adds while loop

learning_python3.md

@@ -1072,7 +1072,155 @@ There are a couple of things you should definitely read up on.
 
 # While loop 
 
-TODO guess the number exercise 
+We started our python journey with fully linear code.
+Next we saw functions which are first **defined** and called afterwards.
+Now we'll have a look at **loops**.
+In python there are **two** types of loops, a **while** and a **for** loop.
+We'll start with the while loop which I see as a loop in *time*.
+The for loop is a loop in *space* but we'll get to that one later.
+
+The concept of a while loop is pretty simple.
+Code **within** the loop will be executed as long as a **condition** is met.
+Consider the code below.
+
+```python3
+import time
+
+counter = 0
+print("before the loop, counter: {}".format(counter))
+
+while counter <= 10:
+	print("inside the loop, counter: {}".format(counter))
+	counter += 1
+	time.sleep(1)
+
+print("after the loop, counter: {}".format(counter))
+```
+
+Two *extra* things might look new to you here.
+First the `import time` and `time.sleep(1)`, can you tell me what it does?
+Next the `counter += 1` which is called [incrementing](https://stackoverflow.com/questions/1485841/behaviour-of-increment-and-decrement-operators-in-python).
+You'll find this feature in most languages.
+You can think of it's syntax as *counter equals itself plus 1*.
+The *1* can be any number you want though!
+
+When learning the `while` [keyword](https://docs.python.org/3/reference/compound_stmts.html#the-while-statement) there is a *second* keyword you should learn.
+It comes in very handy when constructing [infinite loops](https://en.wikipedia.org/wiki/Infinite_loop).
+Consider the following code.
+
+```python3
+import time
+
+counter = 0
+print("before the loop, counter: {}".format(counter))
+
+while True:
+	print("inside the loop, counter: {}".format(counter))
+	counter += 1
+	time.sleep(1)
+
+print("after the loop, counter: {}".format(counter))
+```
+
+The `while True` condition is *always* `True` so the loop will **never** exit!
+This is what we call an infinite loop.
+The `break` keyword was added to the language so we can *break out* of a loop.
+The logic is as follows.
+
+```python3
+import time
+
+counter = 0
+print("before the loop, counter: {}".format(counter))
+
+while True:
+	print("inside the loop, counter: {}".format(counter))
+	counter += 1
+	if counter >= 10:
+		print("I'll break now!")
+		break
+	time.sleep(1)
+
+print("after the loop, counter: {}".format(counter))
+```
+
+Infinite loops are a cornerstone of modern programming.
+While they might look scary, don't overthink it, you'll get used to them very quickly.
+
+⛑ **When testing out an infinite loop it's sometimes handy to insert a `time.sleep` in it to slow down the execution a bit so you can wrap your head around what's happening.**
+
+🏃 Try it
+---
+
+Go back to the Celsius to Farenheit converter and add a while loop to ensure the user put's in only numbers.
+
+
+# Coding challenge - Guess the number
+
+Now that you know how to repeat code execution we can create our first game!
+Everybody knows the *guess the number* game.
+The computer chooses a *random* number and the user has to *guess* which number it is.
+At each try the computer will till you if the user's number is bigger or smaller *than* the one the computer has in mind.
+The flow of the game could be as follows.
+
+```
+I have a number in mind...
+What's your guess? 50
+my number is bigger
+What's your guess? 80
+my number is smaller
+What's your guess? blabla
+that's not a number! try again...
+What's your guess? 76
+yes, that's right! you win!
+bye bye...
+```
+
+<details>
+  <summary>Spoiler warning</summary>
+
+```python3
+import random
+
+
+def ask_for_number():
+    result = input("What's your guess? ")
+    if result.isdigit():
+        number = int(result)
+        return number
+    else:
+        return None
+
+
+if __name__ == "__main__":
+    number_to_guess = random.randint(0, 100)
+    print("I have a number in mind...")
+    while True:
+        user_number = ask_for_number()
+        if user_number is None:
+            print("that's not a number! try again...")
+            continue
+        elif number_to_guess == user_number:
+            print("yes, that's right! you win!")
+            break
+        elif number_to_guess > user_number:
+            print("my number is bigger")
+        elif number_to_guess < user_number:
+            print("my number is smaller")
+    print("bye bye...")
+```
+
+</details>
+
+🏃 Try it
+---
+
+My *solution* is very basic.
+Think of some ways to improve on it.
+Can you limit the number of tries?
+Can you add a feature to let the user play a *second* game after he/she wins or loses?
+Coming up with challenges is on of the most *challenging* aspect op learning how to program.
+Your thought process will send you of into unknown territory and will force you to expand you knowledge!
 
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