Get string value from GUI listbox

I have created a GUI and selected the listbox to list lots of countries.

I then need to know what country the user has selected. The text box should update accordingly. I have used this code:

ListBoxCheck = get(handles.listbox1,'String');
if(ListBoxCheck == Afghanistan)
    set(handles.text3,'String','Working');
elseif(ListBoxCheck == Argentina)
    set(handles.text3,'String','Working2');
end

However, when I click on Argentina or Afghanistan I get this: Undefined function or variable 'Afghanistan'.

Error in VisaProcessor>listbox1_Callback (line 135) if(ListBoxCheck == Afghanistan)

Anyone know why or can help?

Thank.s

[Stephen23]

To get the selected value you need to use the Value property, not the String property.

There are also several major syntax errors in if(ListBoxCheck == Afghanistan) that help generate that error:

• Afghanistan is not a string, so MATLAB expects it to be a variable or function and looks for one with that name, which it then cannot find. To make it a string, use single quotes: 'Afghanistan'.
• ListBoxCheck == 'Afghanistan' using the equivalence operator does an element-by-element comparison of the two strings, checking if each character in each position is the same. It will be an error if the strings are different lengths. To see if strings are the same, use strcmp instead.

Answer 1

[JR]

After reading a little bit more about listboxes I know what the problem is. When you do

ListBoxCheck = get(handles.listbox1,'String'); you get a cell array of strings with all the countries. The if condition will not work this way.

The property that changes when you click on a name is the Value property. So instead of what you have, use

listBoxStrings = get(handles.listbox1,'String');
ListBoxValue = get(handles.listbox1,'Value');
if(find(strcmp(ListBoxString,'Afghanistan')) == ListBoxValue)
  set(handles.text3,'String','Working');
elseif(find(strcmp(ListBoxString,'Argentina')) == ListBoxValue)
  set(handles.text3,'String','Working2');
end

Answered by Yoav Livneh

Answer 2

[Yoav Livneh]

Change the condition on your if to

if strcmp(ListBoxCheck, 'Afghanistan')

Hopefully this should now work.

Answer 3

[JR]

No errors, but with the code you have supplied it doesn't update the text box. I have edited the tag since last time and updated it in the code.

ListBoxCheck = get(handles.listbox1,'String');
if strcmp(ListBoxCheck, 'Afghanistan')
    set(handles.textbox,'String','Working');
elseif strcmp(ListBoxCheck, 'Albania')
    set(handles.textbox,'String','Working2');
end

[Yoav Livneh]

After reading a little bit more about listboxes I know what the problem is. When you do

ListBoxCheck = get(handles.listbox1,'String');

you get a cell array of strings with all the countries. The if condition will not work this way.

The property that changes when you click on a name is the Value property. So instead of what you have, use

listBoxStrings = get(handles.listbox1,'String');
ListBoxValue = get(handles.listbox1,'Value');
if(find(strcmp(ListBoxString,'Afghanistan')) == ListBoxValue)
  set(handles.text3,'String','Working');
elseif(find(strcmp(ListBoxString,'Argentina')) == ListBoxValue)
  set(handles.text3,'String','Working2');
end

Answer 4

[JR]

I have almost come to the end of my code! I now need to save the image. I have two global variables (AdjustedImage and BlankPaper). A lot of countries share the same sizes for my project. Say, UK, Thailand, Australia for example are 45mmx35mm. I wanted to make my code shorter, so thought about using the same style of code above:

global AdjustedImage;
global BlankPaper;
imtodouble = im2double(AdjustedImage);
BPaper = im2double(BlankPaper);
pause(3);
listBoxStrings = get(handles.listbox1,'String'); 
ListBoxValue = get(handles.listbox1,'Value'); 
if(find(strcmp(listBoxStrings,'United Kingdom','Australia','Thailand')) == ListBoxValue) 
set(handles.text3,'String','Saving this image to 2 photos on a 6x4"!');
ImageRowCount = 1;
ImageColumnCount = 2;
end

Other countries, will have different ImageRowCount/ImageColumnCount

The error is 'too many input arguments'. I know that strcmp compares two strings, if strcmp is not a good option. What would be viable?

Thanks!

[Stephen23]

The strcmp documentation states quite clearly that you can compare multiple strings if you put them in a cell array:

strcmp('Australia',{'United Kingdom','Australia','Thailand'})

However in your case you are trying to compare different numbers of strings, which will not work using this method. Instead, you could try using ismember .

