While loop ends unexpectedly

I have an executable file which creates an ascii file containing numbers and characters mixed. I have written a while and if loop combo to read and check each line for data that I want.

while line~=-1;
 line = fgetl(file); % Read next line
 count = count + 1; % Line count
 if length(line)<5; % Skip blank and short lines
 elseif strcmp(line(3:25),'Desired Trigger 1')==1; % Check for first trigger
  % Gather data
 elseif ...
  ...
 end % If check
end % While

This combo reads the data perfectly fine, except when it reaches a blank line where line = ''. This line clears through the if statement properly as a blank line, but for some reason the while loop considers it to be a break condition and exits the while loop. If I make the following adjustment the script runs fine, but I prefer not to have infinite loops in my scripts.

while 1 == 1;
 line = fgetl(file); % Read next line
 count = count + 1; % Line count
 if line == -1; % End of file condition
  break
 elseif length(line)<5; % Skip blank and short lines
 ...

Does anybody know why the first version breaks the loop but the second does not?

Answer 1

[Ameer Hamza]

Because in the first case, the fgetl() function returns an empty vector char vector, and comparison of an empty vector with -1 produce an empty logical vector which MATLAB interpret as false. To avoid this in the first case before the end of while loop add

if isempty(line)
  line = ' '; % add a dummy character
end

this will prevent the while condition to fail.

[Bob Thompson]

So, if I had a line such as below it would interpreted as false, even though the value of line is blank, rather than actually equal to -1?

if line ~= -1;
 ...

[Ameer Hamza]

Actually, it will be evaluated as a 0 by 0 logical variable. Since a 0 by 0 array does not contain any element and the while loop documentation tells that

"while loop to repeat when condition is true"

so it will break the loop on seeing empty comparison matrix in the condition.

[Walter Roberson]

If you want to check for end of file, use isnumeric()