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I am currently working on my matlab final project for school. I consider myself fairly knowledgeable and proficient when it comes to programming.. but Matlab simply has TOO many oddities.

The fundamental question (realized this after finding the answer)! How can I use a variable in a call to a gui handle object without the name of the variable being used instead of the value?

In other words: Use a variable in a field name (If I had known it was this simple I wouldn't of asked)

My project is building a simple rendition of the age old 'Battleship' game.

My issue: I currently 5 objects (axes) for the ship pieces. They are selected one at a time to be moved into another location(the grid). I am able to use the setpixelposition to move each object after a button click.

Right now under the button click, I have something like this

function btnPlaceShip_Callback(hObject, eventdata, handles) 

    %Store the current selected ship(passed from an onclick to a label) 
    ship = get(handles.lblSelectedShip,'string');

    %I have tried everything I could think of, but basically I want to achieve the
    %following

    setpixelposition(handles.ship, [0 250 50 250]) 
    %where the variable 'ship' contains the name of the object.

In other words, the var ship = 'shipAircraftCarrier', and..

    setpixelposition(handles.shipAircraftCarrier, [0 250 50 250])

works!(sets the position of the specific ship indicated). Using the variable ship, however, matlab takes the string literally and not for its value. It would be extremely convenient to use the variable instead!

If anyone have any solutions, I would be grateful. I have scoured the web but perhaps I am missing some fundamental understanding of the Matlab GUI stuff - The matlab help documents are very non-descriptive and are not of much help.

Cœur
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mtotho
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3 Answers3

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You can do this using the eval function, but you need to be careful about string injections:

setpixelposition(eval(strcat('handles.',ship)), [0 250 50 250])
Bruce Hart
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  • Thanks for the answer. I would try this but other users mentioned I can just put the variable in parenthesis, like: setpixelposition(**handles.(ship)**, position) which is shorter. I don't think I need to worry about string injection in my matlab script, especially since this variable is not inputted via input box, but rather gui selection – mtotho Apr 25 '12 at 17:15
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You can use dynamic fieldnames or getfield. Field indexing using identifiers vs strings is quite similar in Matlab structs and Javascript objects.

Matlab:

fromId = handles.shipAircraftCarrier; %identifier
fromString = handles.('shipAircraftCarrier'); %string

Javascript:

var fromId = handles.shipAircraftCarrier; //identifier
var fromString = handles["shipAircraftCarrier"]; //string
AE426082
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  • Thanks! the variable **shipTag** is a string variable that is equal to the tag of the ship. I will try using: **handles.(shipTag)** instead of handles.shipTag – mtotho Apr 25 '12 at 17:11
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As mentioned by others use dynamic fieldnames. Whilst not having the code to test it I believe that simply putting brackets around the ship will substitute the string in to the structure name, thus

setpixelposition(handles.(ship), [0 250 50 250])

Try to avoid the use of eval() if at all possible.

Adrian
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  • Thank you. I will try this shortly. Before I saw this was answered, I had been trying various things all day! But I learned a lot - I am finally understanding how matlab is organizing all this gui stuff! I think everything was confusing because I did not understand the automatically generated code and It took a while to grasp it (i.e I now realize that 'handles' is simply a structure array!) – mtotho Apr 25 '12 at 16:58