# How to create a loop with the sum adding up to a given N?

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Muhammad Akhtar on 5 Oct 2016
Edited: Matthew Eicholtz on 5 Oct 2016
The bread-and-butter of any program language is its ability to perform repeated calculations in a “loop”. Variables are often incremented within loops. The statement x = x+ 5; doesn’t make sense mathematically, but it makes perfect sense to Matlab that evaluates the statement to the right of the equals sign first, and then makes the variable on the left equal to this new value. In other words,
x = 0;
for i=1:10
x = x+1;
end
will start off setting x equal to 10, and then will execute a loop 10 times, where each time through it will increment x by 1. When it finishes, x will equal 10.
Modify the following program below so that it returns the sum of the numbers from one to N. Call the program PyramidSum, and test that PyramidSum(10) returns 1+2+3+4+... 55.
function CountUpToN(N)
for i=1:N
disp(i)
end

Massimo Zanetti on 5 Oct 2016
There is a well-known formula by Gauss
x= n(n+1)/2;
Jan on 5 Oct 2016
+1: Even if Matlab's sum(1:n) looks so nice, using a brain is more efficient.

Matthew Eicholtz on 5 Oct 2016
This sounds like a homework problem. The answer is basically already in the question.
N = 10;
x = 0;
for ii=1:N
x = x+ii;
end
But, for what it's worth, this is not the best way to sum numbers from 1 to N in MATLAB. Try something like:
x = sum(1:N);
Matthew Eicholtz on 5 Oct 2016
Edited: Matthew Eicholtz on 5 Oct 2016
Or if you want to go Massimo's suggested route:
function y = fcn(x)
n = x*(x+1)/2;
y = n*(n+1)/2;
end

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