Set XTick in MATLAB subplot?

Question

I am trying to set the XTick for each of my subplot. After reading the MATLAB documentation here, I decided to do the following, but it is not working.

MWE

subplot(2, 1, 1);
gca.XTick = [0, 6, 12, 18, 24];
subplot(2, 1, 2);
gca.XTick = [0, 6, 12, 18, 24];

My MATLAB version is

>> version

ans =

8.4.0.150421 (R2014b)

Maybe you need to assign to a temporary variable (ax in the documentation example)? I can't test it, because this is new syntax in Matlab 2014b. Or try the old syntax: set(gca, 'XTick', [0, 6, 12, 18, 24]) — Luis Mendo
@LuisMendo Thank you! Your old syntax works. I tried a temporary variable as in the documentation, and it is not working. — Sibbs Gambling
Either syntax will work, as long as it's done in a workspace in which gca.XTick = ... has not already been done (or, rather, seen); that assignment creates a local gca variable that shadows the gca function. Quirk of the M-code parser and syntax. — Andrew Janke

Andrew Janke Andrew Janke · Accepted Answer · 2014-12-17T09:31:17

You can't use gca directly as though it were a handle reference on the left hand side of an assignment operation. You can use either the set(gca, ...) syntax or ax = gca; ax.XTick ..., but only if you avoid the gca.Whatever = ... syntax, which will break gca in the workspace you do it in due to identifier shadowing.

The syntax

gca.XTick = [0, 6, 12, 18, 24];

will not do what you want. Instead of calling the gca() function, this creates a new local variable called gca and populates it with a struct that has a field named XTick. Not only does this not set the ticks in the plot, but the new variable masks the gca function, so subsequent calls to gca in the same workspace will not work until a clear or a restart is done (they'll just access the local struct fields instead).

Using a temporary variable like this

ax = gca;
ax.XTick = [0, 6, 12, 18, 24];

should work, as long as you have not already done a gca.XTick = ... assignment in that workspace, or do one anywhere in the same function.

This is an unfortunate quirk of how Matlab's take on the "uniform access principle" works: you can call an argumentless function or method without parentheses (like set(gca, 'XTick', ...)), but only if you do not also use that same identifier as an lvalue in an assignment statement in the same function, which causes the parser to identify it as a local variable instead of a function call.

In short, don't put gca on the left hand side of a = assignment operation, and it should work.

Demonstration

You can see this in action using whos or which. Throw the code in a function so it gets a clean workspace and use which to see what gca resolves to.

function darnit_gca()

disp('gca is:');
which gca

subplot(2, 1, 1);
gca.XTick = [0, 6, 12, 18, 24];
subplot(2, 1, 2);
gca.XTick = [0, 6, 12, 18, 24];

disp('now gca is:');
which gca

When you run darnit_gca, you can see the resolution of gca change once you use it as an lvalue.

>> darnit_gca
gca is:
built-in (/Applications/MATLAB_R2014b.app/toolbox/matlab/graphics/gca)
now gca is:
gca is a variable.

Set XTick in MATLAB subplot?

1 Answers

Demonstration