documentclass{article}
usepackage{fp}
usepackage{siunitx,amsmath}
%usepackage{xparse}
ExplSyntaxOn
NewDocumentCommand{newarray}{m}
 {
  seq_new:c { l_hafid_array_ cs_to_str:N #1 _seq }
  cs_new:Npn #1 (##1)
   {
    seq_item:cn { l_hafid_array_ cs_to_str:N #1 _seq } { ##1 }
   }
 }
NewDocumentCommand{readarray}{mm}
 {
  seq_set_split:cnn { l_hafid_array_#1_seq } { & } { #2 }
 }
cs_generate_variant:Nn seq_set_split:Nnn { c }
ExplSyntaxOff

begin{document}

newarraysomevalues
readarray{somevalues}{1&2&3.5&4&5}

begin{align*}
v_1 &= somevalues(1)\
v_2 &= somevalues(2)\
v_3 &= somevalues(3)\
v_4 &= somevalues(4)\
v_5 &= somevalues(5)
end{align*}

First number = somevalues(1)\
Second number = somevalues(2)

FPaddxx{somevalues(1)}{somevalues(2)}
FPeval{xx}{round(xx,4)}%
sum of first two numbers = xx

FPevalxx{somevalues(1)+somevalues(2)}
FPeval{xx}{round(xx,4)}%
sum of first two numbers = xx


%
newcountcount
count=5
FPsetans{0}
loop
%somevalues(count)\
FPaddans{ans}{somevalues(count)}
%FPevalans{ans+somevalues(count)}
advance count by -1
unlessifnum count<1
repeat
%
FPeval{ans}{round(ans,4)}%
sum of all numbers = ans

end{document}