Your code does not work because the way that the fp package parses its input. In TeX when you define a delimited macro the delimiter text must always be present, so \somevalues must always be followed by a (, then some text, then a closing ). When fp tries to parse your expression it separates the tokens and adds other tokens in between, then the error you get is:
! Use of \somevalues doesn't match its definition.
<argument> \somevalues
\FP@gen@code {2}
because what follows \somevalues is \FP@gen@code, not a (.
Since you're using expl3 anyway you can use its l3fp module, which offers an expandable FP engine. Being expandable means that you can do
The~value~of~$\pi^2$~is:~\fp_eval:n { pi^2 }
or even
\setlength{\textwidth}{\fp_eval:n{ 10*30 } pt}
Here I emulated fp's macros \FPadd, \FPset and \FPeval using l3fp instead. Note that these emulated macros aren't expandable anymore.
Now you can do, for instance:
\FPset\ans{5}
\FPeval\ans{ans+10}
ans is: \ans % prints 15
Every macro, say, \ans, used as the first argument to \FP<something> is defined with the result of the operation, plus a FP word called ans is defined and will contain that same value, so inside an expression you can use either \ans or ans.
CAUTION! This does not perform any verification whatsoever if the involved macros are already defined, so you can accidentally overwrite something important. \FPeval\pi{6.283185} might not be a good idea.
Here's the code (I only changed the definitions of the FP functions; your array handling and test code is the same):
\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 }
% Stolen from l3fp:
\fp_new:N \l__sandu_temp_fp
\cs_set_protected:Npn \sandu_fp_name:nn #1 #2
{
\cs_undefine:c { c__sandu_#1_fp }
\fp_const:cn { c__sandu_#1_fp } { #2 }
\__sandu_define_fp_name:cn { c__sandu_#1_fp } { #1 }
}
\cs_new_protected:Npn \__sandu_define_fp_name:Nn #1 #2
{
\cs_set:cpn { __fp_parse_word_#2:N }
{ \exp_after:wN #1 \exp:w \exp_end_continue_f:w \__fp_parse_infix:NN }
}
\cs_generate_variant:Nn \__sandu_define_fp_name:Nn { c }
\cs_generate_variant:Nn \sandu_fp_name:nn { f }
% Emulating fp.sty
\NewDocumentCommand \FPadd {m mm}
{
\tl_set:Nx #1 { \fp_eval:n { #2 + #3 } }
\sandu_fp_name:fn { \cs_to_str:N #1 } { #1 }
}
\NewDocumentCommand \FPeval {m m}
{
\tl_set:Nx #1 { \fp_eval:n { #2 } }
\sandu_fp_name:fn { \cs_to_str:N #1 } { #1 }
}
\NewDocumentCommand \FPset {m m}
{
\tl_set:Nx #1 { \fp_eval:n { #2 } }
\sandu_fp_name:fn { \cs_to_str:N #1 } { #1 }
}
\ExplSyntaxOff
\begin{document}
\newarray\somevalues
\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)
\FPadd\xx{\somevalues(1)}{\somevalues(2)}
\FPeval{\xx}{round(xx,4)}%
sum of first two numbers = \xx
\FPeval\xx{\somevalues(1)+\somevalues(2)}
\FPeval{\xx}{round(xx,4)}%
sum of first two numbers = \xx
%
\newcount\counter
\counter=5
\FPset\ans{0}
\loop
%\somevalues(\counter)\\
\FPadd\ans{\ans}{\somevalues(\counter)}
%\FPeval\ans{\ans+\somevalues(\counter)}
\advance \counter by -1
\unless\ifnum \counter<1
\repeat
%
\FPeval{\ans}{round(ans,4)}%
sum of all numbers = \ans
\end{document}
which prints:
P.S.: Don't do \newcount\count because you'll redefine the TeX primitive \count. Use another name.
expl3anyway why don't use usel3fpinstead offp? – Phelype Oleinik Mar 21 '19 at 10:58expl3. I copiedexpl3code, to define array. – sandu Mar 21 '19 at 11:01l3fp's tuples, which are a built-in way of handling arrays, so they have a good support inl3fp. You can take a look at the LaTeX3's interfaces to see what programming facilities it offers. – Phelype Oleinik Mar 21 '19 at 11:42