How to capture an error message to print it out later?

Questions
,
1

I want to capture an error message so I can print it out later. For exmaple, I can capture the the error message for division by zero in the following by setting myerror equal to the $MessageList as long as it is on the same line, apparently. And by repeating myerror on the next line, the output proves that I have captures the “Power::infy” error message. But the specific error statement “Infinite expression 1/0 encountered. >>” I don’t know how to capture.

Executing this code

c = 1/0; myerror= $MessageList

myerror

produces this output:

Power::infy: Infinite expression 1/0 encountered. >>

{Power::infy}

{Power::infy}

So, how do I capture the specific error message "Infinite expression 1/0 encountered. >> " ?
Thanks!

2

Your comment on another post is a separate question and it was re-posted as a new discussion. Please in future do not ask new questions on existing discussions but submit them as new. Also note code-box formatting and please follow it in future.

3

One way would be to add your own output stream to $Messages, which is a list that holds the output streams that messages are written to. Normally $Messages is a list of one output stream, namely the standard output stream (stdout). So this opens an output stream and appends it to $Messages:

$MyMessageLogger = OpenWrite["message.out"];
AppendTo[$Messages, $MyMessageLogger];

After that you can evaluate things that generate messages:

1/0;

And you can look at the content using FilePrint (or Import or OpenRead):

FilePrint["message.out"]

which gives:

                                 1
Power::infy: Infinite expression - encountered.
                                 0

If you don’t want to see any messages on your screen, just set $Messages to your output stream (instead of using AppendTo):

$Messages={$MyMessageLogger];
4

Thanks to the Moderation Team for the tip. Thanks to Arnoud Buzing for the info.

5

How do I get

OpenWrite["message.out"];

to use the FormatType “InputForm” one-dimensional form, so I don’t get the two-dimensional fractions like

                                 1
Power::infy: Infinite expression - encountered.
                                 0

when using

FilePrint["message.out"]
6

You can use $MessagePrePrint to format the message arguments:

$MessagePrePrint = Function[{e}, InputForm[e]]

will make this:

1/0;

give this message:

Power::infy: Infinite expression HoldForm[0^(-1)] encountered.

The HoldForm is required in messages, to avoid an infinite recursion (and InputForm always displays HoldForm as is).
If you don’t like the HoldForm, you could convert the HoldForm to a string and remove that:

$MessagePrePrint = Function[{e}, StringTake[ToString[e, InputForm], {10, -2}]]

which gives:

Power::infy: Infinite expression 0^(-1) encountered.