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()]
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))
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)
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://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!
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]
for c in stringandif not c.isspace()to their own lines, they should line up with theint(c). If you also moveint(c)to a new line, it has to be indented farther than thenum, and the other lines have to match its indentation.stringisn't a great variable name, because it's the name of a module in the standard library. (And presumably it's not actually an extremely large number, but an extremely long string of digits; otherwise you'll get an error because'int' object is not iterable.)