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12.2.2 Enabling and Disabling Warnings

The warning function also allows you to control which warnings are actually printed to the screen. If the warning function is called with a string argument that is either "on" or "off" all warnings will be enabled or disabled.

It is also possible to enable and disable individual warnings through their string identifications. The following code will issue a warning

warning ("example:non-negative-variable", 
         "'a' must be non-negative.  Setting 'a' to zero.");

while the following won’t issue a warning

warning ("off", "example:non-negative-variable");
warning ("example:non-negative-variable", 
         "'a' must be non-negative.  Setting 'a' to zero.");