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I tried to convert a $\LaTeX$ expression in Mathematica, but it shows me a strange result (it shows a strange word: "ghit").

ToExpression["\\frac{s + k_{12} + k_{02}}
 {V_1 \left[ s^2 + s \left(  k_{12} + k_{02} + k_{21} + k_{01} \r\
ight)  + \left(  k_{21} \, k_{02} + k_{01} \, k_{12} + k_{01} \, \
k_{02} \right) \right] }", TeXForm]

enter image description here

Thank you for your help.

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  • $\begingroup$ Shouldn't the single \ be a double \\ ? (You can evaluate the string by itself to see the problem.) $\endgroup$
    – Michael E2
    Dec 7, 2017 at 12:23
  • $\begingroup$ You have to use a backslash \ as escape character for \ (and also for ") when you manipulate strings in Mathematica, exactly as you did in "\\frac{...}". $\endgroup$
    – Henrik Schumacher
    Dec 8, 2017 at 21:24

1 Answer 1

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Backslashes, i.e. \, must be escaped in Mathematica strings.

This is incorrect syntax:

"\xyz"

The correct syntax is

"\\xyz"

Unfortunately, Mathematica tries to be "user-friendly" and when using the notebook interface, it accepts "\xyz" without complaint. (If using the terminal, it at least shows a warning.) This creates the false impression that it's fine to do this. It's not. Sequences like \r (which you used), \b, \f, \n, etc. have special meanings. New escape sequences may also be added in the future, breaking code like "\xyz".

Thus: always, always escape backslashes in strings!.

str = "\\frac{s + k_{12} + k_{02}}{V_1 \\left[ s^2 + s \\left(  k_{12} + k_{02} + k_{21} + k_{01} \\right)  + \\left(  k_{21} \\, k_{02} + k_{01} \\, k_{12} + k_{01} \\, k_{02} \\right) \\right] }";

ToExpression[str, TeXForm]

enter image description here

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  • $\begingroup$ thank you very much for your excellent solution and explanation! $\endgroup$
    – Gennaro Arguzzi
    Dec 7, 2017 at 13:05
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    $\begingroup$ Is there a way to prevent MMA from dropping the zero in $k_{02}$? $\endgroup$
    – Janosh
    Dec 7, 2017 at 13:38
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    $\begingroup$ @Casimir Good point. I did not notice that. I do not know how to do what you are asking. It is not even about dropping it, but getting it to understand that $0$ and $2$ are two separate numbers. This is a good example of notation that is clear to a human, but not at all to a computer. k_{0,2} would produce a better result. $\endgroup$
    – Szabolcs
    Dec 7, 2017 at 13:49
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    $\begingroup$ @GennaroArguzzi Be aware of the problem pointed out by Casimir above. $\endgroup$
    – Szabolcs
    Dec 7, 2017 at 13:49
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    $\begingroup$ @Casimir You could use k_{\"02\"} instead of k_{02}. $\endgroup$
    – Carl Woll
    Dec 7, 2017 at 15:32

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