2 个投票

I'm trying to have a plot with a constant line at value y=39. I'm using function fplot() with a constant double as argument. This works for any value (e.g. fplot(38), fplot(40)) except for fplot(39). Anyone has an idea or ad alternative way to get a constant line plot on y=39. Thanks A
The error message while running fplot(39) is the following Error using symvar>isquoted (line 154) Quotes in S are not in pairs.
Error in symvar (line 49) q = isquoted(s);
Error in countvars>symvarMulti (line 17) vars = reshape(symvar(c),1,[]);
Error in countvars>symvarMulti (line 10) vars = cellfun(@symvarMulti, c, 'UniformOutput', false);
Error in countvars (line 5) n = numel(symvarMulti(outer));
Error in fplot>singleFplot (line 225) nvars = countvars(fn);
Error in fplot>@(f)singleFplot(cax,{f},limits,extraOpts,args) (line 193) hObj = cellfun(@(f) singleFplot(cax,{f},limits,extraOpts,args),fn{1},'UniformOutput',false);
Error in fplot>vectorizeFplot (line 193) hObj = cellfun(@(f) singleFplot(cax,{f},limits,extraOpts,args),fn{1},'UniformOutput',false);
Error in fplot (line 163) hObj = vectorizeFplot(cax,fn,limits,extraOpts,args);

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Star Strider
Star Strider 2018-11-3

1 个投票

The fplot function is interpreting ‘39’ as a single quote, its ASCII equivalent:
chr39 = char(39)
chr39 =
'''
Using an anonymous function plots the lines:
fplot(@(x)39+x-x)
The strange appearance avoids several lines of warning messages that would otherwise result.

5 个评论
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Alessandro Tufano
Alessandro Tufano 2018-11-3
Thank you, it does work!
John D'Errico
John D'Errico 2018-11-3
Just an addendum to Stars answer:
If you want to plot a constant line at some value, you can also use refline. Thus,
plot(rand(1,100),'o')
% now a constant line, at y == 0.75, slope == 0
hold on
refline(0,0.75)
Star Strider
Star Strider 2018-11-3
As always, my pleasure!
I may submit an ‘enhancement request’ to MathWorks to see if they are interested in ‘trapping’ fplot to avoid this problem.
Steven Lord
Steven Lord 2018-11-3
An addendum to John's addendum: if you're using release R2018b or later you can use yline. yline is in MATLAB, while refline is in Statistics and Machine Learning Toolbox.
madhan ravi
madhan ravi 2018-11-4
编辑：madhan ravi 2018-11-4
An addendum to Steven's addendum:
fplot(subs(sym('y'),39)) %does the same
NOTE:
fplot(subs(sym('x'),39)) %won't plot a vertical line
the above will work the same that's why I made a FEX for this purpose

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