# if, else, and elif Statements

Site: | Saylor Academy |

Course: | CS105: Introduction to Python |

Book: | if, else, and elif Statements |

Printed by: | Guest user |

Date: | Friday, June 21, 2024, 12:00 AM |

## Description

Read this for more on conditional 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!**

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