Messages
Mathematica has a general mechanism for handling messages generated during computations. Many built-in
Mathematica functions use this mechanism to produce error and warning messages. You can also use the mechanism for messages associated with functions you write.
The basic idea is that every message has a definite name, of the form
symbol::tag. You can use this name to refer to the message. (The object
symbol::tag has head
MessageName.)
| Quiet[expr] | evaluate expr without printing any messages |
| Quiet[expr,{s1::tag,s2::tag,...}] | evaluate expr without printing the specified messages |
| Off[s::tag] | switch off a message, so it is not printed |
| On[s::tag] | switch on a message |
Controlling the printing of messages.
As discussed in
"Warnings and Messages", you can use
Quiet to control the printing of particular messages during an evaluation. Most messages associated with built-in functions are switched on by default. If you want to suppress a message permanently, you can use
Off.
This prints a warning message. Also, the front end highlights the extra argument in red.
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This suppresses the warning message.
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The message reappears with the next evaluation.
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You can use
On and
Off to make global changes to the printing of particular messages. You can use
Off to switch off a message if you never want to see it.
You can switch off the message like this. |
Now no warning message is produced.
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Although most messages associated with built-in functions are switched on by default, there are some which are switched off by default, and which you will see only if you explicitly switch them on. An example is the message
General::newsym, discussed in
"Intercepting the Creation of New Symbols", which tells you every time a new symbol is created.
| s::tag | give the text of a message |
| s::tag=string | set the text of a message |
| Messages[s] | show all messages associated with s |
Manipulating messages.
The text of a message with the name
s::tag is stored simply as the value of
s::tag, associated with the symbol
s. You can therefore see the text of a message simply by asking for
s::tag. You can set the text by assigning a value to
s::tag.
If you give LinearSolve a singular matrix, it prints a warning message.
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Here is the text of the message.
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This redefines the message.
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Now the new form will be used.
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Messages are always stored as strings suitable for use with
StringForm. When the message is printed, the appropriate expressions are "spliced" into it. The expressions are wrapped with
HoldForm to prevent evaluation. In addition, any function that is assigned as the value of the global variable
$MessagePrePrint is applied to the resulting expressions before they are given to
StringForm. The default for
$MessagePrePrint uses
Short for text formatting and a combination of
Short and
Shallow for typesetting.
Most messages are associated directly with the functions that generate them. There are, however, some "general" messages, which can be produced by a variety of functions.
If you give the wrong number of arguments to a function
F,
Mathematica will warn you by printing a message such as
F::argx. If
Mathematica cannot find a message named
F::argx, it will use the text of the "general" message
General::argx instead. You can use
Off[F::argx] to switch off the argument count message specifically for the function
F. You can also use
Off[General::argx] to switch off all messages that use the text of the general message.
Mathematica prints a message if you give the wrong number of arguments to a built-in function.
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This argument count message is a general one, used by many different functions.
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If something goes very wrong with a calculation you are doing, it is common to find that the same warning message is generated over and over again. This is usually more confusing than useful. As a result,
Mathematica keeps track of all messages that are produced during a particular calculation, and stops printing a particular message if it comes up more than three times. Whenever this happens,
Mathematica prints the message
General::stop to let you know. If you really want to see all the messages that
Mathematica tries to print, you can do this by switching off
General::stop.
| $MessageList | a list of the messages produced during a particular computation |
| MessageList[n] | a list of the messages produced during the processing of the nth input line in a Mathematica session |
Finding out what messages were produced during a computation.
In every computation you do,
Mathematica maintains a list
$MessageList of all the messages that are produced. In a standard
Mathematica session, this list is cleared after each line of output is generated. However, during a computation, you can access the list. In addition, when the
nth output line in a session is generated, the value of
$MessageList is assigned to
MessageList[n].
This returns $MessageList, which gives a list of the messages produced.
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The message names are wrapped in HoldForm to stop them from evaluating.
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In writing programs, it is often important to be able to check automatically whether any messages were generated during a particular calculation. If messages were generated, say as a consequence of producing indeterminate numerical results, then the result of the calculation may be meaningless.
| Check[expr,failexpr] | if no messages are generated during the evaluation of expr, then return expr, otherwise return failexpr |
| Check[expr,failexpr,s1::t1,s2::t2,...] | check only for the messages si::ti |
Checking for warning messages.
Evaluating 1^0 produces no messages, so the result of the evaluation is returned.
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Evaluating 0^0 produces a message, so the second argument of Check is returned.
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Check[expr, failexpr] tests for all messages that are actually printed out. It does not test for messages whose output has been suppressed using
Off.
In some cases you may want to test only for a specific set of messages, say ones associated with numerical overflow. You can do this by explicitly telling
Check the names of the messages you want to look for.
The message generated by Sin[1, 2] is ignored by Check, since it is not the one specified.
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| Message[s::tag] | print a message |
| Message[s::tag,expr1,...] | print a message, with the expri spliced into its string form |
Generating messages.
By using the function
Message, you can mimic all aspects of the way in which built-in
Mathematica functions generate messages. You can for example switch on and off messages using
On and
Off, and
Message will automatically look for
General::tag if it does not find the specific message
s::tag.
This defines the text of a message associated with f.
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When the argument of f is greater than 10, the message is generated.
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This switches off the message. |
Now the message is no longer generated.
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When you call
Message, it first tries to find a message with the explicit name you have specified. If this fails, it tries to find a message with the appropriate tag associated with the symbol
General. If this too fails, then
Mathematica takes any function you have defined as the value of the global variable
$NewMessage, and applies this function to the symbol and tag of the message you have requested.
By setting up the value of
$NewMessage appropriately, you can, for example, get
Mathematica to read in the text of a message from a file when that message is first needed.
