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Hi

Currently working through some of Project Eulers problems as mentioned in the forum discussion 'Best way(s) to master MATLAB'. I have created this code, it works but there is something about it I find messy. For a start I don't like all the declared variables.

function eusol2 = euler2(euin)

% for No's in fibonacci sequence < 4mil, sum all even No terms.

x = [1 2];

y = 1;

xout = 2;

fibMax = 0;

while fibMax < euin

fibMax = x(1) + x(2);

if fibMax < euin

if mod(fibMax, 2) == 0;

xout = xout + fibMax;

else

end

else

end

x(y) = fibMax;

y = 3 - y;

eusol2 = xout;

end

Any suggestions or remarks

AD

Jan
on 26 Oct 2011

You started the sequence with [1,2] accidently instead of [1,1]. Using xout=2 instead of 0 compensates this such that the correct answer is calculated.

A very nice example for a "stealth bug": No effect for standard usage, generalising the function for any Fibonacci sequence [a,b] fails.

Jan
on 25 Oct 2011

Your function looks fine already. Most of all it is very fast, e.g. twice as fast as Daniel's program.

The result is wrong, because you start with xout=2. The empty ELSE branch wastes time. The "if fibMax < euin" check can be omited, if fibMax is increased after adding the new term. I prefer meaningful names instead of x and y.

function out = euler2(limit)

accum = [1, 1]; % [EDITED], was [1,2] as in the original question

index = 1;

out = 0;

fibMax = 0;

while fibMax < limit

if mod(fibMax, 2) == 0

out = out + fibMax;

end

fibMax = accum(1) + accum(2);

accum(index) = fibMax;

index = 3 - index;

end

Of course Binet's formula would be more sophisticated. But calling this function 10'000 times with the input 4e6 needs 0.15438 seconds only.

The MOD(fibMax, 2) fails after 75 terms due to the limited precision. Exploiting the simple pattern of even terms might be interesting, but a further "improvement" of the function is of accademical interest only. For a practical use this function is eitehr fast enough, of a lookup table with the 75 values is faster brute-force approach.

bym
on 24 Oct 2011

Daniel Shub
on 25 Oct 2011

x = [1 2];

eusol2 = 0;

while sum(x) < euin

if mod(sum(x), 2) == 0;

eusol2 = eusol2 + sum(x);

end

x = [x(2), sum(x)];

% k = sum(x);

end

This will likely be a little slower since it calculates the sum(x) 4 times per loop. If you add another declared variable (k) you can make it so sum(x) is calculated only once. I eliminated y, because it doesn't seem to do anything.

bym
on 26 Oct 2011

just to let you know where I was headed:

clc;clear;tic

phi = (1+sqrt(5))/2;

i = 1:11; % 11 even terms < 4e6

fib = (phi.^(i.*3)-(-1/phi).^(i.*3))/sqrt(5);

fprintf('%6.0f\n',sum(fib))

toc

as Jan suspected, it runs slower than yours (about .8 msec vs .07 msec). +1 vote for the question

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