How do I verify user input from prompt/inputdlg and go back to same prompt if input is invalid?

We are in the process of adding some gui elements to update one of our text based scripts. Some of the input values must be a number or an integer. With the input command at the command line, there are various ways to validate the input and have the question be asked again before the rest of the program proceeds.

While I can do a similar validation for the prompt command, where I am stuck is how to get the script to loop back to the same prompt if the response to the prompt is not valid. In the example below, the script displays an error message and then ends if the user did not enter an integer for the matrix size. That's okay for small programs, but in our project, it will be frustrating for a user to have to start over if they make a mistake towards the end of the script. How do I get the script to loop back to the prompt where the mistake occurred instead of having to start over (it is probably a good idea to still display the error message though). Thanks!

% input dialog box example
% do something with the input. Display an error message if matrix size
% input is not an integer

close all
clear
clc

promt={'Enter matrix size as an integer:','Enter your figure name:'};
title='Input';
dims=[1 35];
definput={'200','RGB noise'}; %makes a square matrix to keep things simple
answer=inputdlg(promt,title,dims,definput); %creates an array with input. Next is to extract input and do something
matrixsizeinput=answer{1,1}; % if everything in array is in rows, specifying columns is not absolutely necessary
matrixsize=str2double(matrixsizeinput); %str2double is preferred over str2num
if isreal(matrixsize) && rem(matrixsize,1)==0 %evaluates whether matrix size entered is an integer
else
       msgbox('The value you entered for the matrix size was not an integer. Please rerun program.','Error!','error');
       return %halts the program since maxtrixsize is not an integer
end %ends integer evaluation. Rest of program runs if maxtrixsize is an integer
figtitle=answer{2,1}; %gets the figure title which is a string
% result=zeros(matrixsize,matrixsize); %this makes a matrix filled with zeroes depending on the input
result=randi(256,matrixsize,matrixsize,3);
figure('Name',figtitle);
imshow(uint8(result)); %must specify that values are 8 bit RGB, otherwise numbers in array are read as intensity

[Rik]

Just some small remarks:

The uint8 data type only supports digits up to 255, so your random RGB matrix will be slightly biased, as 256 will be rounded to 255.

You should use functions to keep your workspace clean, instead of using the clear function. You should also consider if it is reasonable for the user to have other figures open, which they might not want to close.

[Jason Newman]

Thank you for pointing out the unit8 issue.

Answer 1

[Rik]

One of the ways to solve this is with a recursive function. (another solution you could choose is a while loop)

function matrixsize=prompt_user__matrix_size
promt={'Enter matrix size as an integer:','Enter your figure name:'};
title='Input';
dims=[1 35];
definput={'200','RGB noise'}; %makes a square matrix to keep things simple
answer=inputdlg(promt,title,dims,definput); %creates an array with input. Next is to extract input and do something
matrixsizeinput=answer{1,1}; % if everything in array is in rows, specifying columns is not absolutely necessary
matrixsize=str2double(matrixsizeinput); %str2double is preferred over str2num
if ~( isreal(matrixsize) && rem(matrixsize,1)==0 )
    %show error and restart prompt
    msgbox('The value you entered for the matrix size was not an integer.','Error!','error');
    matrixsize=prompt_user__matrix_size;
end
end

[Jason Newman]

I'll check this out to see how it works. I do have a while recursive loop in the text version, so the user us asked the question again if they enter a non integer value. Adding gui elements makes things a litte more complicated.

[Jason Newman]

I tried this out and it is pretty close. I was banging my head against the wall trying to figure this one out, and didn't think of defining prompt as as a self contained function. My guess is that I will need to do this for any prompt that requires an aswer to meet certain conditions.

The only issue I can see from a user stand point is the prompt dialog box appears again right over the msgbox for the error. Ideally, I would like them to see the error, press okay, and then have the prompt appear again.

I put a pause in after msgbox. Now that works if someone has the command window open and makes it active as hitting any key will bring up the original prompt. However, if a user has other scripts open and one of them is in focus (active) over the command prompt, they will just start typing in that window while the running function is still waiting.

commandwindow and uiwait don't seem to be the answer.

