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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?

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3 Answers 3

up vote 15 down vote accepted

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:

enter image description here

\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}

a is \IsInteger{a}{an integer}{not an integer}

Empty String is \IsInteger{}{an integer}{not an integer}

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}
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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 3:31
    
Peter, thank you. :) –  Kaveh Mar 31 '12 at 4:56
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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).

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\mycommand{\undefined\@undefined} will fail. I am sure you know why I have used two undefined tokens. –  Ahmed Musa Mar 31 '12 at 0:27
    
Thank you. :) I decided to use the solution given by Peter. –  Kaveh Mar 31 '12 at 4:56
    
@AhmedMusa All the proposed macros fail with only one undefined token. –  egreg Mar 31 '12 at 8:26
    
Very nice solution. Your first example is exactly what I needed today (with slight modification to handle negative numbers too). –  A.Ellett Mar 27 at 2:57
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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
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