How to check if the value of a parameter is a number?

Question

I have a command with a parameter. I want to check if its value is a positive number and perform actions based on that.

\newcommand{\mycommand}[1]{
    \ifnum#1>0%
        %some actions
    \fi
}

But I receive errors when the value of the parameter is not a number, e.g.:

\mycommand{*}

How can I check if the value of #1 is a number?

See http://tex.stackexchange.com/questions/7180/testing-for-number perhaps? — Joseph Wright, Mar 30 '12 at 21:03
In many situations, e.g., with \ifnum, (La)TeX does form numbers by expanding expandable tokens. Thus your request implies also checking whether the tokens forming the "given parameter" at the stage of expansion form an algorithm which terminates at all and which does not trigger errors. This is the halting problem. — Ulrich Diez, May 18 '19 at 08:45

score 26 · Accepted Answer · edited Jun 10 '20 at 12:32

26

Update:

Since it appears that the \IfInteger from the xtring package perceives blank strings as integers (the empty string {} is ok, but not { }), I have defined a modified macro \IsInteger which handles that case:

\documentclass{article}
\newcommand*{\IsInteger}[3]{%
    \IfStrEq{#1}{ }{%
        #3% is a blank string
    }{%
        \IfInteger{#1}{#2}{#3}%
    }%
}%
\usepackage{xstring}
\begin{document}
$2$ is \IsInteger{2}{an integer}{not an integer}\par
$2.0$ is \IsInteger{2.0}{an integer}{not an integer}
$-7$ is \IsInteger{-7}{an integer}{not an integer}\par
$-7.0$ is \IsInteger{-7.0}{an integer}{not an integer}
$2.1$ is \IsInteger{2.1}{an integer}{not an integer}\par
$-7.1$ is \IsInteger{-7.1}{an integer}{not an integer}
a is \IsInteger{a}{an integer}{not an integer}
Empty String is \IsInteger{}{an integer}{not an integer}\par
Blank String is \IsInteger{       }{an integer}{not an integer}
\end{document}

You can use IfInteger from the xstring package to test if it is an integer number:

enter image description here

There is also \IfDecimal which works similarly.

\documentclass{article}
\usepackage{xstring}
\begin{document}
2 is \IfInteger{2}{integer}{not an integer}
a is \IfInteger{a}{integer}{not an integer}
\end{document}

edited Jun 10 '20 at 12:32

Community

1

answered Mar 30 '12 at 18:24

Peter Grill

223,288

Thank you, this solved the problem. By the way, is there a TeX macro similar to \ifnum that I can use? I would prefer to keep the number of packages I am using to minimum. – Kaveh Mar 30 '12 at 19:10
I just noticed that \IfInteger interprets the empty string as an integer. – Kaveh Mar 30 '12 at 19:16
1

@Kaveh: Had created a modified macro \IsInteger to compensate for that. – Peter Grill Mar 31 '12 at 03:31
It also appears that \ifinteger interprets negative numbers as non-integer. – Rob Hall May 06 '20 at 16:50
@RobHall: Seems to work fine for negative values as well. See updated MWE. – Peter Grill May 06 '20 at 18:25

score 22 · Answer 2 · answered Mar 30 '12 at 20:25

22

It depends mostly on the expected input and also on the context where you want to use the command.

If your expected input is either a number or something that doesn't start with digits, then

\newcommand{\mycommand}[1]{%
  \ifnum0<0#1\relax
    #1 is a positive number%
  \else
    #1 is not a positive number%
  \fi}

will do. For example, \mycommand{42} will do the comparison 0<042 which is true; instead, with \mycommand{*} TeX will see \ifnum0<0*\relax and it will test 0<0, which is false, so the * will be ignored as part of the "true text". Also the test from \mycommand{0} will evaluate to false.

On the other hand, with \mycommand{1x} the test will evaluate to true and give wrong results.

Another expandable way can be

\def\mycommand#1{%
  \if\relax\detokenize\expandafter{\romannumeral-0#1}\relax
    #1 is a number%
  \else
    #1 is not a number%
  \fi
}

but \mycommand{0} would test true. Here \mycommand{1x} would answer that 1x is not a number.

However, the argument should not contain "dangerous" items: \mycommand{\textbf{x}} would fail miserably.

A non-expandable test can be

\makeatletter
\def\mycommand#1{%
  \afterassignment\get@args\count@=0#1\hfuzz#1\hfuzz}
\def\get@args#1\hfuzz#2\hfuzz{%
  \if\relax\detokenize{#1}\relax
    #2 is a number%
  \else
    #2 is not a number%
  \fi
}
\makeatother

This works also with input such as \mycommand{\textbf{1}} (the test will evaluate to false).

answered Mar 30 '12 at 20:25

egreg

1,121,712

\mycommand{\undefined\@undefined} will fail. I am sure you know why I have used two undefined tokens. – Ahmed Musa Mar 31 '12 at 00:27
Thank you. :) I decided to use the solution given by Peter. – Kaveh Mar 31 '12 at 04:56
@AhmedMusa All the proposed macros fail with only one undefined token. – egreg Mar 31 '12 at 08:26
Very nice solution. Your first example is exactly what I needed today (with slight modification to handle negative numbers too). – A.Ellett Mar 27 '14 at 02:57
How would it be possible to use this command as a typical if-then-else command? E.g. \mycommand{\textbf{1}}{<Output if number>}{<Outpuf if not number>} – jessepeng Jun 14 '19 at 14:56
1

@jessepeng In the first solution, substitute #1 is a number by \expandafter\@firstoftwo and #1 is not a number by \expandafter\@secondoftwo. Similarly for the second solution. – egreg Jun 14 '19 at 15:25
And could this now be easily extended to also handle floating point numbers? Or would an entirely different approach be necessary. – jessepeng Jun 15 '19 at 15:00
Nevermind, I asked a seperate question for this: https://tex.stackexchange.com/questions/495919/checking-if-argument-is-a-floating-point-without-breaking-on-control-sequences-i – jessepeng Jun 15 '19 at 15:12

score 4 · Answer 3 · answered Mar 31 '12 at 15:48

Here is an expandable solution that accepts even undefined control sequences. It is admittedly acrobatic.

