is true x >= 4 is false

Operator chaining

Python supports operator chaining.
Example:
a < b < c
determines whether b is greater-than a but less-than c.
Chaining performs comparisons left to right,
evaluating a < b first.

• If the result is true, then b < c is evaluated next.
• If the result of the first comparison a < b is false, then there is
• no need to continue evaluating the rest of the expression.

4. Nested if-else statements

A branch's statements can include any valid statements, including another if-else statement, which are known as nested if-else statements.

if grade >= 90:
  if grade < 93:
    print("that's an A-")
  elif grade >= 97:
    print("that's an A+")
  else:
    print("that's an A")
  else:
    print("not an A grade")

if grade = 78

if grade >= 90:
  if grade < 93:
    print("that's an A-")
  elif grade >= 97:
    print("that's an A+")
  else:
    print("that's an A")
  else:
    print("not an A grade") # not an A grade

if grade = 95

if grade >= 90:
  if grade < 93:
    print("that's an A-")
  elif grade >= 97:
    print("that's an A+")
  else:
    print("that's an A") # that's an A
  else:
    print("not an A grade") 

5. Multiple if statements

For the code snippets below, make sure to note that multiple if statements can execute when a given condition is satisfied.

Consider the following code fragment:

if num >= 10:
   print("A")
if num >= 0:
   print("B")
if num < 0:
   print("C")
if num < -10:
   print("D")

What would the program output if num = 12?

Consider the following code fragment:

if num >= 10:
   print("A")
if num >= 0:
   print("B")
if num < 0:
   print("C")
if num < -10:
   print("D")

What would the program output if num = 1?

Consider the following code fragment:

if num >= 10:
   print("A")
if num >= 0:
   print("B")
if num < 0:
   print("C")
if num < -10:
   print("D")

What would the program output if num = -1?

Consider the following code fragment:

if num >= 10:
   print("A")
if num >= 0:
   print("B")
if num < 0:
   print("C")
if num < -10:
   print("D")

What would the program output if num = -12?

6. Boolean operators and expressions

Booleans and Boolean operators

A Boolean refers to a value that is either True or False. These two are constants in Python. 

• we can assign a Boolean value by specifying True or False, 

x = True

• an expression can evaluate to a Boolean value

y > 10

and operator

The Boolean expression a and b is True if and only if both a and b are True.

a	b	a and b
True	True	True
True	False	False
False	True	False
False	False	False

Examples: assume that a = 8 and b = 3, then the Boolean value of     
1)  ( a > 10 ) and ( b < 5 ) is False
2) ( a != 10 ) and ( b > 1 ) is True

or operator

The Boolean expression a or b is False if and only if both a and b are False.

a	b	a or b
True	True	True
True	False	True
False	True	True
False	False	False

Examples: assume that a = 8 and b = 3, then the Boolean value of     
1)  ( a > 10 ) or ( b < 5 ) is True
2) ( a == 10 ) or ( b > 1 ) is True

not operator

The Boolean expression not a  is False when a is True, and is True when a is False.

a	not a
True	False
False	False

Examples: assume that a = 8 and b = 3, then the Boolean value of     
1) not ( a > 10 ) is True
2) not ( a * 10 > 20 ) is False

Booleans and Boolean operators

Consider the following code fragment:

if letter == 'a' or letter == 'b':
   print("Help!")
elif letter == 'c' or letter == 'd':
   print("We are in trouble!")
else:
   print("We are good!")

If letter = "a", then we will get: Help!

Consider the following code fragment:

if letter == 'a' or letter == 'b':
   print("Help!")
elif letter == 'c' or letter == 'd':
   print("We are in trouble!")
else:
   print("We are good!")

If letter = "c", then we will get: We are in trouble!

7. Order of evaluation

Precedence rules

The order in which operators are evaluated in an expression is known as precedence of operators.

operator	description	Example
()	parentheses are evaluated first	(2+5*3) - (5/6+2*4)
**  //  %  /  *  -  +	arithmetic operations next (in order)	10-2**5 >= 10%7
<  <=   >   >=   ==   !=	then comparisons and membership operators	a > 9 and b in [1, 2, 3]
not	negation operator next	not (a > 9) or b == 2
and	conjunction (and) next	a > 9 or a < 0 and b > 1
or	disjunction (or) last	a > 9 or a < 0 and b > 1

 

Example: Let's evaluate the Boolean expression below for:

• g = 12, b = True, and a = 17
• g >= 90 or b and a > 10
• (g >= 90) or (b and a > 100
• (g >= 90) or (b and a > 100)
• F or (T and F)
• F or F

8. Membership and identity operators

Membership operators: in/not in

Quite often we need to check if a value can be or cannot be found within a container, such as a list or dictionary. 

in and not in operators, known as membership operators, can help us!

Example:

num = int(input("Enter an integer:"))
myContainer = [1,2,3,4,5,6,7]
if num in myContainer:
	print("Found it! It is in myContainer!")
else: print("Nope. It is not in myContainer.")

Example:

name = input("Enter a name:")
MyNamesContainer = {
"Maria" : 23,
"Anna" : 19,
"Jack" : 5,
"Alex" : 12,
"John" : 18}
if name in MyNamesContainer:
  print("Found it! It corresponds to", MyNamesContainer[name])
else: 
  print("No such name in the container.")

** Note that the keys are matched, not the values!

Identity operators: is/is not

Sometimes we want to determine whether two variables are the same object.

is and is not operators, known as identity operators, can help us out!
Identity operators return True only if the operands reference the same object (they do not compare object's values).

Example:

myContainer = [1,2,3,4,5,6,7]
otherContainer = [9,8,7,6,5,4,3,2,1]
a = myContainer
b = otherContainer
a = b
if a is myContainer:
	print("a is myContainter!")
elif a is otherContainer:
	print("a is otherContainter!")
else: 
	print("I have no idea what a is!")

9. Code blocks and indentation

Consider the following code fragment:

a = int(input("Enter a value:"))
if a > 5:
# Code blocks
      myString = input("Enter a word:")
      print(myString*a)
else:
# Code blocks  - 3-4 Spaces,  Tab: 3 spaces
      myNum = int(input("Enter an integer:"))
      print(myNum-a)
print("That's it!")

* Caution: be consistent! Either use 4 spaces or a Tab (3 spaces)

Consider the following code fragment:

a = 3

a = int(input("Enter a value:"))
if a > 5:
      myString = input("Enter a word:")
      print(myString*a)
else:
      myNum = int(input("Enter an integer:"))
      print(myNum-a)
print("That's it!")

Enter an integer: 10
7
That's it!

Consider the following code fragment:

a = 6

a = int(input("Enter a value:"))
if a > 5:
      myString = input("Enter a word:")
      print(myString*a)
else:
      myNum = int(input("Enter an integer:"))
      print(myNum-a)
print("That's it!")

Enter a word: mymymymymymymy
That's it!

10. Conditional expressions

A conditional expression has the following form:

<expr_t> if <condition> else <expr_when_f>

Example:

x=5 if a<10 else x=6