Solving Boolean Vector: Position of "1" Values

[phillyj]

Hi, I needed to write a function that takes a boolean vector and returns the position of the "1" values only. The correct way it was done was like this :

function res = tmp(x)
res=[ ];
for k=1:length(x)
if x(k) 
    res = [res,k];
end
end

I don't understand why the "if" statement returns only the value "1"? I thought that I might have to make a boolean statement that checks if the value at "k" is equal to "1" like this "if x(k) ==1"

Please help, it's very confusing to me

 

[trambolin]

the function you are trying to write is a MATLAB function already. Try

find(x)

or read the documentation from

doc find

 

[phillyj]

the function you are trying to write is a MATLAB function already. Try

find(x)

or read the documentation from

doc find

You misunderstand me. I know this is a built-in fuction but this was an exercise in learning MATLAB conditionals. Can you explain my original question on why the output works that way? My professor isn't around this week otherwise I might ask him.

Thanks

 

[trambolin]

actually your code works fine
I did this

x = [true false true]
res = [];
for k=1:length(x)
if x(k)
res = [res,k];
end
end

and the result is res = [1,3]

 

[phillyj]

actually your code works fine
I did this

x = [true false true]
res = [];
for k=1:length(x)
if x(k)
res = [res,k];
end
end

and the result is res = [1,3]

Yes, the code works. I want to know why the if statement returns only the true values. I'm still learning. When I read the code in my mind this is what I understand: From index 1 to the end of the vector, result is index. Isn't that what

if x(k)

means? This makes no sense to me. Conditionals confuse me so much. Why aren't the false elements outputted. How does MATLAB differentiate between the true and false in this statement?

 

[Mark44]

"if x(k)" evaluates to true when x(k) is 1 (and possibly any other nonzero value), and evaluates to false when x(k) is 0.

This code could be written more clearly as

if x(k) == 1
  res = [res, k];
end

 

[trambolin]

Ah, now I got it. It works because you should read the if clause as follows

if (value that is declared here is nonzero)
...
end

For example

if 5
...
end

always executes but

if 0
...
end

never executes. See "doc if" in Matlab for details.