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Logical conditions in Python explained

Introduction to conditionals in Python
Logical Operators in Python
  • AND & OR Operators in Python

Introduction to conditionals in Python

There are 5 skills you need to master in your tool set for building algorithms with python:

  1. Variables.
  2. Conditionals.
  3. Lists (or arrays).
  4. Loops.
  5. Functions.

The use of conditionals is the easiest way we have to make decisions when coding our algorithms.

Let's say we are building a program to help us pick what to wear, but we hate the blue color, we could tell the computer to avoid that color using a statement like this:

1if color == 'blue': 2 #do something 3else: 4 #do something else or do nothing

Condicionales

🔗 Using switch is not available in python.

What is a logical expression in Python?

Logical expressions let you conditionally skip a series of lines of your code. The best way to understand them, is to think of them like questions you can ask the computer about your variables, for example:

  1. if user_age > 21:
  2. if day == "tuesday"
  3. if car_model == "toyota" and number_of_tires == 6:

In order to ask a question, or conditionally excuse a particular set of lines, you first need to have data (information) stored on useful variables, above we had the variables user_age, day, car_model and number_of_tires.

If we don't have the information pre-stored in variables we are not able to ask any questions, it is all about strategy and planning!

For example, if we have the user's age stored in a variable age then, and only then, we are able to code for something like:

1# We use two equal signs (==) when we want to compare two variables for equality 2if age == 21: 3 print("You are old enough!!")

What type of conditions/questions can we use/ask?

The previous example was a simple condition, but in real life picking what to wear involves a combination of several conditions to make the final decision, for example: Let's look at this algorithm that tells you if you have the flu

Hit by car or have a flu

If you want to represent this algorithm in Python, it will look something like this:

1feels_like_hit_by_train = True 2you_were_hit_by_train = False 3 4if feels_like_hit_by_train == True: 5 if you_were_hit_by_train == True: 6 print("You don't have a flu") 7 else: 8 print("You have a flu") 9else: 10 print("You don't have a flu")

Basically, this algorithm has two variables to consider: feels_like_hit_by_train and you_were_hit_by_train. Our job as developers is to sit down and try to prepare a strategy and come up with an algorithm that solves a problem.

Logical Operators in Python

Mostly all the questions can be asked using the following comparisons: ==, >, <, !=, is None, is not None, in:

OperatorExampleDescription
==if a == bif the value of variable A is identical to the value of B (same data-type and value)
<if a < bif the value of variable A is less than the value of B
>if a > bif the value of variable A is greater than the value of B
!=if a != bif the value of variable A is different (not identical) from the value of B
is not Noneif a is not NoneThis is self-explanatory, isn't it?
is Noneif a is NoneThis is self-explanatory, isn't it?
inif name in ['bob','maria','nancy']If the value of name is contained inside the list of names

AND & OR Operators in Python

Another way to write the algorithm is to combine questions in the same condition using the AND and OR:

1feels_like_hit_by_train = True 2you_were_hit_by_train = False 3 4if feels_like_hit_by_train and you_were_hit_by_train: 5 print("You don't have a flu") 6elif feels_like_hit_by_train: 7 print("You have a flu")

As you can see we are using elif together for the first time, for faster coding. Another trick you can use for faster coding:

OriginalEquivalent
instead of if(feels_like_hit_by_train == true)you write if(feels_like_hit_by_train)
instead of if(you_were_hit_by_train == false)you write if(!you_were_hit_by_train)

If...else in Python

You can also use the else expression to refer to the negation of the first condition:

1if color == "blue": 2 # Discard this clothing item 3else: 4 # Put it in your closet 5 6age = 12 7if age < 18: 8 print("Old enough") 9else: 10 print("Not old enough")

You can also nest several if...else conditions on top of one another, like this:

1if age < 16: 2 # You cannot do anything 3elif age < 18: 4 # At this point, we know it's older than 15 because if not it would have entered 5 # into the first condition 6elif age < 21: 7 # If the algorithm enters here, we know its older than 17 8else: 9 # If the algorithm enters here, we know its older than 20

Here is another example that runs an algorithm to find out if a number is in the "hundreds".

1value = 14 2 3if value < 10: 4 print("Single unit value") 5elif value < 100: 6 print("dozens") 7elif value < 1000: 8 print("hundreds") 9elif value < 10000: 10 print("thousands") 11else: 12 print("hundrends of thousands, or maybe more")

The switch statement in Python

Python does not have a switch statement.

Conclusion

It's all about what question to ask. The previous example was a simple condition, but real life is not that simple. There are lots of nested conditions and complicated flows that will challenge your skills to the limit. For example:

This will be the algorithm to pick what to wear on Valentine's Day:

What to ware in valentine's day

1if going_out: 2 if can_I_get_burger: 3 if place_bottle_white: 4 if cool_mix: 5 # do something 6 else: 7 if blazers > 3: 8 # do something 9 else: 10 # do something 11 elif she_pants: 12 # do something 13 else: 14 # do something 15else: 16 if naked_she_door: 17 # do something 18 elif blazers > 3: 19 # do something 20 else: 21 # do something