How to return to a previous point in the code based on result of 'if' command

Programming Help
1

Hi, I have an experiment wherein subjects must press either the left, right or up arrow button. If they choose another keyboard button, an error message should come up reminding them of the restricted choice and they choose again. If they choose an arrow button then, the rest of the trial continues. However, if they again choose a keyboard button other than an arrow key, the error message appears again and they choose again and so on until they choose correctly. Here is some mock code to demonstrate:

if arrow key pressed
   proceed to next part of trial
else
  error message
  ask for key to be pressed
  check key again
  return to if statement
end

I am unsure how to code for this since a ‘for’ loop would require a fixed index number to count through, but they could press the wrong key an undefined number of times. Is there a way of going back to a previous point in the code like a ‘flag’ without using a for loop?

Any suggestions would be much appreciated. Thanks in advance!

2

for or while loop with break statement would do it. You simply break from the loop when if is true.

3

Hi, thanks for your answer, I was wondering what I would use to count up in the for loop though? As in, how do I specify

for i in (1:until condition is true)
if else statement
end

Thanks again.

4

You decide how many tries you like, or infinite by while loop

    while 1 % or for iTry = 1:10
        key = getResponse;
        if key is valid
            record the response
            break; % break while loop, go for next trial
        else
            error message
        end
    end
5

Hi, thanks for the answer. I have attempted to implement it as shown:

number_tries = 1;
while number_tries < Inf
    if KbName(keyCode1) == "LeftArrow"
    elseif KbName(keyCode1) == "RightArrow"
    elseif KbName(keyCode1) == "UpArrow"
    break;
    else
        Screen('TextSize', window, 17);
        Screen('DrawText', window, 'Please only select an arrow key to indicate your choice', screenXpixels*0.1, screenYpixels*0.5, white);
        Screen('Flip', window);
        WaitSecs(2.0);
        
        % Question mark
        Screen('TextSize', window, 120);
        Screen('DrawText', window, '?', screenXpixels*0.48, screenYpixels*0.45, white);
        Screen('Flip', window)

        % Timestamp
        questionmarkOnset1 = GetSecs();

        % Check the keyboard to see if a button has been pressed
        [secs1, keyCode1] = KbStrokeWait;
        
        number_tries = number_tries + 1;
    end
end

However, it keeps crashing when the question mark is displayed when it is supposed to wait for the next keyboard button pressed and then test the ‘if’ condition again.

6

Should be changed to

    if ismember(KbName(keyCode1), {'LeftArrow' 'RightArrow' 'UpArrow'})
        break;

This will treat all three keys as valid, but it looks unrelated to the crashing issue.