# if Statements In Python

#### A SIMPLE EXAMPLE

**What we want to do?** We have a list of `cars` that contains the name of cars in *lowercase* letter. We want to print the names of all in *title case* expect from ‘byd’, which should be all in *upper case* To achieve this we will combine a `for` loop with `if-else` statements

```python
cars = ['toyota','honda','byd','buick']
for car in cars:
    if car == 'byd':
        print(car.upper())
    else:
        print(car.title())
```

```
Toyota
Honda
BYD
Buick
```

**How it works?** The loop in this example first checks `if` the current value of `car` is ‘byd’ .If it is, the value is printed in uppercase. `else` (If the value of `car` is anything other than ‘byd’) it’s printed in title case.

## 1. CONDITIONAL TESTS

At the heart of every `if` statement is an expression that can be evaluated as *True* or *False* and is called a **conditional test**

* If a conditional test evaluates to *True*, Python executes the code following the `if` statement.
* If the conditional test evaluates to *False*, Python ignores the code following the `if` statement. If we have provided `else` statement, then that code will be executed

### 1.1. Checking for Equality

Single equal sign `=` is used *to assign* a value to a variable, whereas double equal sign `==` is `to check` for equality — this equality operator returns *True* if the values on the left and right side of the operator match, and *False* if they don’t match.

```python
car = 'BYD'
car == 'byd'
```

```
False
```

#### Ignoring Case When Checking for Equality

Testing for equality is case sensitive in Python as we seen above

```python
car = 'BYD'
car.lower() == 'byd'
```

```
True
```

This test would return *True* no matter how the value ‘BYD’ is formatted because the test is now case insensitive.

### 1.2. Checking for Inequality

When you want to determine whether two values are *not equal*, we use the conditional operator `!=` — the exclamation point represents *not*

```python
username = 'thor'
if username != 'spiderman':
    print('Sorry, Only Spiders are welcomed!')
```

```
Sorry, Only Spiders are welcomed!
```

### 1.3. Numerical Comparisons

We can compare the numerals the same way as we did the strings.

```python
your_answer = 95
correct_answer = 100
if your_answer == correct_answer:
    print("You've passed the test.")
else:
    print("Your answer is wrong.")
```

```
Your answer is wrong.
```

#### Using various mathematical operators

We can use following comparison operators :

| Operator | Meaning                  |
| -------- | ------------------------ |
| >        | greater than             |
| <        | less than                |
| >=       | greater than or equal to |
| <=       | less than or equal to    |

### 1.4. Checking Multiple Conditions

We can use `and` or `or` to check to check the *True* or *False* status of multiple conditions:

* `and` — when you might need two conditions to be True to take an action
* `or` — when you might need either of two conditions to be True to take an action

#### Using `and` condition

To check whether two conditions are *True* simultaneously, we use the keyword `and` to combine the two conditional tests.

* If both conditional tests pass, the overall expression evaluates to *True*
* If either or both conditions fail, the expression evaluates to *False*.

```python
age_1 = 15
age_2 = 21
if age_1 >= 18 and age_2 >= 18:
    print("You are elible to register as team.")
else:
    print("Sorry, you are not eligible to register.")
```

```
Sorry, you are not eligible to register.
```

To improve readability of our code, we can use parentheses `( )` around the individual tests, but they are not required

```
if (age_1 >= 18) and (age_2 >= 18):
```

#### Using `or` condition

The `or` condition returns *True* when either or both of the individual tests pass. It will return *False* only when both individual tests fail.

```python
age_1 = 15
age_2 = 21
if age_1 >= 18 or age_2 >= 18 : 
    print("You are elible to register as team.")
else:
    print("Sorry, you are not eligible to register.")
```

```
You are elible to register as team.
```

### 1.5. Checking Whether a Value is `in` a List

Sometimes, it’s important to check whether a list contains a certain value before taking an action. To find out whether a particular value is already in a list, we use the keyword `in`.

```python
guests = ['ironman','spiderman','superman']
on_door = 'antman'
if on_door in guests:
    print(f"Welcome to the Club, {on_door.title()}")
else:
    print(f"Sorry {on_door.title()}, you are not in the guests list")
```

```
Sorry Antman, you are not in the guests list
```

### 1.6. Checking Whether a Value is `not in` a List

Other times, we would like to know if a value does not appear in a list. In this case, we can use the keyword `not in` this situation.

```python
banned_codes = ['code5','code10','code15']
user_code = 'code10'
if user_code not in banned_codes:
    print("Your code is valid")
else:
    print("Your code is invalid")
```

```
Your code is invalid
```

### 1.7. Boolean Expressions

A Boolean expression is just another name for a conditional test whose value is either *True* or *False*. We have studied earlier in part 1 about Boolean data type and the same logic applies here.

```python
has_arrived = True
if has_arrived:
    print("Welcome to the club")
```

```
Welcome to the club
```

## 2. Types of if STATEMENTS

There are few types of `if` statements, and your choice of which to use depends on the number of conditions you need to test.

### 2.1 Simple `if` Statements

Some basic examples of `if` statements have been discussed, the general syntax of `if` statement is as follows:

```python
if conditional_test: 
	things to do something
```

**Logic** → If the conditional test evaluates *True*, Python executes the code following the if statement. If the test evaluates to *False*, Python ignores the code following the if statement

```python
user_age = 31
required_age = 21
if user_age >= required_age:
    print("You are elible to enter the arena")
```

```
You are elible to enter the arena
```

NOTE: All indented lines after an `if` statement will be executed if the test passes, and ignored if the test fails

### 2.2. `if-else` Statements

An `if- else` block is similar to a simple `if` statement, but the `else` statement allows you to execute some code when the conditional test under `if` fails

```python
user_age = 17
required_age = 21
if user_age >= required_age:
    print("You are elible to enter the club")
else:
    print("You are not eligible to enter the club")
```

```
You are not eligible to enter the club
```

### 2.3. The if-elif-else Chain

> Often, you’ll need to test more than two possible situations, for these situations you can use Python’s `if- elif- else` syntax.

