如何使用Python打印偶数列表

Question

I'm trying to write a python program that will prompt the user for an integer greater than 1 and reads those integers into a list followed by producing two lists and returning those values. Where should I start?

A list titled "listOfNumbers" has been created. I need to produce a new list that only contains integers from listOfNumbers that have even induces in listOfNumbers.

def make_a_list(*listOfNumbers):
    for x in numList:
        if x % 2 == 0:
        print x
        listOfNumbers.append(x)

print(make_a_list(4, 5, 6, 7, 9, 11, 10, -12, -18, 1, 0)

I excepted a list that has the user type in a series of numbers but I'm not sure how this will turn out.

Answer 1

As @geckos pointed out you can just use indexing to specify this range. Supposing we have:

my_list = [4, 5, 6, 7, 9, 11, 10, -12, -18, 1, 0]

And we want evens using a counting scheme of [1, 2, 3, 4, 5, ...] , then we would call:

print(my_list[1::2])

Outputting:

[5, 7, 11, -12, 1]

This is because you specify the index as [start:stop:step size] but normally the step size is omitted, and therefore defaults to 1.

If instead you mean a counting scheme of [0, 1, 2, 3, 4, 5, ...] then just adjust the index to [::2]

Note you can also use negative step sizes. For instance:

print(my_list[::-1])

Outputs the list in reverse order, or:

[0, 1, -18, -12, 10, 11, 9, 7, 6, 5, 4]

To put it into a function would look like:

def make_a_list(input_list):
    even_index_list = input_list[1::2]
    return even_index_list

Answer 2

this will give you the list of all even numbers

def make_a_list(*listOfNumbers):
    even_list =[]
    for num in listOfNumbers:
        if num%2==0:
            even_list.append(num)
    return even_list
print(make_a_list(4, 5, 6, 7, 9, 11, 10, -12, -18, 1, 0))
# output [4, 6, 10, -12, -18, 0]

this will give you numbers at even indices

def even_indices(*listOfNumbers):
    return listOfNumbers[2::2]


print(even_indices(4, 5, 6, 7, 9, 11, 10, -12, -18, 1, 0))
# output (6, 9, 10, -18, 0)

Answer 3

Use range() to increment by two while looping.

def make_a_list(*listOfNumbers):
    output_list = []
    for i in range(0, len(listOfNumbers), 2):
        output_list.append(listOfNumbers[i])
    return output_list