Using loops to create tables

First implementation:

\documentclass{article}
\usepackage{array,booktabs}
\usepackage{xparse}

\newcommand{\myhrule}{\rule{1cm}{0.4pt}}

\ExplSyntaxOn

\NewDocumentCommand{\makeruledtabular}{mm}
 {% #1 = number of rows, #2 = header
  \begin{tabular}{r w{c}{2cm} w{c}{2cm} @{}}
  \toprule
  #2 \\
  \midrule
  \int_step_function:nN {#1} \__hafid_ruledtabular_line:n
  \bottomrule
  \end{tabular}
 }

\cs_new_protected:Nn \__hafid_ruledtabular_line:n
 {
  #1 & \myhrule & \myhrule \\
 }

\ExplSyntaxOff

\begin{document}

\makeruledtabular{10}{\multicolumn{1}{c}{\(i\)} & \(P\) & \(y_{m}\) }

\end{document}

More general implementation: the number of columns is determined from the header.

\documentclass{article}
\usepackage{array,booktabs}
\usepackage{xparse}

\newcommand{\myhrule}{\rule{1cm}{0.4pt}}

\ExplSyntaxOn

\NewDocumentCommand{\makeruledtabular}{mm}
 {% #1 = number of rows, #2 = header
  \hafid_ruledtabular:nn { #1 } { #2 }
 }

\seq_new:N \l__hafid_ruledtabular_header_seq
\tl_new:N \l__hafid_ruledtabular_first_tl

\cs_new_protected:Nn \hafid_ruledtabular:nn
 {
  % absorb the header as a sequence
  \seq_set_split:Nnn \l__hafid_ruledtabular_header_seq { & } { #2 }
  % split off the first item, which should be centered
  \seq_pop_left:NN \l__hafid_ruledtabular_header_seq \l__hafid_ruledtabular_first_tl
  % Define the auxiliary function based on the number of items in the header
  % At the end, if the header is 'A & B & C & D`, the function will be
  % defined to do '#1 & \myhrule & \myhrule & \myhrule \\', where #1 stands
  % for the current index in the loop
  \cs_set_protected:Nx \__hafid_ruledtabular_line:n
   {
    ##1
    \prg_replicate:nn { \seq_count:N \l__hafid_ruledtabular_header_seq }
     { & \exp_not:N \myhrule }
    \exp_not:N \\
   }
  \begin{tabular}
   {
    r
    *{ \seq_count:N \l__hafid_ruledtabular_header_seq } { w{c}{2cm} }
    @{}
   }
  \toprule
  \multicolumn{1}{c}{\tl_use:N \l__hafid_ruledtabular_first_tl} &
  \seq_use:Nn \l__hafid_ruledtabular_header_seq { & } \\
  \midrule
  \int_step_function:nN {#1} \__hafid_ruledtabular_line:n
  \bottomrule
  \end{tabular}
 }

\ExplSyntaxOff


\begin{document}

\makeruledtabular{10}{\(i\) & \(P\) & \(y_{m}\) }

\bigskip

\makeruledtabular{15}{A & B & C & D}

\end{document}

The idea is that loop of the form ..._function:... will deliver the whole loop before anything else is processed, so in the case of a three column table with three rows we will get

\__hafid_ruledtabular_line:n { 1 }
\__hafid_ruledtabular_line:n { 2 }
\__hafid_ruledtabular_line:n { 3 }

and only after this TeX will start to process the first item, therefore getting

1 & \myhrule & \myhrule \\
2 & \myhrule & \myhrule \\
3 & \myhrule & \myhrule \\

yielding the full table body.

Here's a LuaLaTeX-based answer. No need for the pgffor package.

% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{array,booktabs}
\newcolumntype{C}[1]{>{\centering\arraybackslash}p{#1}}
\newcommand{\myhrule}{\rule{1cm}{0.5pt}}

\usepackage{luacode}
\begin{luacode}

function print_line (i)
  tex.sprint(i.."&\\myhrule&\\myhrule&\\myhrule\\\\")
end

\end{luacode}

\begin{document}
\begin{tabular}{@{} p{0.5cm} *{3}{C{2cm}} @{}}
\toprule
\(i\) & \(m\) & \(P\)  & \(y_{m}\) \\
\midrule
\directlua{ for i = 1 , 10 do print_line ( i ) end }
\bottomrule
\end{tabular}
\end{document}

---

Addendum: Suppose you wanted to generalize the Lua code so that it prints J rather than always 3 copies of &\\myhrule before forcing a line break. Assuming that J=5, say, all you'd have to do is (a) replace

function print_line (i)
  tex.sprint(i.."&\\myhrule&\\myhrule&\\myhrule\\\\")
end

with

function print_line (i,J)
  tex.sprint(i)
  for j=1,J do tex.sprint("&\\myhrule") end
  tex.sprint("\\\\")
end

and (b) change the function call

print_line ( i )

to

print_line ( i , 5)

in the argument of the subsequent \directlua directive in the tabular environment.

A simple solution with pgfplotstable.

I've created a table of the desired number of rows from scratch with \pgfplotstablenew and then display it with \pgfplotstabletypeset.

The column styles set with \pgfplotstableset are fully customizable.

\documentclass{article}
\usepackage{array,booktabs,pgffor}
\newcommand{\myhrule}{\rule{1cm}{0.5pt}}
\usepackage{pgfplotstable}
\pgfplotsset{compat=1.14}
\begin{document}
%set the columns style and content
\pgfplotstableset{
    create on use/new/.style={create col/expr={\pgfplotstablerow+1}},
    columns/new/.style={column name={\(i\)},column type={p{0.5cm}}},
    create on use/emme/.style={create col/set={\myhrule}},
    columns/emme/.style={column name={\(m\)},string type, column type={>{\centering\arraybackslash}p{2cm}}},
    create on use/pi/.style={create col/copy={emme}},
    columns/pi/.style={column name={\(P\)},string type, column type={>{\centering\arraybackslash}p{2cm}}},
    create on use/yup/.style={create col/copy={emme}},
    columns/yup/.style={column name={\(y_{m}\)},string type, column type={>{\centering\arraybackslash}p{2cm}}},
    }
%create a table with the desidered number of rows
\pgfplotstablenew[
    columns={new, emme, pi, yup}
    ]{9}% <--- put here the number of rows you like
    \loadedtable
%display the table
\pgfplotstabletypeset[
    every last row/.style={after row=\bottomrule},
    every head row/.style={before row=\toprule,after row=\midrule},
    ]\loadedtable

\end{document}