CODEWITHSUNDEEP
pythonsyntax
Hummus
Hummus

Reputation: 365

Complex syntax- Python

I am quite new to programming and don't understand a lot of concepts. Can someone explain to me the syntax of line 2 and how it works? Is there no indentation required? And also, where I can learn all this from?

string = #extremely large number

num = [int(c) for c in string if not c.isspace()]

Upvotes: 7

Views: 664

Answers (4)

John La Rooy
John La Rooy

Reputation: 304185

These examples from the PEP are a good place to start. If you are not familiar with range and % you need to take a step back and learn more about the foundations.

>>> print [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> print [i for i in range(20) if i%2 == 0]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

>>> nums = [1,2,3,4]
>>> fruit = ["Apples", "Peaches", "Pears", "Bananas"]
>>> print [(i,f) for i in nums for f in fruit]
[(1, 'Apples'), (1, 'Peaches'), (1, 'Pears'), (1, 'Bananas'),
 (2, 'Apples'), (2, 'Peaches'), (2, 'Pears'), (2, 'Bananas'),
 (3, 'Apples'), (3, 'Peaches'), (3, 'Pears'), (3, 'Bananas'),
 (4, 'Apples'), (4, 'Peaches'), (4, 'Pears'), (4, 'Bananas')]
>>> print [(i,f) for i in nums for f in fruit if f[0] == "P"]
[(1, 'Peaches'), (1, 'Pears'),
 (2, 'Peaches'), (2, 'Pears'),
 (3, 'Peaches'), (3, 'Pears'),
 (4, 'Peaches'), (4, 'Pears')]
>>> print [(i,f) for i in nums for f in fruit if f[0] == "P" if i%2 == 1]
[(1, 'Peaches'), (1, 'Pears'), (3, 'Peaches'), (3, 'Pears')]
>>> print [i for i in zip(nums,fruit) if i[0]%2==0]

Upvotes: 0

Jason White
Jason White

Reputation: 686

Just to expand on mgilson's answer as if you're fairly new to programming, that can also be a bit obtuse. Since I started learning python a few months back, here's my annotations.

string = 'aVeryLargeNumber'
num = [int(c) for c in string if not c.isspace()] #list comprehension

"""Breakdown of a list comprehension into it's parts."""
num = [] #creates an empty list
for c in string: #This threw me for a loop when I first started learning
     #as everytime I ran into the 'for something in somethingelse':
     #the c was always something else. The c is just a place holder
     #for a smaller unit in the string (in this example). 
     #For instance we could also write it as:
     #for number in '1234567890':, which is also equivalent to 
     #for x in '1234567890': or 
     #for whatever in '1234567890'
             #Typically you want to use something descriptive.
     #Also, string, does not have to be just a string. It can be anything
     #so long as you can iterate (go through it) one item at a time
     #such as a list, tuple, dictionary.

if not c.isspace(): #in this example it means if c is not a whitespace character
        #which is a space, line feed, carriage return, form feed,
                #horizontal tab, vertical tab.

num.append(int(c))  #This converts the string representation of a number to an actual
        #number(technically an integer), and appends it to a list. 

'1234567890' # our string in this example
num = []
    for c in '1234567890':
        if not c.isspace():
            num.append(int(c))

The first iteration through the loop would look like:

num = [] #our list, empty for now
    for '1' in '1234567890':
        if not '1'.isspace():
            num.append(int('1'))

Note the ' ' around the 1. Anything between ' ' or " " means this item is a string. Although it looks like a number, as far as Python is concerned it isn't. An easy way to verify that is to type 1 + 2 in the interpreter and compare the result to '1' + '2'. See a difference? With numbers it adds them together as you'd expect. With strings it joins them together.

On to the second pass!

num = [1] #our list, now with a one appended!
    for '2' in '1234567890':
        if not '2'.isspace():
            num.append(int('2'))

And so it will continue until it runs out of characters in the string, or it produces an error. What would happen if the string was '1234567890.12345'? We can safely say that '.' isn't a whitespace character. So when we get down to int('.') Python is going to thrown an error:

Traceback (most recent call last):
    File "<string>", line 1, in <fragment>
builtins.ValueError: invalid literal for int() with base 10: '.'

As far as resources for learning Python, there's a lot of free tutorials such as:

http://www.learnpython.org

http://learnpythonthehardway.org/book

http://openbookproject.net/thinkcs/python/english3e

http://getpython3.com/diveintopython3

If you want to buy a book for learning then: http://www.amazon.com/Learning-Python-Powerful-Object-Oriented-Programming/dp/0596158068 is my favourite. Not sure why the ratings are low but I think the author does an excellent job.

Good luck!

Upvotes: 1

Karl Knechtel
Karl Knechtel

Reputation: 61526

It means exactly what it says.

num   =        [          int(                      c)

"num" shall be a list of: the int created from each c

for   c                           in string

where c takes on each value found in string

if        not                     c .isspace() ]

such that it is not the case that c is a space (end of list description)

Upvotes: 4

mgilson
mgilson

Reputation: 309929

That is a list comprehension, a sort of shorthand for creating a new list. It is functionally equivalent to:

num = []
for c in string:
    if not c.isspace():
       num.append(int(c))

Upvotes: 14

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