Friday, September 9, 2011

Manipulate data using ternary operator in gunplot

In gnuplot there is an operator "? :" which is ternary. It behaves as it does in C language. For example, "a?b:c" means if a is true then return 'b', otherwise return 'c'. Using this operator we can do some complex operation on input data. Let us see an example.

We have a data file as follows:
#x  y1   y2
0.1	0.2	0.2
0.2	0.4	0.4
0.3	0.6	0.6
0.4	0.8	0.8
0.5	1	1
0.6	1.2	1.2
0.7	1.4	1.4
0.8	2.4	0.6
0.9	2.7	0.675
1	3	0.75
1.1	3.3	0.825
1.2	3.6	0.9
1.3	3.9	0.975
1.4	4.2	1.05
1.5	4.5	1.125
We want to plot the points y1=y2 with blue pints, and other points y1!=y2 (y1 not equals y2) with some other colors. To realize this goal, we can use ternary operator. According to the definition of ternary operator, expression "$2==$3?$2:1/0" will return the points satisfy $2=$3, and when $2!=$3 it returns undefied value(1/0 is an undefied value). So when we use command "plot 'data.dat' u 1:($2==$3?$2:1/0) w p", we plot the points with y1=y2. In a simillar manner, we can plot the points with y1!=y2. Now we come to our final plotting script.
set term png font "Times,20"
set output "ternaty.png"
set xlabel "x"
set ylabel "y"
plot "data.dat" u 1:($2==$3?$2:1/0) w p lc rgb"blue" notitle,\
     "data.dat" u 1:($2!=$3?$2:1/0) w p lc rgb"green" notitle,\
     "data.dat" u 1:($2!=$3?$3:1/0) w p lc rgb"red" notitle
The picture comes out to be like follows.

Manipulate data using ternary operator in gnuplot

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