September 20th, 2013, 02:17 PM
Question About Fixing Simple While Loop
Hello, So I'm trying to fulfill the following requirements:
What is the sum of the even numbers from 524 through 10508, inclusive?
Hint: write a while loop to accumulate the sum and print it.
Then copy and paste that sum. For maximum learning, do it with a for loop as well, using range
Here is what I have thus far. My main issue is how to make i change through each succession of the loop. I think the problem lies within the fact that I initialized i before the loop began, and its constantly referring to its initial value of 524 and never fully changing i each time:
i = 524
while(i < 10508):
j = i + 2
result = i + j
i = i + 2
September 20th, 2013, 04:29 PM
What is j for? Let's use your algorithm to sum the even numbers from 2 to 6, inclusively. We do this with pen and paper. We know the answer, 12, ahead of time because
2 + 6 equals 8, and there are 1½ of these.
Trying the algorithm by hand on a small data sample is an approach. The debugger (module pdb) automates this which really helps with accuracy. I did not use pdb. This shows us that the algorithm is incorrect, the final value printed is 10 which is not 12. You already knew your program is wrong.
# read from top to bottom. Only recomputed values are shown.
# i j result i<6 print expression(reason)
i = 2 # 2 i=2
while(i < 6): # True 2<6
j = i + 2 # 4 j=2+2
result = i + j # 6 result=2+4
print(result) # 6 result
i = i + 2 # 4 i=2+2
while(i < 6): # True 4<6
j = i + 2 # 6 j=4+2
result = i + j # 10 result=4+6
print(result) # 10 result
i = i + 2 # 6 i=4+2
while(i < 6): # False 6<6
j = i + 2 #
result = i + j #
i = i + 2 #
In this case, solving a small problem with pen and paper expedites our algorithmic discovery. You're likely to follow this:
Hmm. This doesn't work. Can you fix it? Maybe the computer doesn't understand "doh!"
# i result reason
# 2 start value
# 2 doh!
# 4 increment i by 2
# 6 2 + 4 is 6
# 6 add another 2 to i
# 12 6 + 6 is 12
# which, I'm sure you can see translates to the algorithm
i = 2 # start value
while i <= 6:
result = result + i
i = i + 2
sum(range(524, 10508+1, 2))
In your case the sum of the first and last terms is 11032 and there are 4993 halves of these giving an expected result of 27541388.
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September 20th, 2013, 06:15 PM
Somehow you have to keep adding the even numbers to a sum that starts with zero. The first number added is 524 then 526 and so on.
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