Remember that Python executes *only one* block in an `if- elif- else chain` depending on which condition is *True*. That is, it runs each conditional test in order until one passes, when a test passes, the code following that test is executed and Python skips the rest of the tests For example, the price for museum varies with age:

* For kids under 4 age, price is $0
* For kids and teen above 4 years and less than 18 years, ticket price is $5
* For anyone above 18 years, price is $10

```python
entrant_age =12

if entrant_age < 4:
    price = 0
elif entrant_age < 18:
    price = 5
else:
    price = 10

print(f"Your price is ${str(price)}")
```

```
Your price is $5
```

You can see how the logic of `if-elif-else` works :

* for `elif` code block, we don’t need to mention that price is $5 for age greater than 4 years but less than or equal to 18 years. Because, if the `if` condition passes, the elif and else code block will be ignored
* `else` block serves as fallback, if none of above conditions are *True*

### 2.4. Using Multiple elif Blocks

> To check multiple cognitions , you can use as many `elif` blocks in your code as you like

For example, for the above example, we can create another age group — price is $2 for anyone with age greater than 60 years

```python
entrant_age =65

if entrant_age < 4:
    price = 0
elif entrant_age < 18:
    price = 5
elif entrant_age < 60:
    price = 10
else:
    price = 2

print(f"Your price is ${str(price)}")
```

```
Your price is $2
```

### 2.5. Omitting the else Block

> Python does not require an `else` block at the end of an `if- elif` chain.

Sometimes an *else block* is useful; sometimes it is more meaningful to use an additional *elif* statement that catches the specific condition of interest, in this way you exhaust all the available options that your program should run on:

```python
entrant_age =65

if entrant_age < 4:
    price = 0
elif entrant_age < 18:
    price = 5
elif entrant_age < 60:
    price = 10
elif entrant_age >=60:
    price = 2

print(f"Your price is ${str(price)}")
```

```
Your price is $2
```

💡The `else` block is a *catchall statement*. It means, it will only be executed if all earlier conditions are *False* , which can lead to invalid results in our code. Therefore, to avoid this situation, if you have a specific final condition you are testing for, consider using a final `elif` block and omit the `else` block.

### 2.6. Testing Multiple Conditions

Until now, we only executed one set of code and ignore all others. However, we can use more than one `if` statements to run more than one block of code. This leads us to define some rules:

* If you want only one block of code to run, use an `if- elif-else chain`
* If more than one block of code needs to run, use a series of independent `if` statements

```python
vips = ['superman','spiderman']

if 'superman' in vips:
    print ("Welcome, Superman")
if 'spiderman' in vips:
    print("Welcome, Spiderman")
if 'antman' in vips:
    print ("Welcome, Antman")

print("\nWelcome to all VIPs to join our event")
```

```
Welcome, Superman
Welcome, Spiderman

Welcome to all VIPs to join our event
```

### 2.7 STYLING YOUR `if` STATEMENTS

PEP 8 recommends to use a single space around comparison operators:

```python
if code > 8
```

is better than

```python
if code>8
```

## 3. USING if STATEMENTS WITH LISTS

### 3.1 Checking: an item inside list

Let’s first make a list of few items and store it inside the variable `cart_items` Then we can use the `for` loop to print the items inside `cart_items`

```python
cart_items = ['bananas','apples','oranges']

for item in cart_items:
    print (f"Shipping {item.title()}") 

print("\nAll requested items are shipped")
```

```
Shipping Bananas
Shipping Apples
Shipping Oranges

All requested items are shipped
```

#### `if` statement inside the `for` loop

Now we will use the `if-else` statement with in a `for` loop

```python
cart_items = ['bananas','apples','oranges']

for item in cart_items:
    if item == 'apples':
        print("Sorry, apples are out of stock")
    else:
        print(f"Shipping {item.title()}")

print("\nShipped all items")
```

```
Shipping Bananas
Sorry, apples are out of stock
Shipping Oranges

Shipped all items
```

### 3.2. Checking: List Is Not Empty

It’s useful to check whether a list is empty before running a `for` loop. This will avoid getting any errors for performing operation on empty list

```python
cart_items = []

if cart_items:
    for item in cart_items:
        print ("Shipping {item.title()}")
    print("\nAll items are shipped")
else:
    print("Your basket is empty")
```

```
Your basket is empty
```

#### Combining two if-else statements block

We can combine the above two examples in the following way:

```python
cart_items = ['bananas','apples','oranges']

if cart_items:
    for item in cart_items:
        if item == 'apples':
            print("Sorry, apples are out of stock")
        else:
            print(f"Shipping {item.title()}")
    print("\nShipped all items")
else:
    print("Your basket is empty")
```

```
Shipping Bananas
Sorry, apples are out of stock
Shipping Oranges

Shipped all items
```

### 3.3. Using Multiple Lists

We can use two list to check whether content of one list is available `in` second list:

```python
cart_items = ['bananas','apples','oranges']
available_items = ['bananas','oranges','pears']

for item in cart_items:
    if item in available_items:
        print(f"Shipping {item.title()}")
    else:
        print(f"Sorry, {item} is out of stock")

print("\nShipped all items")
```

```
Shipping Bananas
Sorry, apples is out of stock
Shipping Oranges

Shipped all items
```