reading-notes

List Comprehensions

for the reading task for this class we go through some resocres about the lists handling , This is a very important subject , because after reading and skimming the ideas ,examples, etc…. I figure out we can do A lot of functoinllty by using thses information This is way I will show you the most important ideas and some examples also with some description.

• Old way example:

  new_list = []
  for i in old_list:
    if filter(i):
        new_list.append(expressions(i))
  

• New way example:

  new_list = [expression(i) for i in old_list if filter(i)]
  

• Basic syntax:

  [ expression for item in list if conditional ]
  

• Equiv to:

  for item in list:
    if conditional:
        expression
  

• Breaking the below example down:

  new_list = [expression(i) for i in old_list if filter(i)]
  

• new_list: The new list (result).
• expression(i): Expression is based on the variable used for each element in the old list.
• for i in old_list: The word for followed by the variable name to use, followed by the word in the old list.
• if filter(i): Apply a filter with an If-statement.
• Another example: ``` new_range = [i * i for i in range(5) if i % 2 == 0]

Which corresponds to:

result = [transform iteration filter ]

- Creating a simple list:

x = [i for i in range(10)] print x

This will give the output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

- Create a list using loops and list comprehension:

You can either use loops:

squares = []

for x in range(10): squares.append(x**2)

print squares [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Or you can use list comprehensions to get the same result:

squares = [x**2 for x in range(10)]

print squares [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

- Multiplying parts of a list

list1 = [3,4,5]

multiplied = [item*3 for item in list1]

print multiplied [9,12,15]

- Show the first letter of each word

listOfWords = [“this”,”is”,”a”,”list”,”of”,”words”]

items = [ word[0] for word in listOfWords ]

print items

- Lower/Upper case converter

[x.lower() for x in [“A”,”B”,”C”]] [‘a’, ‘b’, ‘c’]

[x.upper() for x in [“a”,”b”,”c”]] [‘A’, ‘B’, ‘C’]

- Print numbers only from a given string

string = “Hello 12345 World” numbers = [x for x in string if x.isdigit()] print numbers

[‘1’, ‘2’, ‘3’, ‘4’, ‘5’]

- Parsing a file using list comprehension

In this example, we can see how to get specific lines out from a text file.

Create a text file and put in some text in it.

this is line1 this is line2 this is line3 this is line4 this is line5

Save the file as test.txt

Then create the filter by using list comprehension:

fh = open(“test.txt”, “r”)

result = [i for i in fh if “line3” in i]

print result Output: [‘this is line3 ‘]

- Using list comprehension in functions

Now, let’s see how we can use list comprehension in functions.

Create a function and name it double:

def double(x): return x*2

If you now just print that function with a value in it, it should look like this:

print double(10) 20 We can easily use list comprehension on that function.

[double(x) for x in range(10)]

print double [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

You can put in conditions:

[double(x) for x in range(10) if x%2==0] [0, 4, 8, 12, 16]

You can add more arguments:

[x+y for x in [10,30,50] for y in [20,40,60]] [30, 50, 70, 50, 70, 90, 70, 90, 110] ```

Things I want to know more about

1- as usual Ineed to know and understand BigO fully without any mistakes. 2- I know this is far from the reading for this class but the thing I want to know more and more about the linked-list..