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I want to label a scale based on a variable value and I wrote a metapost code like this.

$ := 1;
for i = -.3cm step .6cm until 3.6cm:
    label.bot(str$,(i,-3.7cm));
    $ := incr($);
endfor;

It just labels "$" instead of its value. How to output the variables value?

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2 Answers 2

up vote 6 down vote accepted

To get the value of the variable, use the decimal macro. The following works:

$:=1;
for i = -.3cm step .6cm until 3.6cm:
    label.bot(decimal $,(i,-3.7cm));
    $ := incr($);
endfor

To quote the MetaPost manual:

The str operator is generally for emergency use only.

further

The decimal operator takes a number and returns the string representation.

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If you need a label, which is in TeX (between btex and etex), and you want to use value of a variable there, you can do something like this:

a[0]:=1/2;
for t=0 upto 7:
  a[t+1]:=sqrt(a[t]);
  draw (a[t]*ux,a[t]*uy)--(a[t]*ux,a[t+1]*uy);
  draw (a[t]*ux,a[t+1]*uy)--(a[t+1]*ux,a[t+1]*uy);
  draw (a[t]*ux,-dx)--(a[t]*ux,dx);
  write "label.bot (btex $x_{"&decimal(t)&"}$ etex,(a["&decimal(t)&"]*ux,-dx));"
  to "mptextmp.mp";
endfor;

write EOF to "mptextmp.mp";
scantokens "input mptextmp"  

See also this post on SO: METAPOST: using loop variables in labels

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The code I posted above was part of the metapost source used to obtain this. –  Martin Apr 26 at 12:30

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