# [NTG-context] running lua in metafun and in ConTeXt

Hans Hagen j.hagen at xs4all.nl
Sat Apr 11 19:08:38 CEST 2020

On 4/11/2020 4:15 PM, Jeong Dal wrote:
> Dear all,
>
> The problem is solved by using the namespace of lua as below:
> I am not sure what I did is the right way.
> If it is not the right way, please let me know.
>
>
> Best regards,
>
> Dalyoung
>
>
>
> \startluacode
> P={}
> combi = P
>
> function P.fact (n)
> if n <= 0 then
> return 1
> else
> return n * P.fact(n-1)
> end
> end
>
> function P.ncr(n,r)
> return P.fact(n)/(P.fact(r)*P.fact(n-r))
> end
> combi = {
> fact = fact,
> ncr = ncr,
> }
> \stopluacode
>
> \startbuffer[fig121]
> numeric n,r,s,u,dx,dy,tt; u := 1.8cm;
> path p, q;
> pair A,B,start,now;
> A := dir(210)*u;
> B := dir(-30)*u;
> dy := sind(30)*u;
> dx := 2*cosd(30)*u;
> for n=0 upto 4:
> start := n*dir(210)*u;
> for r=0 upto n:
> s := n-r;
> % tt := lua("mp.print(P.fact(" & decimal n & ")/(P.fact(" & decimal r &
> ")*P.fact(" & decimal s &" )))");
> tt := lua("mp.print(P.ncr(" & decimal n & "," & decimal r & " ))");
> now := start+r*right*dx;
> dotlabel.top(textext("$\displaystyle {" & decimal n & "\choose" & > decimal r & "} = "& decimal tt & "$"),now);
> draw now -- (now+A);
> draw now -- (now+B);
> endfor;
> endfor;
> \stopbuffer
Sunday afternoon educational moment (that you can wikify), four variants:

1 : A more metafunish alternative of your example.
2 : The same but avoiding a temporary variable.
3 : Less code and clutter, the real deal.
4 : Idem, but permits more tuning at the TeX end.

\unexpanded\def\MyWhatever#1#2#3%
{$\displaystyle{#1\choose #2} = #3$}

\startluacode
local function fact(n)
if n <= 0 then
return 1
else
return n * fact(n - 1)
end
end

local function whatever(n,r)
return fact(n) / (fact(r) * fact(n-r))
end

MP.WhateverA = whatever

function MP.WhateverB(n,r)
mp.quoted("%.0f",whatever(n,r))
end

function MP.WhateverC(n,r)
mp.quoted([[$\displaystyle{%.0f\choose %.0f} = %.0f$]],n,r,whatever(n,r))
end

function MP.WhateverD(n,r)
mp.quoted([[\MyWhatever{%.0f}{%.0f}{%.0f}]],n,r,whatever(n,r))
end
\stopluacode

\startbuffer[fig121]
numeric n, r, s, u, dx, dy, tt;
path p, q ; pair A, B, start, now;
u := 1.8cm;
A := dir(210)*u;
B := dir(-30)*u;
dy := sind(30)*u;
dx := 2*cosd(30)*u;
for n = 0 upto 4:
start := n*dir(210)*u;
for r = 0 upto n:
s := n - r;
now := start + r*right*dx;
draw (now + A) -- now -- (now + B);

tt := lua.MP.WhateverA(n,r) ;

dotlabel.top(textext("$\displaystyle {" & decimal n & "\choose" & decimal r & "} = "& decimal tt & "$"),now);

dotlabel.top(textext("$\displaystyle {" & decimal n & "\choose" & decimal r & "} = "& lua.MP.WhateverB(n,r) & "$"),now);

dotlabel.top(textext(lua.MP.WhateverC(n,r)),now);

dotlabel.top(textext(lua.MP.WhateverD(n,r)),now);

endfor;
endfor;
\stopbuffer

\starttext

{\switchtobodyfont[11pt]\processMPbuffer[fig121]}

\stoptext

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