Troubleshooting code for prompt and inputdlg

The following code works fine for any user input for N:

    N = input('Number of pieces to break into ');
    rowgroups = diff( round(linspace(0,r,N+1)));

But when I try to use the following code, I get an error:

Undefined function 'plus' for input arguments of type 'cell'.

Error in sharebased_nshares (line 62) rowgroups = diff( round(linspace(0,r,N+1)));

     prompt = {'Enter no. of shares:'};
     dlg_title = 'Input';
     num_lines = 1;
     defaultans = {'2'};
     answer = inputdlg(prompt,dlg_title,num_lines,defaultans); 

     N=answer;
     rowgroups = diff( round(linspace(0,r,N+1)));

What is going wrong and where?

Answer 1

[Star Strider]

You need to convert ‘answer’ to a number:

N=str2double(answer);

That should work with your code.

[fiona rozario]

Thank you!

[Image Analyst]

Users don't always enter what you told them to. See my code below for a way to authenticate what they inputted.

[Star Strider]

@fiona rozario — My pleasure!

[fiona rozario]

Thank you... I hadn't thought about this.

Answer 2

[Image Analyst]

N is a cell so use str2double(). See this robust snippet:

% Ask user for one integer number.
defaultValue = 2;
titleBar = 'Enter an integer value';
userPrompt = 'Enter the integer';
caUserInput = inputdlg(userPrompt, titleBar, 1, {num2str(defaultValue)});
if isempty(caUserInput),return,end; % Bail out if they clicked Cancel.
% Round to nearest integer in case they entered a floating point number.
integerValue = round(str2double(cell2mat(caUserInput)));
% Check for a valid integer.
if isnan(integerValue)
    % They didn't enter a number.  
    % They clicked Cancel, or entered a character, symbols, or something else not allowed.
    integerValue = defaultValue;
    message = sprintf('I said it had to be an integer.\nTry replacing the user.\nI will use %d and continue.', integerValue);
    uiwait(warndlg(message));
end