Two problems with foreach - TikZ

Question

Note: This is a major edit to my original question. I intended to ask another question for the second problem separately, but it has already been initiated in the comment and probably already answered here. So I have edited this question instead to cover all two of them.

FIRST Problem: Illegal unit of measure: (pt inserted)

I am facing a similar problem as "Illegal unit of measure (pt inserted)" using arithmetic in TikZ foreach. Except that I am using negative decimal numbers as well as positive decimal numbers and I have used three dots here for the interval. The MWE is as below:

\documentclass[border=3mm,tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach \x/\y in {-1.4/-1.4,-1.2/-1.2,...,1.6/1.6}{
\filldraw (\x,\y) circle [radius=2pt];}
\end{tikzpicture}
\end{document}

How do I fix this? I know there is an alternative way to get the same picture. But I am experimenting with this for some purpose. I have passed here equal values of x and y. The problem is not that if there is an alternative way to make the same picture. The problem is that if there is a way to workaround with this error and get rid of it. This is because I may change the values of y and may still find the same error lurking in.

SECOND Problem: Using two foreach to make an ordered pair

The second problem is how I can use two foreach commands to make an ordered pair so that one doesn't iterate over the iteration of the other, but forms a one-to-one correspondence or something like that. Some suggests to use evaluate in foreach. This is okay when I know y depends on x. But how about when y doesn't depend on x at all? When y has some random values for example? The MWE is

\documentclass[border=3mm,tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach \x  in {1,...,9}
\foreach \y  in {2,5,4,6,9,1,3,8,7}
\filldraw (\x,\y) circle [radius=10pt];
\end{tikzpicture}
\end{document}

which gives:

What I want is:

Answer 1

Maybe something like this as a workaround...

\documentclass[border=3mm,tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach \x [count=\xcnt] in {-1.4,-1.2,...,1.6}{%
\foreach \y [count=\ycnt] in {-1.4,-1.2,...,1.6}{%
\ifnum\xcnt=\ycnt\filldraw (\x,\y) circle [radius=2pt];\fi
}}
\end{tikzpicture}
\begin{tikzpicture}
\foreach \x [count=\xcnt] in {-1.4,-1.2,...,1.6}{%
\foreach \y [count=\ycnt] in {-2.8,-2.4,...,3.2}{%
\ifnum\xcnt=\ycnt\filldraw (\x,\y) circle [radius=2pt];\fi
}}
\end{tikzpicture}
\end{document}

Answer 2

Why don't you just do this?

\documentclass[border=3mm,tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach \x in {-1.4,-1.2,...,1.6}{
\filldraw (\x,\x) circle [radius=2pt];}
\end{tikzpicture}
\end{document}

Answer 3

Just mentioning this as well. If you have a list of values that you want to place at regular intervals, one loop with a counter is enough:

\documentclass[border=3mm,tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach [count=\x] \y  in {2,5,4,6,9,1,3,8,7}
   \filldraw (\x,\y) circle [radius=10pt];
\end{tikzpicture}
\end{document}

If the values should start at some specific value, you can say e.g. count=\x from 10, and if the \x should have a different spacing, you can probably use evaluate, e.g. \foreach [count=\x, evaluate={\NewX=2*\x + 4;}] which will make \NewX 6, 8, 10, ...

\documentclass[border=3mm,tikz]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\foreach [count=\x, evaluate={\NewX=2*\x+4;}] \y  in {2,5,4,6,9,1,3,8,7}
   \filldraw (\NewX,\y) circle [radius=10pt];
\end{tikzpicture}
\end{document}

Answer 4

Now that you edited your question, it seems that you want to draw points at every \x integer coordinates but with randomly chosen \y coordinates.

In my opinion, this is what [count=\x from 1] is made for:

\documentclass[border=3.14mm,tikz]{standalone}

\begin{document}
    \begin{tikzpicture}
        \foreach \y [count=\x from 1] in {2,5,4,6,9,1,3,8,7}
            \filldraw (\x,\y) circle [radius=10pt];
    \end{tikzpicture}
\end{document}

Answer 5

For the second problem, this works:

\begin{tikzpicture}
\foreach \coords in {(1,2),(2,5),(3,4),(4,6),(5,9),(6,1),(7,3),(8,8),(9,7)}
\filldraw \coords circle [radius=10pt];
\end{tikzpicture}

See the section "Special handling of pairs" on page 1002 of the PGF manual, under section 88 "Repeating Things: The Foreach Statement".