> For the complete documentation index, see [llms.txt](https://codingnotes.gitbook.io/coding_notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://codingnotes.gitbook.io/coding_notes/coding/python/if-statements-in-python.md). # 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 ``` --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://codingnotes.gitbook.io/coding_notes/coding/python/if-statements-in-python.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.