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58 chapter 33.4.1 Gnuplot Input Data Format ⊙The format of the data file that Gnuplot can read is not confined to (x, y) values.You may also read in a data file as a C language scanf format string xy by invokingthe using option in the plot command. (Seeing that it is common for Linux/Unix programs to use this format for reading files, you may want to read moreabout it.)plot ‘datafile’ { using { xy | yx | y } {"scanf string"} }This format explicitly reads selected rows into x or y values while skipping pasttext or unwanted numbers:gnuplot> plot "data" using "%f%f" Default, 1st x, 2nd y.gnuplot> plot "data" using yx "%f %f" Reverse, 1st y, 2nd x.gnuplot> plot "data" xy using "%*f %f %*f %f" Use row 2,4 for x, y.gnuplot> plot "data" using xy "%*6c %f%*7c%f"This last command skips past the first six characters, reads one x, skips the nextseven characters, and then reads one y. It works for reading in x and y from filessuch as✞☎theta : −20.000000 Energy : −3.041676 theta : −19.000000 Energy :−3.036427 theta : −18.000000 Energy : −3.030596 theta : −17.000000Energy : −3.024081 theta : −16.000000 Energy : −3.016755✝Observe that because the data read by Gnuplot are converted to floating-pointnumbers, you use %f to read in the values you want.Besides reading data from files, Gnuplot can also generate data from userdefinedand library functions. In these cases the default independent variable is xfor 2-D plots and (x, y) for 3-D ones. Here we plot the acceleration of a nonharmonicoscillator:gnuplot> k=10 Set value for kgnuplot> a(x) = .5 * k * x ** 2 Analytic expressiongnuplot> plot [–10:10] a(x) Plot analytic functionA useful feature of Gnuplot is its ability to plot analytic functions along withnumerical data. For example, Figure 3.7 compares the theoretical expression forthe period of a simple pendulum to experimental data of the form✞☎✝# length (cm) period ( sec ) # length (cm) period ( sec )10 0.8 60 1.620 0.9 70 1.730 1.2 80 1.840 1.3 90 1.950 1.5 100 2.0−101COPYRIGHT 2008, PRINCET O N UNIVE R S I T Y P R E S SEVALUATION COPY ONLY. NOT FOR USE IN COURSES.ALLpup_06.04 — 2008/2/15 — Page 58