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To save time when debugging with Print statements, I'd like to define a function -- call it PrintVal -- that prints both the name and value of a variable defined locally within the calling function. The function would be called with a single argument, which would be the name of the variable in question (expressed in the form of a string). To provide a concrete example, if fooVar is a variable defined as local to a module within the calling function, and if the current value of fooVar is 9, then

PrintVal[ "fooVar" ]

should print

fooVar = 9

The new function would thus have the same effect as including the statement

Print[ "fooVar = ", fooVar ]

directly within the calling function itself, but without the need to type the variable name twice, to include the equal sign, and (most important from a time-saving perspective) to include some other formatting stuff that I haven't mentioned here (because it's not relevant to the part I'm struggling with).

Although there may well be an embarrassingly simple solution that I don't know enough to have thought of, after a considerable amount of experimentation, documentation-reading, and web-searching, I've been unable to find one. With apologies in advance if I'm missing something obvious, I'd be very grateful to anyone who might provide a (detailed and concrete) solution.

Many thanks!

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  • $\begingroup$ Is it necessary for the argument to be passed as a String? Could you use PrintVal[fooVar] instead if the output were the same? $\endgroup$ – Mr.Wizard Nov 6 '14 at 9:07
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    $\begingroup$ Mr. Wizard: No need for the argument to be specified as a string. Indeed, specifying the argument to PrintVal without having to include the quotes will save me even more time, and the solution you've provided below seems to work perfectly! $\endgroup$ – grateful user Nov 6 '14 at 22:34
  • $\begingroup$ I'm glad I could help, and thanks for the Accept. :-) $\endgroup$ – Mr.Wizard Nov 7 '14 at 1:21
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Here is a method that is not exactly to your specification but which may serve you anyway. It uses a Symbol argument. Because a Symbol is passed it is automatically modified by Module and does not need the guesswork of $ModuleNumber etc. but it does need string conversion and cleanup.

SetAttributes[printVal, HoldFirst];

printVal[s_Symbol] := 
  Print @ Row[{StringTrim[SymbolName@Unevaluated@s, "$" ~~ DigitCharacter ..], s}, "="]

An example:

b = "Fail!" (*this should not print!*);

Module[{b},
 b = RandomInteger[9, 3]^2;
 printVal[b];
 b
]

b = {16, 4, 36}

{16, 4, 36}

Regarding why naive attempts fail you must consider the mechanism of Module; only visible (explicit) Symbols in the body are substituted by localized equivalents. A String that is later converted to a Symbol is not included in this localization:

b = "Fail!";

Module[{b},
 b = RandomInteger[9, 3]^2;
 Print[b];
 Print[Unevaluated@b];
 Print[Symbol["b"]];
]

{81, 64, 36}

b$2061

Fail!

Note that Symbol["b"] evaluates to the global b rather than the localized b$2061.

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  • $\begingroup$ You may add .. to "$" so it works well in DynamicModule too. One should be careful where it is called there anyway. $\endgroup$ – Kuba Nov 6 '14 at 10:01
  • $\begingroup$ @Kuba I hadn't thought about DynamicModule. (Thanks for reminding me.) I don't know how this would behave there; I need to think about that. $\endgroup$ – Mr.Wizard Nov 6 '14 at 10:02
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    $\begingroup$ Printing should be held till DM content is displayed. DynamicModule[{b}, { Slider[Dynamic[b, (b = #; printVal[b];) &]], printVal[b]; SymbolName[Unevaluated@b], Button["print b", printVal[b]] }] $\endgroup$ – Kuba Nov 6 '14 at 10:07
  • $\begingroup$ As noted above, in the quick tests I've just run, @Mr.Wizard's solution seems to work perfectly for me. I'm also grateful to Kuba for his comment. Many thanks to both of you! $\endgroup$ – grateful user Nov 6 '14 at 22:46
  • $\begingroup$ I tried this solution on a Manipulate Control variable, and it gave unexpected results ("b\$" instead of "b"). The reason was that these variables use two "\$"s and not just one. The easy fix is to use ("\$"..) instead of just "\$" in the String pattern. $\endgroup$ – Spencer Rugaber Jan 23 at 20:26

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