Help me differentiate between two codes??

I'm trying to understand how to use for loops and if statements in school.

I wrote a function to try to sum certain elements in a vector that are between two set parameters.

The correct function code is

function [Sum] = summing (x,a,b)

%x being a vector, a&b are the parameters

Sum = 0;

for k = 1: length(x)

if ((x(k)>=a) & (x(k)<=b))

Sum = Sum + x(k);

end

end

end

so for example is I wanted to add all the elements of vector x that are between 1 and 3 I would enter

x = [0,1,2,4,3];

summing(x,1,3)

ans = 6

I've tested this and I am content with it.

However my first attempt was to create the function

function [Sum] = Summing (x,a,b)

Sum = 0;

for k = 1:length(x)

if (a<=x(k)<=b)

Sum = Sum + x(k);

end

end

end

using the same input as before:

x = [0,1,2,4,3];

summing(x,1,3)

ans=10

I get a totally different wrong answer. Obviously my if statement is different, but I am curious as to

why the output is so different.

Can anyone enlighten me as to how my second code works? like what exactly is happening in it?

Answer 1

[Matt Tearle]

Ah, yes. This curious bit of syntax. What you ended up with was sum(x)! Why? Well, MATLAB evaluates the first condition a <= x(k). This returns either 1 (true) or 0 (false). Then there's a second condition (previous result) <= b. Given that b = 3, and the previous result is 0 or 1, this is always true! Hence all the elements of x are added to your sum.

IOW, a <= x(k) <= b is parsed as ((a <= x(k)) <= b), and T/F in MATLAB is equivalent to 1/0.

[Note: it's possible that the order of operations is the other

As an aside, I hope they will teach you this, but the best way to do this operation in MATLAB is

idx = (a <= x) & (x <= b);
mysum = sum(x(idx))

Logical indexing is sweet!

[Radhika]

Thanks so much!