Can someone please explain in detail the below type of for loop?
primes = [x for x in range(2, 50) if x not in noprimes]
or
le_list = [i for i in getfiles(path) if i != 'fred']
The bit i dont get is the i for i, or x for x. I do not understand what its actually saying.
variable for variable in a list if variable whatever.
I dont see why you need variable twice at the start. The for loop goes through each item in list anyway, how is it treated differently?
The reason for the apparently redundant extra mention of the variable x when writing x for x is that the first x does not need to be x. It just happens to be in the examples you give. Here are a few more examples which should clarify the difference between the first and second x in your question:
ones = [1 for x in range(10)]
This simply gives a list of 10 ones, the same as [1] * 10.
squares = [x*x for x in range(10)]
This gives x squared for each x in the specified range.
In your example, the second x is the variable used by the for loop, and the first x is simply an expression, which happens in your case to be just x. The expression can be whatever you like, and does not need to be in terms of x.
results = [expression for x in range(10)]
expression can include anything you like - a string, a calculation, a function - whatever you choose. If the expression happens to be just x then it looks unusual if you are not used to it, but it's the same as the following:
results = []
for x in range(10):
results.append(expression)
The good way to understand it is to read it a bit different. So if we take your piece of code:
primes = [x for x in range(2, 50) if x not in noprimes]
We can read is as:
Primes = All "x for" which "x in range(2,50)" but only "if x not in noprimes"
I hope this helps you understand the functionality better.
It is a placeholder for a transformation of the element.
Suppose you wanted a list of primes multiplied by 2. Then you could replace the first x with x*2.
primes = [x*2 for x in range(2, 50) if x not in noprimes]
You write the following to say that you do not want a transformation, and just original x.
primes = [x for x in range(2, 50) if x not in noprimes]
Another perspective: Consider the x in the statement primes.append(x). That is the same as the first x in the loop in your question. In fact, this is the exact same loop as the one above.
primes = []
for x in range(2,50):
if x not in noprimes:
primes.append(x)
Here is the other example, with x*2.
primes = []
for x in range(2,50):
if x not in noprimes:
primes.append(x*2)
This syntax is called a "comprehension" (e.g. list comprehension, dictionary comprehension, etc.). It's a compact way of specifying the contents of the list/dictionary/other data structure programmatically. The Python docs explain it pretty well.
