Error using && in if conditional statements

cont=1;
for x=0:N
    for y=0:N
        for z=0:N
            
            fh(cont) = ( c/2 ) * ( sqrt((x/lx)^2 + (y/ly)^2 + (z/lz)^2));
            cont=cont+1;
            
        end
    end
end

Here the frequency for different normal vibration modes is calculated and saved in fh

b1=0;
	co=cont;
	while co>0
		if fh<50 && fh>25
			b1=b1+1;
		end
		co=co-1;
	end

but when I try to count how many modes have a frequency between 2 parameters by using &&, I get the following error

Operands to the || and && operators must be convertible to logical scalar values.

Error in ModosNormales (line 91)
		if fh<50 && fh>25

I tried the conditional AND, &, ||, but it does not give me the result that should

Answer 1

[Matt J]

I believe your entire code can be replaced with 3 lines,

[x,y,z]=ndgrid(0:N);

fh=( c/2 ) * ( sqrt((x(:)/lx)^2 + (y(:)/ly)^2 + (z(:)/lz)^2));

b1=sum(fh<50 & fh>25)

No need for loops and if statements.

But to answer your question, you are not indexing the vector fh in your loop, so the entire vector quantity fh>50 is being given to the && operator. The && operator does not work with vectors and, even if it did, it's rare that you would want to give a vector expression to an if statement (I've never done it).