Some of the commands in catoptions package are redefined here for the sake of speed and experimentation.

\documentclass[a4paper]{article}
\usepackage{catoptions}
\makeatletter
\new@def\cptifdef#1{%
  \cptifblank{#1}{%
    \@secondoftwo
  }{%
    \csname @\ifx#1\@undefined second\else
    \ifx#1\relax second\else first\fi\fi oftwo\endcsname
  }%
}
\new@def\cptifundef#1{\cptifdef{#1}\@secondoftwo\@firstoftwo}
\new@def\cptifleftbraced#1{%
  \cptifblank{#1}{%
    \@secondoftwo
  }{%
    \csname @\if\expandafter\cpt@car\detokenize{#1}\car@nil
      \expandafter\cpt@car\string{\car@nil\ifnum0=`}\fi
      first\else second\fi oftwo\endcsname
  }%
}
\new@def\cptifxpandable#1{%
  \cptifleftbraced{#1}{%
    \@secondoftwo
  }{%
    \expandafter\ifx\noexpand#1#1%
      \expandafter\@secondoftwo
    \else
      \expandafter\@firstoftwo
    \fi
  }%
}
\newcommand\cpttwooftwo[2]{#1#2}
\newcommand\cptifblank[1]{%
  \expandafter\ifx\expandafter\noboundary\@gobble#1.\noboundary
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\newcommand\cptifcmdeq[2]{%
  \ifx#1#2\cpt@quark
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\long\def\cptifsolo#1{%
  \if0\pdfstrcmp{\detokenize\expandafter
    {\cpttwooftwo#1{}{}}}{\detokenize{#1{}}}%
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\new@def\cptxpand#1{%
  \romannumeral-`\q\cptifblank{#1}{\space}{\cpt@xpand#1\xpand}%
}
\def\cpt@xpand#1#2\xpand{%
  \cptifsolo{#1}{%
    \cptifundef{#1}{%
      \cpt@@xpand{#2}{\noexpand#1}%
    }{%
      \cptifxpandable{#1}{%
        \cpt@@xpand{#2}{\expandafter\cpt@xpand#1\xpand}%
      }{%
        \cpt@@xpand{#2}{\noexpand#1}%
      }%
    }%
  }{%
    \cpt@@xpand{#2}{\cpt@xpand#1\xpand}%
  }%
}
\def\cpt@@xpand#1{%
  \expandafter\cptswap\expandafter{\romannumeral-`\q
  \cptifblank{#1}{\space}{\cpt@xpand#1\xpand}}%
}
\def\ifinteger#1{%
  \if\cptifblank{#1}{1}{\expandafter\ifinteger@a\romannumeral-`\q
  \cpt@xpand#1\xpand\cpt@nnil}00%
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\def\ifinteger@a#1{%
  \cptifcmdeq#1\cpt@nnil{}{%
    \ifinteger@b#1\cpt@nil0123456789\cpt@nnil
  }%
}
\def\ifinteger@b#1\cpt@nil#2#3\cpt@nnil{%
  \expandafter\cptifcmdeq\cpt@car#1\car@nil#2{%
    \ifinteger@a
  }{%
    \cptifblank{#3}{%
      1\cpt@removetonnil
    }{%
      \ifinteger@b#1\cpt@nil#3\cpt@nnil
    }%
  }%
}

% Tests
\def\cmda{01}
\def\cmdb{xy}
\let\cmdc\undefined
\edef\cmdd{\ifinteger{\cmda}{True}{False}}
\edef\cmde{\ifinteger{\cmdc}{True}{False}}
\edef\cmdf{\ifinteger{\cmda\cmdb\cmdc}{True}{False}}
\show\cmdf

score 2 · Answer 4 · answered Feb 04 '20 at 10:01

2

I know, i'm "a bit late" to the party, but this seems to work fine:

\makeatletter
\def\mycommand#1{%
  \sbox\z@{\@tempcnta=0#1\relax}%
  \expandafter\ifdim\wd\z@>\z@\relax
    #1 is NOT an integer
  \else
    #1 is an integer
  \fi}
\makeatother

This works as long as you try to find positive integers (i.e., counters), not floats or negative numbers.

answered Feb 04 '20 at 10:01

Lupino

2,772

Didn't work for me. Instead, the page I used the command on disappeared and it gave no error messages :/ – MD004 Sep 13 '23 at 19:18
@MD004 Sounds like \box0 is already occupied in your context. Try replacing \sbox\z@ with \sbox\@tempboxa and \wd\z@ with \wd\@tempboxa or maybe declare your own box register (\newbox\mybox) and use that for measuring, instead. – Lupino Sep 14 '23 at 20:23

How to check if the value of a parameter is a number?

4 Answers4

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