how do i sum the first 100 even numbers on python?

Create a for loop from 1 to 100, then within the loop, check if the iterated number is even. If even, then add it to the total. If not, then do nothing.

even_sum = 0

for i in range(100):

if (i+1) % 2 > 0:

even_sum = even_sum + (i+1)

print even_sum

This isn't the way you would normally do this, but a clever way of doing this is by recognizing that the sum of the first n even numbers is the same as double the sum of the first n numbers.

Now, you might wonder how that helps. The sum of [1,2,3,...n] is actually really easy to compute - it's just (n+1)*(n/2). So, if you double that, you just get n*(n+1) as the sum of the first n even numbers.

So, the sum of the first 100 even numbers would be 100(101), which I can do in my head as 10100.

The sum of the first `n' multiples of `k' can be expressed as

Sigma from i = 1 to n of (ki);

Factor `k' from the summation

k ((Sigma from i = 1 to n) of i)

Expand the summation

Which yields the general formula

k n (n + 1) (1 / 2)

So let k = 2 and n = 100

100 * 101 = 10100

So in answer to your question:

print (10100)

I think you're safe, because I don't think the sum of the first 100 even numbers is going to change, ever.

Although it might be considered better practice to use a symbolic constant instead of a magic number:

Sum100Even = 10100

print (Sum100Even)

EDIT:

this is my program

evenNumber = int(input("enter a number: "))

for i in range(2,evenNumber):

.... evenNumber += 2

print(evenNumber)

Is a bug. evenNumber is the upper bound on the loop (it's `n' in the above problem). You shouldn't add to the upper bound. Instead, you should start out with a new variable to keep the sum, starting from 0.

upperBound = int (input ("enter the upper bound"))

sum = 0

for i in range (2, upperBound): sum += 2

print sum

Now that's almost right --- I changed the name of `evenNumber' to `upperBound', because if you do enter an even number, it will not be included in the generated range.

That is

for i in range (2, 4): print i

would print

2

3

And not 4. Mathematically, the values assigned to i are integers in [lower, upper).

You don't care about `i' in your loop, so don't give it a name, and use "blank" (`_') as a name instead:

for _ in range (2, upperBound): sum += 2

And if you had to do it this way, it would also be considered better style to write just

print (sum (range (2, 200 + 1, 2)))

I will tell you two tricks that I use. First, even numbers

are numbers of the form 2k, alright? So you might just

think of i as beginning with 2, and ending with some number

called 2n.

A second trick I use is to write the program for the first

10 even numbers, and see if I can get that right; then if it

works, I have confidence that the same logic will work for

100. You see? Get things right for a small group, then go

for the prize. Good luck.

>

> John (gnujohn)