在python中打印列表的第3和第4项

Question

Suppose I have the below list in python: myList = ['a','b','c','d','e','f','g','h','i',1,2,3]

Now, i basically want a function which will take the list and a enter code here range as input and print below as the output:

['c','g',1]

def alternate_num(l,r):
    if r >=3 :
        if r <= len(myList):
            myResult = l[0:r]
            first_loc = myResult.index(myResult[2])
            second_loc = myResult.index(myResult[6])
            newList = list(myResult[first_loc:second_loc + 1:4])
            len_list = len(myResult)
            print(newList)
            newList.append(myResult[i] for i in first_loc)
            print(newList)
        else:
            print("The range provided is greater than the length of the list")
    else:
        print("Please provide a range greater than 3")

eg suppose i have given a range input as 10, in that case it will take first the 3rd element from the list, then next,it should pick up the alternate 4th element and then it should the next 3rd element and so on.

so, in this example, my third element is c, hence it is picked up, then it iterates through the list and pick the alternate fourth element ie g and then it picked the alternate 3rd element which is 1.

Answer 1

you can use itertools.cycle and itertools.accumulate to get a generator that yields the desired indexes.

try this:

myList = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 1, 2, 3]


def alternate_num(l, r):
    if r < 3:
        print("Please provide a range greater than 3")
        return
    if r > len(l):
        print("The range provided is greater than the length of the list")
        return
    from itertools import cycle, accumulate
    indexes = accumulate(cycle([3, 4]))
    new_list = []
    for i in indexes:
        if i > r:
            return new_list
        new_list.append(l[i-1])


print(alternate_num(myList, 10))