如何在 Python 中创建按 2 计数的 x 元素列表

Question

I'm taking an introductory Python course, so this question might be pretty easy for most of you all. An assignment I'm working on is introducing us to lists. We are given the variable b2 = 5 and are tasked with using b2 to create a list c2 such that c2 = [0, 2, 4, 6, 8] which has b2 number of elements and counts by 2. How would I do this?

Answer 1

Lets break down the problem. First let us cover how lists are created in python.

#will create a list variable
c2 = [] 

Each list variable has the append method that you can readily use. So in the event you need to add the first element into the list, just append it to the list that contains no values.

c2.append(value)

The thing is, your problem will probably want you to automatically increment the counter and place the new value, without having to hard code each value into the list. This is how to solve the problem if you decide to hardcode.

c2.append(0)
c2.append(2)
c2.append(4)
c2.append(6)
c2.append(8)

Now the reason the question gives you value b2 = 5 is because that is your limit, you want to loop, increment a counter by a value of 2 until you have incremented the counter by 2 five times.

Increment of a counter can be done as such:

b2 = 0
b2 =+ 2 #or any numerical value you would like to increment by
#b2's new value will be 2 if you print it. 

I assume you have already covered loops in this course you are taking. Take the info I covered and place them into a for loop, using your b2 as your limit, your knowledge of increment by 2 every time the loop comes around, and then append the new counter value into the list.

If you want go ahead and update your code with what you think is your answer and we can work through it, any questions feel free to commend and we can go over then.

There are much faster ways of answering the question such as:

c2 = range(0,b2*2,2)

The range function works like this . range() can take 3 parameters:

1. start: staring number in the sequence
2. stop: Generate numbers up to, but not including this number, hence why you multiply b2 * 2 in order to generate numbers up to 10 but not 10 leaving the last number at 8 due to the next parameter.

3. Step: Difference between each number in sequence

   range(start,stop,step)

But I assume the problem wants you to get familiar with using for loops, list functions, and operators.

Answer 2

b2=5
c2=[]
for i in range(b2):
    i=i*2
    c2.append(i)
#Print for Pyton 2:
print c2
#Print for Python 3:
print(c2)

Variable i will be 0 when is first appended to c2, than it will take the value 2 (range(b2) is same as range(5) is same as [0,1,2,3,4]) and so on while there are values to take. When the loop is finished it will print out c2. Hope I helped you :D