#### 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.");