"List Comprehensions" in python is a concept used to construct lists in a easy way, like a mathematician does on a piece of paper. These listcomp (I'll be using listcomp instead of List Comprehensions in the remaining of my description) are useful when there is a requirement of doing certain kind of operations on all the list elements. Say that you are having list 'a' as shown below:
Using listcomp we can do that easily.
Let us take another example. Say we have 2 lists 'a' and 'b' and wish to build another list 'c' containing tuples of 'a' and 'b' i.e.,
Note: I've used 3.3 python interpreter to run the code
>>>a = [1,2,3,4,5,6] >>>You want to create another list b which contains squares of elements in list a. We can do that using the below python script:
a = [1,2,3,4,5,6]
for x in a:
b.append(x*x)
print(b)
Using listcomp we can do that easily.
a = [1,2,3,4,5,6] b = [ x*x for x in a] print(b)Listcomp conssts of an expression followed by a for clause, then zero or more for or if clauses. The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it.
>>>a = [1,2,3] >>>b = [10,20,30] >>>c = [(1,10),(1,20),(1,30),(2,20),(2,30),(2,30),(3,10),(3,20),(3,30)]This can be done using listcomps as below:
a = [1,2,3]
b = [10,20,30] c = [(x,y) for x in a for y in b if x!= y ] print(c)Let us look into some complex listcomps. Observe the below piece of code
nonprimes = [j for i in range(2, 8) for j in range(i*2,50,i)] #above listcomp creates a list with all nonprime numbers between 2 and 50 primes = [x for x in range(2, 50) if x not in noprimes] #above listcomp creates a list all prime numbers between 2 and 50 by checking # into nonprimes list print(primes)The comments in the above code snippet explains the liscomp generated. We can even make the above code simple by using nested listcomps:
primes = [x for x in range(2, 50) if x not in [j for i in range(2, 8) for j in range(i*2, 50, i)]] print(primes)As shown above we can build very complex lists using listcomps. In coming post I'll discuss how to convert the listcomp to iterator loops. Happy coding....:)
Note: I've used 3.3 python interpreter to run the code
