Assign a value to a gnuplot function out of a data file

Question

In Gnuplot, I try to assign a value to a function f(x) from a two column data file in order to plot that function as a horizontal line.

f(x)=value of $2 at $1==2 from filename.dat
plot 'filename.dat' , ' ' x<=2?f(x):1/0

I tried :

awk '$1==2,print{$2}' filename.dat

but it says :

Warning: encountered a string when expecting a number Did you try to generate a file name using dummy variable x or y?".

Any suggestions?

P.S.: I know that there should be a < sign after awk, but it would not display it here.

See e.g. stackoverflow.com/q/18583180/2604213 or stackoverflow.com/q/15007620/2604213 — Christoph
– Christoph, Commented Nov 21, 2015 at 18:53

F. Knorr · Accepted Answer · 2015-11-21 15:49:52Z

0

I suggest this approach:

filename = "test.txt"
y = system(sprintf("awk '$1==2{print $2}' %s", filename))
f(x)=real(y)
plot filename u 1:2 w l, f(x) t ''

It seems that gnuplot is interpreting the value of y as a string. Therefore, we have to enforce a conversion to a number, which I do by converting it to a real number. (Another possibility f(x)=y+0)

Here is the ouput for my demo test.txt with the following content:

1 1
2 0.5
3 0

answered Nov 21, 2015 at 15:49

F. Knorr

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2 Comments

Paul Rousseau Over a year ago

Thank you so much, I did not think it would be that easy, but every time you search for "basic" stuff like this, it is always stumbling upon very advanced stuff. Do you know a place where I can get more basic information about awk like this one ??

F. Knorr Over a year ago

@PaulRousseau: Glad that I could help. If you are satisfied with the answer, please mark the question answered. For more information an awk see Awk by example or the Awk Introduction Tutorial

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