Allowing backspace only on the last item entered

Hi, I've been having trouble figuring out how to allow backspace only on the last item entered. I was trying to modify some codes on the psychtoolbox function 'GetEchoString'. In this function, it allows backspacing for all items entered:

string = '';
while true
    char = GetChar;
    switch (abs(char))
        case {13, 3, 10}
            % ctrl-C, enter, or return
            break;

          case 8
              % backspace
              if ~isempty(string)
                  string = string(1:length(string)-1);
              end
          otherwise
              string = [string, char ' '];
      end

But I just want to allow backspacing only on the last item entered. (i.e.: case 8 cannot be entered twice in a row).

How do I do that?

Thanks in advance!

Answer 1

[Walter Roberson]

string = '';
last_was_bs = false;
while true
  char = GetChar;
  switch (abs(char))
      case {13, 3, 10}
          % ctrl-C, enter, or return
          break;

          case 8
            if ~last_was_bs
              % backspace
              if ~isempty(string)
                  string = string(1:length(string)-1);
              end
            end
            last_was_bs = true;

          otherwise
              string = [string, char ' '];
              last_was_bs = false;
      end

However, I don't think you are going to find this very suitable. The extra space that is inserted after every character increases the length of the string, and would thus be what was trimmed out by the first backspace. In order to be able to actually remove the last character typed, the user needs to be able to type at least two backspaces. Unless, that is, the extra space was not added.

It is poor programming practice to name a variable "char", as that interferes with using the MATLAB char() function.

[Aishah]

Thanks for your help! It works out well.

I just had to change the length(string) to minus 2 instead of minus 1.