Update list-comprehension.md

Added information to list-comprehension.md

Author: myckellr

basics/list-comprehension.md

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+# List Comprehension
+
+
+## Definition
+List comprehensions creates a new list from an existing list that is a shorter syntax than normal. It also processes the list much faster than using a for loop.
+
+
+## Without List Comprehension
+```python
+animals = ["dog", "cat", "giraffe", "donkey", "ape"]
+arr = []
+
+for i in animals:
+  if len(i) == 3:
+    arr.append(i)
+    
+print(arr)
+```
+```python
+['dog', 'cat', 'ape']
+```
+The example above shows a list of animals and the for loop iterates through the list of animals and the if statement filters the list and only adds the animal if their name is exactly 3 characters long.
+
+## With List Comprehension
+```
+animals = ["dog", "cat", "giraffe", "donkey", "ape"]
+
+arr = [i for i in animals if len(i) == 3]
+print(arr)
+```
+```python
+['dog', 'cat', 'ape']
+```
+The example above does exactly the same thing but is in a much more concise syntax where it iterates through the list of animals and appends to the new list if their name is exactly 3 characters long.
+
+## Syntax
+
+newlist = [**expression** for **item** in **iterable** if **condition**]
+This is the usual form in which list comprehension is used.
+
+**Expression:** It is the current item for each iteration but also serves as the outcome which can be manipulated before it becomes added to the new list.
+```python
+animals = ["DOG", "CAT", "GIRAFFE", "DONKEY", "APE"]
+
+arr = [i.lower() for i in animals if len(i) == 3]
+print(arr)
+```
+```python
+['dog', 'cat', 'ape']
+```
+The **i** is essentially an animal each iteration where it checks if the animal's name is 3 characters long and if it does it **also** changes that animal name to all lower case before it finally appends or adds it to the list.
+
+
+