[JR]

Thank you for your comment. I have had a look at the 'ismemeber' function and have almost implemented it correctly.

global AdjustedImage;
global BlankPaper;
global listBoxStrings;
global ListBoxValue;
imtodouble = im2double(AdjustedImage);
BPaper = im2double(BlankPaper);
pause(3);
listBoxStrings = get(handles.listbox1,'String');  
ListBoxValue = get(handles.listbox1,'Value'); 
if(ismember(listBoxStrings,'Afghanistan'));
set(handles.text3,'String','Saving this image to 2 photos on a 6x4"!');

This works, but when I put:

if(ismember(listBoxStrings,'Afghanistan','Albania'));
I get an error, stating that 'Albania' is an unrecognised flag. From what I have been able to gather, ismember compares A and B, not a further C in the Albania part. Not sure how to get around it.

[Stephen23]

The same thing again: you are trying to enter each string as a separate arguments. Your code

ismember(listBoxStrings,'Afghanistan','Albania')

has three input arguments to ismember: the first two are supposed to be the data you want to compare, the third is an optional flag. So ismember thinks that 'Albania' is an option flag, but it does not match with any flags that it accepts. Thus the error.

To fix this simply place the strings in a cell array. This array is then entered as the second argument of ismember, exactly as the documentation clearly requires:

ismember(listBoxStrings,{'Afghanistan','Albania'})

Note the curly brackets which define the cell array.

Tip: don't clutter your work with unnecessary parentheses and semi-colons. It is much clearer to write this, for example:

if ismember(listBoxStrings,'Afghanistan')

and

if ismember(listBoxStrings,{'Afghanistan','Albania'})

[JR]

Thankyou for the detailed explanation, I now understand where I was going wrong! I have adjusted the code slightly, as it would not work with some variables I have defined, after this piece of code.

Using:

if find(ismember(listBoxStrings,{'Afghanistan','Albania'}));

[Stephen23]

Using find does not really help this situation. You probably want to use all or any instead:

if any(ismember({'Afghanistan','Albania'}, listBoxStrings))

Answer 5

[JR]

The ismember code, perhaps isn't doing the correct trick.

listBoxStrings = get(handles.listbox1,'String'); % 
ListBoxValue = get(handles.listbox1,'Value'); % 
if find(ismember(listBoxStrings,{'Afghanistan'}));
set(handles.text3,'String','Saving this image to 6 photos on a 6x4"!'); % SETS THE TEXTBOX TO THE DEFINED VALUE
ImageRowCount = 2;
ImageColumnCount = 3;
ADDITIONAL CODE.......;

then

elseif find(ismember(listBoxStrings,{'Nepal'}));
set(handles.text3,'String','Saving this image to 4 photos on a 6x4"!'); % SETS THE TEXTBOX TO THE DEFINED VALUE
ImageRowCount = 2;
ImageColumnCount = 2;

When I choose Afghanistan when I process the image, the code still works and I get 6 photos on a sheet. :)

When I choose Nepal instead, I get 6 photos per sheet :(, not 4!

Interestingly, I put an end at the end of the first if, before else if and changed elseif to if and put another end at the end. It does the code for the first if statement, then does the second. Which to me indicates that the ismember code isn't being registered?

Cheers.

[Stephen23]

ismember works just fine for locating which strings are members of another cell array of strings, just like we want to do here.

There are two issues that should be fixed to make this work properly. Both of them are clearly described in the documentation, so lets have look:

The documentation for ismember states: ismember(A,B) returns an array containing 1 (true) where the data in A is found in B. Elsewhere, it returns 0 (false). So actually your comparisons have the arguments around the wrong way: the string/s that you want to check for should be the first argument to ismember, whereas listBoxStrings should be the second.

The documentation for find states: find(X) returns a vector containing the linear indices of each nonzero element in array X. Why did you add find? How do you expect indices to work in an if statement? Because MATLAB indices start from one, and all non-zero values are considered to be true, then the only case where if [some indices} is not going to evaluate the following statement is when some_indices is empty. For any indices at all this statement is going to be true, which is not very useful for this situation.

You might like to consider using any instead of find, which probably provides the output condition that you want. Altogether we get something like this:

if any(ismember({'Afghanistan','Albania'}, listBoxStrings))

PS: Although you keep writing them, the semicolons are not required after an if statement.