List comprehension

Instead of for loops, we should use list comprehension, equivalently,

[expression for item in iterable if condition == True]

Why? so simple. Let’s just assume that we want to print out all the letters in ‘google’ as a list.

If we use for loop,

a = []

for letter in "google":
  a.append(letter)

On the other hand, with the list comprehension,

a = [letter for letter in "google"]

As we can see clearly, the latter is simpler and it is definitely powerful when the function gets more complex. Thus, we should be accustomed to the list comprehension.

Let’s practice with some examples.

l = [22, 13, 45, 50, 98, 69, 43, 44, 1]
j = [12, 42, 52, 13, 24, 53, 94, 24, 25]
k = [['A', 'B', 'C'] ,['D', 'E', 'F'], ['G', 'H', 'I']] 1. Subtract elements smaller than 30 in l  

   [x for x in l if x < 30]

the output is,

    [22, 13, 1]

Tell if an element is smaller than 30 in l l

[x < 30 for x in l]

the output is,

 [True, True, False, False, False, False, False, False, True]

Extract elements in l which are bigger than those in j element-wisely
```
[x for i, x in enumerate(l) if x > j[i]]
```
the output is,
```
  [22, 50, 98, 69, 44]
```

Return element+10 if it is bigger than j element-wisely, otherwise return element-10

[x+10 if x > j[i] else j[i]-10 for i, x in enumerate(l)]

the output is,

[32, 32, 42, 60, 108, 79, 84, 54, 15]

Change the middle value of each nested list in k to lower case

[[val.lower() if i ==1 else val for i, val in enumerate(x)] for x in k]

the output is,

  [['A', 'b', 'C'] ['D', 'e', 'F'], ['G', 'h', 'I']]

List comprehension

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