[Rik]

You could always design a prompt window yourself that has room for reporting the error, although I would have expected uiwait to have been the solution.

[Jason Newman]

I expected uiwait to be the solution to, but I get a figure that is empty and the program will not continue until it is closed.

Also, I went back in and put the rest of my code, and got an interesting result. If I input an integer the first time around, I get the figure with the RGB noise as expected. If I enter in text or a non integer, the function does loop back to ask the questions again. However, if I fix my error and enter an integer, I get ans=(integer entered), but no expected figure with the RGB noise. That area of the code looks like this:

if ~(isreal(matrixsize) && rem(matrixsize,1)==0) %evaluates whether matrix size entered is an integer
       % Show the error and restart the propmt.
       msgbox('The value you entered for the matrix size was not an integer. Please dismiss this box, highlight the command window, and press any key to continue.','Error!','error');
       pause %pauses the program since maxtrixsize is not an integer
       matrixsizefunc=prompt_user__matrix_size
elseif isreal(matrixsize) && rem(matrixsize,1)==0
    figtitle=answer{2,1}; %gets the figure title which is a string
    % result=zeros(matrixsize,matrixsize); %this makes a matrix filled with zeroes depending on the input
    result=randi(255,matrixsize,matrixsize,3);
    figure('Name',figtitle);
    imshow(uint8(result)); %must specify that values are 8 bit RGB, otherwise numbers in array are read as intensity

[Rik]

This works as expected on my copy of R2018a:

uiwait(msgbox('The value you entered for the matrix size was not an integer. Please rerun program.','Error!','error'))

As for your error: why are you mixing input and processing again? The whole point of such an input function is to separate them, so you can keep everything clear. The functions below show what I mean. I also shortened the lines to a max of 75 chars (you can do a bit larger if you like, but short lines keep code clear). I also added a comment about clc,clear,close all. It might also be usefull to know that there is a difference between a string and a char. It is a bit annoying that Mathworks created a data type with this name a few years back, because now it is not clear if someone means a char array or a string.

function SomeFunction
% input dialog box example
% do something with the input. Display an error message if matrix size
% input is not an integer

%Don't use this outside of debugging. Use functions to keep your workspace
%clean, use explicit figure/axes handles to create graphics objects (your
%user might have other relevant figures open), and don't rely on the
%command window output to show vital information to your user.
close all
clear
clc

[matrixsize,figtitle]=prompt_user__matrix_size;
result=uint8(randi(255,matrixsize,matrixsize,3));
figure('Name',figtitle);
imshow(result);
end

function [matrixsize,figtitle]=prompt_user__matrix_size
promt={'Enter matrix size as an integer:','Enter your figure name:'};
title='Input';
dims=[1 35];
%makes a square matrix to keep things simple
definput={'200','RGB noise'};
%creates an array with input. Next is to extract input and do something
answer=inputdlg(promt,title,dims,definput);
matrixsizeinput=answer{1}; %get the size (still as a char array)
figtitle=answer{2}; %gets the figure title which is a char array
matrixsize=str2double(matrixsizeinput); %str2double is better than str2num
if ~( isreal(matrixsize) && rem(matrixsize,1)==0 )
    %show error and restart prompt
    uiwait(msgbox(...
        'The value you entered for the matrix size was not an integer.',...
        'Error!','error'));
    [matrixsize,figtitle]=prompt_user__matrix_size;
end
end

[Jason Newman]

Thanks. I accidently put uiwait on it's own line, so that was the reason for the problem.

You are correct... I should start with a script that calls upon the function rather than mixing the function and the results. I suspect that if I spend a little more time working that out, I should be able to get it to work,

I've only been doing this for the last 6-8 months, and the online class I took was a bit weak on the fucntion side of this to get input and pass the results on to something else.

This was proof of concept, and I feel your replies have answered my question. Thanks.

[Rik]

You're welcome.

As a sidenote I would urge you to keep up with this amount of comments in your code. I can imagine it might feel a bit excessive at times, but it really helps other people understand the code you wrote. And you in 6 months or a year is also counts as someone else.