Help me differentiate between two codes??

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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);
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];
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);
using the same input as before:
x = [0,1,2,4,3];
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?

Accepted Answer

Matt Tearle
Matt Tearle on 22 Oct 2014
Edited: Matt Tearle on 22 Oct 2014
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!

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