Controlling Warning Messages and State

Contents

Evolution of Warnings

In older versions of MATLAB, you couldn't control the warnings at all. We followed this with the ability to control seeing warnings or not, an all or nothing deal. So warning had a global state. More recently, we added message identifiers and the ability to control warnings more finely. We introduced these message ids was specifically so you could control the messages you see. In addition, they help us supply translated messages for the Japanese version of MATLAB on Windows.

Capture warning state for later use.

sw = warning;

Poor Programming Pattern

Let me illustrate one of the programming patterns I've seen. Here's a line of code that normally gives a warning in MATLAB R2006b:

1/0

Warning: Divide by zero.
ans =
   Inf

Suppose I don't want to see that message. I can turn all warnings off and then enable them after I finish.

warning off
1/0
warning on

ans =
   Inf

The reason this is problematic is that I may have some other warnings in some non-default state. But by typing warning on, I have obliterated that information. Let me look at the state of warnings now:

swn = warning

swn = 
2x1 struct array with fields:
    identifier
    state

We can see there are two elements to my warning state and they are

swn(1)
swn(2)

ans = 
    identifier: 'all'
         state: 'on'
ans = 
    identifier: 'MATLAB:nonScalarConditional'
         state: 'off'

Compare this to the original warning state I saved at the beginning of this article.

sw

sw = 
8x1 struct array with fields:
    identifier
    state

The original settings were

for ind = 1:length(sw)
    sw(ind)
end

ans = 
    identifier: 'all'
         state: 'on'
ans = 
    identifier: 'MATLAB:intConvertNonIntVal'
         state: 'off'
ans = 
    identifier: 'MATLAB:intConvertNaN'
         state: 'off'
ans = 
    identifier: 'MATLAB:intMathOverflow'
         state: 'off'
ans = 
    identifier: 'MATLAB:intConvertOverflow'
         state: 'off'
ans = 
    identifier: 'MATLAB:nonScalarConditional'
         state: 'off'
ans = 
    identifier: 'MATLAB:mir_variable_with_script_name'
         state: 'error'
ans = 
    identifier: 'MATLAB:mir_warning_unrecognized_pragma'
         state: 'off'

Return warning state to initial one.

warning(sw)

Better Code : First Option

If you know the message id for the warning you want to suppress, simply turn that warning off. To find the id, if it exists, use lastwarn. Then set the state explicitly for that id.

1/0
[msg, msgid] = lastwarn

Warning: Divide by zero.
ans =
   Inf
msg =
Divide by zero.
msgid =
MATLAB:divideByZero

Note that the state s captured below is the state prior to resetting the warning state. In this way, you can preserve the original state and set to a new state in one statement. Later, you can use s to restore to the original state.

s = warning('off',msgid);
1/0
warning(s)  % restore state

ans =
   Inf

Better Code : Second Option

Not all warning messages have identifiers associated with them. If you know of a warning you don't want to see, but it does not have an associated id, you can do this.

s = warning('off','all'); % turn all warnings off
if true,
    disp('I am running this code!')
    warning('Here is a warning that doesn''t have an id.')
end
warning(s)  % restore the warning state

I am running this code!

I turned all the warnings off so you didn't see the message. So, did I actually trigger the warning? Yes, and here's how I know.

[msglast, msgidlast] = lastwarn

msglast =
Here is a warning that doesn't have an id.
msgidlast =
     ''

Summary

There are set of functions in MATLAB that allow you control the state of warnings for your work without disrupting how all functions behave. Any thoughts on when and how you use this set of features? Let me know.

Loren Shure

Copyright 2006 The MathWorks, Inc.

Published with MATLAB® 7.3