Prepend Information to Warning Messages

Question

I have a function running within a Do loop that sometimes issues a warning. I'd like to prepend the warning with the loop ctr so that I can go back and debug that instance later.
Basically, I would like to modify the following line,

Do[i^0, {i, -1, 1}]

so that instead of displaying the warning:

Power::indet: Indeterminate expression 0^0 encountered. >>

it displays:

i=0, Power::indet: Indeterminate expression 0^0 encountered. >>

Where i==0 is the iteration that i^0 issues the warning.

Thanks

Answer 1

Here is my proposal for tagging messages with (the value of) an arbitrary expression at the time of message generation. The tag is placed inside the the message itself.

ClearAll[withTaggedMsg]
SetAttributes[withTaggedMsg, HoldAll]

withTaggedMsg[exp_, label_: "When"] := Function[,
   Internal`InheritedBlock[{MessagePacket},
     Unprotect @ MessagePacket;
     mp : MessagePacket[__, _BoxData] /; !TrueQ[$tagMsg] :=
   Block[{$tagMsg = True},
         Style[Row[{label, HoldForm[exp], "=", exp, " "}, "  "], "SBO"] /. tag_ :>
           MapAt[RowBox[{ToBoxes @ tag, #}] &, mp, {-1, 1}] // Identity
       ];
     #
   ],
  HoldAll]

Usage:

Do[i^0, {i, -1, 1}] // withTaggedMsg[i]

Do[i^0, {i, -1, 1}] // withTaggedMsg[i, "At iteration"]

---

Note: this only works with variables that are either globally accessible or are scoped using Block. For example,

f[x_] := Message[f::brains, x]
f[5] // withTaggedMsg[x]
(* At iteration x = x f::brains: -- Message text not found -- (5) *)

Module[{x = 5},
 Message[f::brains, x]
] // withTaggedMsg[x]
(* At iteration x = x f::brains: -- Message text not found -- (5) *)

With[{x = 5},
 Message[f::brains, x]
] // withTaggedMsg[x]
(* At iteration x = x f::brains: -- Message text not found -- (5) *)

Block[{x = 5},
 Message[f::brains, x]
] // withTaggedMsg[x]
(* At iteration x = 5 f::brains: -- Message text not found -- (5) *)

This means that any variable that is scoped using Block can be used to tag a message. So, loop variables from Do and Table are accessible via this method, in addition to any Block variable. This makes it indispensable as a debugging tool.

Answer 2

I cannot seem to make it do exactly what you want do to how messages are created, but here is a serviceable alternative using $MessagePrePrint. $MessagePrePrint formats the variables specified in the message string, and in your example, the message has the form

General::indet = "Indeterminate expression `1` encountered."

where the `1` will be replaced by $0^0$, or whatever else you pass to it. It is that argument that $MessagePrePrint operates on, and we can change it to suit us, as follows

Block[{$MessagePrePrint},
     $MessagePrePrint := ToString[StandardForm[#]] <> " at i = " <> ToString[i] &;
 Do[i^0, {i, -1, 1}]
]

where the output will now read "$0^0\text{ at i} =1$".

---

(Edit from Mr.Wizard)

The code above fails in Mathematica 7. It seems that v7 doesn't like the fact that

InputForm @ ToString @ StandardForm @ HoldForm[0^0]

 "\!\(\*TagBox[\(0\^0\), HoldForm]\)"

Here is a variation that is compatible with version 7:

Block[{$MessagePrePrint},
      $MessagePrePrint := Row@{#, " at i = ", i} ~ToString~ StandardForm &;
  Do[i^0, {i, -1, 1}]
]

This is still not ideal however, as every field in the message gets this tag. For example, if you enter the spurious Inner[f, {{1, 2}}, {3}] you get:

Inner::incom: Length 2 at i = 1 of dimension 2 at i = 1 in {{1,2}} at i = 1 is incommensurate with length 1 at i = 1 of dimension 1 in {3} at i = 1. >>

Answer 3

rcollyer has a nice solution. Here's another possibility using Check and printing the list of messages generated at the current evaluation.

Quiet@Block[{$OldMessages = 0}, 
        Do[Check[#^#/# &@Mod[i, 2], 
            Print@StringForm["At i=``, ``", i, $MessageList[[$OldMessages + 1 ;;]]]; 
            $OldMessages = Length@$MessageList;], 
        {i, 0, 5}]
   ];

(* At i=0, {Power::indet,Power::infy}
   At i=2, {Power::indet,Power::infy}
   At i=4, {Power::indet,General::stop,Power::infy,General::stop} *)

Remove the Quiet@ if you want to see the actual messages generated.