Warning: Integer operands are required for colon operator when used as index

For this code:

40: y1_x=y_data(1)
41: y1=round(y1_x)
42: y2_x=y_data(2)
43: y2=y2_x-y1_x+y1

I get this output(and warning):

y1_x =

     57.3517

y1 =

      57

y2_x =

     83.3517

y2 =

      83

Warning: Integer operands are required for colon operator when used as
index 
> In sonar_prob at 43
  In robot_diferential at 60
  In Painel>RunSimulation at 259
  In Painel>pushbuttonReactive_Callback at 154
  In gui_mainfcn at 96
  In Painel at 16
  In @(hObject,eventdata)Painel('pushbuttonReactive_Callback',hObject,eventdata,guidata(hObject))

I couldn't find any reason for this warning...what can it mean?

[Fangjun Jiang]

Does the warning really happen on line 43? I know sometimes the Matlab warning or error message is misleading. Put a break point around line 43 and step through to see when exactly the warning happened.

Answer 1

[Matt Fig]

This is caused by something like:

T = rand(1,10);
T(2.2:7.99)   % Note the indexing by:  2.2:7.99

So you need to look through your code and spot where you are using a colon operator like the above. BTW, this fixes the warning message and produces the same output.

T(floor(2.2:7.99))

%

%

%

%

%

EDIT In response to your comments below:

Do this at the command line:

dbstop if warning

Then run the code again. The debugger will stop on the line that causes the warning...

[Nuno]

Your example will give the error at the line "T(2.2:7.99)...".

But at my code the is no indexing operation at that line. I'm sure the code is saved and that I'm running the correct version of the file!

Regards,

Nuno

[Matt Fig]

No, my example does not give an error at that line. A warning is not an error. Look below, pasted from the command line.

>> T = rand(1,10);

>> T(2.2:7.99)

Warning: Integer operands are required for colon operator when used as index

ans =

0.7792 0.9340 0.1299 0.5688 0.4694 0.0119

Now, I suggest you enter this at the command line:

dbstop if warning

Then run the code again. The debugger will stop on the line that caused the warning...

[Nuno]

Sorry, I called error to the warning.

With "dbstop if warning" now I get the warning on the correct line. Let's start debugging :)

Is there any logic reason for the warning to appear on an incorrect line or there is simply no assurance that it will signal the exact line?

[Matt Fig]

Good question! I don't know the answer. Was the warning at least in the same file as reported?

[Nuno]

Yes. 14 lines after on this "example".

[Matt Fig]

Perhaps a bug report is in order. I have seen this issue myself, but I don't have an example anymore. If you submit a bug report, TMW will look at your code and see what went wrong. At least calling DBSTOP gets it right!

[Walter Roberson]

The wrong line can get reported if you have modified the file since it was last saved, Or if the file is being invoked through a handle that was created before the latest modification was made.

[Matt Fig]

Good thinking Walter! However, I have experienced the same issue when those two conditions weren't in play.

[Walter Roberson]

Hmmm. An old "object" would be an example of something that could have a handle to old code, I suspect.

[Nuno]

Are you talking of some kind of cache? My files are saved, I'm sure!

Answer 2

[the cyclist]

This warning normally occurs when doing something like:

>> y = x(index)

and the indexing vector has something other than a positive integer as an element.

I don't see how this could happen on the line of code you posted, so I guessing maybe the code was edited and not saved before you ran it. (Look for a star after the file name, in the editor.) Maybe the error really happened on line 41?

Try saving the code and trying again. Also, use breakpoints as suggested by Fangjun Jiang.

[Matt Fig]

If the indexing vector has something other than a positive integer, an error will result, not a warning. MATLAB is smart, and when an index is created with the colon operator the results are floored before indexing. For example:

T = rand(1,10);

T(2.2:7.99) % A warning

idx = 2.2:7.99; % Now do this without the context.

T(idx) % An error.