Defining variables outside of the 'while' loop

Question

[Background info: I am currently completing a course on Python programming from the MIT edx site, and am working on the section on while loops.]

The question I have been struggling with is as follows: "Write a while loop that sums the values 1 through end, inclusive. end is a variable that we define for you."

When I tried answering the question, I put:

while end != 0:
    total = 0 + end
    end = end-1
print total

The return result for any value I put in for 'end' was 1, which obviously is incorrect.

However, when I tried again, I defined 'total' outside of the loop, and put:

total = 0
while end != 0:
    total = total + end
    end = end-1
print total

This works!

My question is: why does the first code that I put in not work? What is the significance of defining 'total' outside of the loop?

Answer 1

The problem lies with total = 0 + end . What this does is assign the current value of end to total . Since end eventually becomes 1 , that's what total is. You need to add end to the running total :

total = total + end

or:

while end != 0:
    total += end
    end -= 1
print total

Answer 2

In your first code block, each time you run total = 0 + end , you reset total

total = 0 + 10
total = 0 + 9
total = 0 + 8
...
total = 0 + 1

At the end of this, the last line run was total = 0 + 1 which equals 1

In the second loop, you are utilizing the previous value of total :

total = 0
total = 0 + 10
total = 10 + 9
total = 19 + 8
...
total = 54 + 1

With each pass through the loop, total is incremented and utilized. In the first one, you are over writing the total in each loop.

Answer 3

When you define total inside the while loop, it reassigns itself to 0 + end each time it loops, which is 1 by the end of the program. When you define it outside, it sets total to 0, then for each loop within while , it continues to add the value to it.

Also, sidenote, you can write total = total + end as total += end and it means the same thing but is actually a (very) small bit faster.

Answer 4

In the example with total inside the while loop, it is being redefined each time end != 0 , and you are printing it right after end was equal to 1 ( end = end-1 will make break you out of the while loop after end is equal to 1).

In the other example, you are not redefining total each trip through the while loop, and this is accumulating the answer you expect.