Transposing regression table in Latex - r

Question

I have a regression table in latex that is the result of output from applying the stargazer package on my regression results in R. I am wondering how to transpose the table such that the columns switch place with rows. In other words, I want the Lags to be where the variables are and vice versa.
Moreover, I am wondering how to reduce the space between the first and second columns as well as how to add a vertical line between columns. I've tried adding | in between, but sometimes this doesn't cover well and leaves in some space in between the top and bottom hline.
\usepackage{dcolumn}
\begin{table}[!htbp] \centering
\caption{Regression results}
\label{}
\setlength{\tabcolsep}{0pt}
\begin{tabular}{lD{.}{.}{-2} D{.}{.}{-2} D{.}{.}{-2} D{.}{.}{-2} D{.}{.}{-2} D{.}{.}{-2}}
\\[-1.8ex]\hline
\hline \\[-1.8ex]
& \multicolumn{6}{c}{Characteristics} \\
\cline{2-7}
\\
Lags: & \multicolumn{1}{c}{1} & \multicolumn{1}{c}{2} & \multicolumn{1}{c}{3} & \multicolumn{1}{c}{4} & \multicolumn{1}{c}{5} & \multicolumn{1}{c}{6} \\
\hline \\[-1.8ex]
Variable 1 & -1.14^{***} & 0.04 & -1.73^{***} & -0.24 & -0.59 & -1.64^{***} \\
Variable 2 & 0.16 & -0.25 & 0.12 & -0.12 & 0.14{**} & 0.04 \\
Variable 3 & 0.46^{*} & -0.61 & 0.17 & -0.98^{***} & -0.32 & -0.09\\
Variable 4 & -0.02 & -0.005 & -0.03^{***} & -0.32 & -0.11 & 0.015 \\
\hline \\[-1.8ex]
\hline
\hline \\[-1.8ex]
\textit{Note:} & \multicolumn{6}{r}{$^{*}$p$<$0.1; $^{**}$p$<$0.05; $^{***}$p$<$0.01} \\
\end{tabular}
\end{table}

Answer 1

So, a few LaTeX basics:
(1) LaTeX is unsophisticated. In R, you can transpose a table or matrix or whatever with t(); there is no such analogue in LaTeX. (Caveat, maybe it exists in a package or something, but personally I wouldn't trust such an approach.)
You'll have to manually retype the table -- or, alternatively, convince R to massage the table before passing it to LaTeX output. Since you didn't discuss any R code, I'll skip over any discussion about that and focus on the LaTeX part.
(2) The building blocks of the tabular environment are: // breaks to a new line, and & (the alignment indicator) skips to a new tab. So if you have a table like
\begin{tabular}{c c c}
a & b & c \\
d & e & f
\end{tabular}
the only way to get its transpose is just to retype it:
\begin{tabular}{c c}
a & d \\
b & e \\
c & f
\end{tabular}
The number of columns is denoted by the markings immediately after \begin{tabular}; for instance, {c c c} indicates three columns, all centered. In yours, the D tab stands for a column aligned to a decimal point per the dcolumn package, so you'll need to remove two of these when you transpose your table since you'll have four column variables instead of six. After that, it will just come down to a matter of retyping the contents of the matrix.
For your other questions:
I am wondering how to reduce the space between the first and second columns
That's an interesting one that seems not to be contained to the code you posted. When I posted that exact same code into an empty LaTeX document, it didn't have that spacing that yours has; what this means is that the table is being ballooned out by some other code, most likely to justify out to a given page width or something. I can't help since I can't reproduce the artifact, but you might try enclosing the entire code block with \begin{center} and \end{center}, maybe? This might let LaTeX break the rules of how big the box containing this table is supposed to be. That's a total guess, though, and the folks at https://tex.stackexchange.com/ might be able to help more on that issue.
as well as how to add a vertical line between columns. I've tried adding | in between, but sometimes this doesn't cover well and leaves in some space in between the top and bottom hline.
First response: consider not doing vertical lines at all.
Second response: You've got the right idea, but the reason it's leaving breaks is that you don't have enough alignment tabs (&) in certain lines. For instance, putting | between the third and fourth D{}{}{} in your tabular options makes this monstrosity:
So, what went wrong? Referring to the regions I annotated in blue:
A: When you break a line with \\, it only print vertical alignment bars if you put the proper number of column advancement tokens. Fix by replacing the line \hline \\[-1.8ex] immediately before Variable 1 with \hline & & & & & & \\[-1.8ex]. (One & per desired column advancement!)
B: In this case, the use of \multicolumn overrides any other vertical alignment options. Fix by replacing
Lags: & \multicolumn{1}{c}{1} & \multicolumn{1}{c}{2} & \multicolumn{1}{c}{3} & \multicolumn{1}{c}{4} & \multicolumn{1}{c}{5} & \multicolumn{1}{c}{6} \\
with either
Lags: & \multicolumn{1}{c}{1} & \multicolumn{1}{c}{2} & \multicolumn{1}{c|}{3} & \multicolumn{1}{c}{4} & \multicolumn{1}{c}{5} & \multicolumn{1}{c}{6} \\
or
Lags: & \multicolumn{1}{c}{1} & \multicolumn{1}{c}{2} & \multicolumn{1}{c}{3} & \multicolumn{1}{|c}{4} & \multicolumn{1}{c}{5} & \multicolumn{1}{c}{6} \\
and note where the | was in either of the two fixes.
C: Same issue as A. Fix by replacing the \\ immediately after the \cline{2-7} with & & & & & & \\ to advance through columns and render vertical markers.
D: Don't do anything here, obviously.