# sum of multiples of 3 from 0 to 100

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Mina on 4 Jun 2021
Commented: Walter Roberson on 5 Jun 2021
Calculate the sum of all the multiples of 3 from 0 to 100 without using an if construct or vectorization.
I'm not sure how to do it without using an if construct.
sum = 0;
for x = 0:100
if mod(x,3) == 0
sum = sum + x;
end
end
fprintf ('The sum is %4d \n', sum);
David Fletcher on 4 Jun 2021
Edited: David Fletcher on 4 Jun 2021
You're using an if construct - the only reason you need an if construct is to filter out the numbers that you don't need. So, don't generate the useless numbers in the first place.
As a hint - you can use a step size when writing this: for x = 0:100

Rik on 5 Jun 2021
If I was your teacher I would just tell you to use a loop, leaving you to decide what you aren't allowed to do.
In this case you need to read the documentation for the colon function. It will tell you how you can introduce a step size.
doc colon
You should also avoid the use of sum as a variable name, as you are likely to use the sum function elsewhere in your code. If it already is a variable, that will cause strange issues.
Walter Roberson on 5 Jun 2021
I think the colon would count as vectorization.
It is then questionable whether using a colon in a for counts as vectorization. The model MATLAB uses is as if the limit expression was evaluated and all of the terms were stored ahead of time, but there is evidence that the implementation does not do that in the case of using a colon range.

Manas Minnoor on 5 Jun 2021
Edited: Manas Minnoor on 5 Jun 2021
Hey Mina,
You can generate only the multiples of 3 in the for loop using the colon operator.
Please refer to the following documentation to view the different ways to utilize a for loop:
Hope this helps.
Rik on 5 Jun 2021
Please refrain from posting complete solutions to homework questions. That only encourages cheating.

Walter Roberson on 5 Jun 2021
s = 0;
for x = 0 : 100
s = s + (mod(x,3) == 0).*x;
end
s
s = 1683
sum(0:3:100)
ans = 1683
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Walter Roberson on 5 Jun 2021
Does a while loop count as an if statement ?