How can I make a conditional statement when using variables?

In my script I first defined a for loop:

for n = 1:inf                  
    a = 10^(n+1);
    b = 10*a;

so the for-loop assigns every iteration new values to my variables a and b.

Right after it I made an conditional statement:

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

I want my value x (which I assign when activating the script) to lie between a and b. But matlab won't agree with me. It says:

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

Error in palin (line 10)
if (a <= x)&&(x < b)

Though I thought values to the variables a and b were assigned earlier in the script, matlab would recognize these values. I was wrong. Does anybody has a suggestion what I could do to fix this error? I hope it is very easy to solve, so that soon someone will help me out of this. Thanks in advance.

Answer 1

[Wayne King]

I'm guessing from the error message you report that x is a vector, not a scalar. From the code snippet you show, both a and b are scalars, x should be a scalar too.

So for example, look at the following

   x = 10001;
   for n = 1:3
   a = 10^(n+1);
   b = 10*a;

   if (a <= x)&&(x < b)
      disp('yes');
   else
      disp('no');
   end
   end

Now, if you try the above with x a vector

x = 10000:10004;

You'll get the error message you are seeing.

[Maik Petersen]

Actually I worked with scalars too, but now it seems like the error was due to the fact that I sent n to infinity;

n = 1:inf

Because when I tried the same script with n = 1:3 it worked out well... It also seems to depend on which value for x I take, for some values I get the error, for others I don't. So now I'm guessing it has something to do with the rest of my script, so I will put some magic in there and I hope it then works out well. Thanks for your answer!

[Image Analyst]

I find it hard to believe that having the upper limit as 3 or something else, like inf, would cause it to work or fail. My guess is that you're changing x inside the loop and that if n is more than 3 somehow x turns into a vector. Does x appear at all in the loop other than the if statement? If so, are you concatenating/appending to it?

[Maik Petersen]

Yes, I am doing some calculations with x inside the loop, but I'm not appending to it, in contrast I'm making the scalar a smaller scalar inside the loop. But for now I don't get the error anymore, so I guess I made some small mistake in the script. Thanks for thinking along with me!