Space after macro involving a TikZ picture

Question

I wrote a macro to print linear combinations and related things using TikZ (pgffor really, but I need TikZ for other things in a big document). Here's a MWE:

\documentclass[11pt]{article}

\usepackage{tikz,spalign,xspace,amsmath}

\newcommand\lincomb[3]{% 3args: list of coeffs, vector symb, operation
%   \begin{tikzpicture}
    \tikz{
        \foreach \x [count=\j] in {#1}{
            \ifnum\j=1
                $\x{#2}_{\j}$
            \else
                ${#3}\x{#2}_{\j}$
            \fi
        }\xspace
%   \end{tikzpicture}
    }
}


\begin{document}

\begin{equation}
    \spalignmat{0.7;0.5;0.1}={\lincomb{0.7,0.5,0.1}{\mathbf{e}}{+}}\quad
    \;\text{and}\:
    \spalignmat{0.1;0.5;0.7}=\lincomb{0.1,0.5,0.7}{\mathbf{e}}{+}
\end{equation}

$\lincomb{a,b,c}{f}{-}$ and then some text.

\begin{tikzpicture}
    \foreach \x [count=\j] in {a,b,c,d}{
        \ifnum\j=1
            $\x f_{\j}$
        \else
            $+ \x f_{\j}$
        \fi
    }
\end{tikzpicture}\ and then some text

\end{document}

The output is at the end. What I want is to have proper spacing between two of these. As you can see, I even tried using the code as a standalone piece of text, but got the same result; I also tried using the tikzpicture environment, but nothing changes. After seraching I've seen many questions asking to remove space after a tikzpicture, but I want to add it. As you can see also, xspace doesn't work here either. Thanks in advance!!

Answer 1

You don't need put the commands in tikz or tikzpicture, as the code is just a text, not drawing commands. In fact TeX is indeed trying to typeset these letters, but since it is in the middle of a tikzpicture, it has been instructed to use the nullfont, which is an internal font that has no character at all and produces no interword space either. So \tikz or tikzpicture environment just drop the text as it is not in a node.

\documentclass[11pt]{article}

\usepackage{tikz,spalign,amsmath}

\newcommand\lincomb[3]{% 3args: list of coeffs, vector symb, operation
%   \begin{tikzpicture}
%    \tikz{
        \foreach \x [count=\j] in {#1}{
            \ifnum\j=1
               \x{#2}_{\j}
            \else
               {#3}\x{#2}_{\j}
            \fi
%        \xspace
%   \end{tikzpicture}
        }
}

\begin{document}

\begin{equation}
  \spalignmat{0.7;0.5;0.1}={\lincomb{0.7,0.5,0.1}{\mathbf{e}}{+}}\quad
    \;\text{and}\:
  \spalignmat{0.1;0.5;0.7}=\lincomb{0.1,0.5,0.7}{\mathbf{e}}{+}
\end{equation}

$\lincomb{a,b,c}{f}{+}$ and then some text.

\end{document}