Comparison of derivatives of the sinc function

dsinc.cpp

#include <iostream>
#include <cmath>

using namespace std;

double f(double x)
{
    if(x == 0)
        return 1;
    return sin(x) / x;
}

double df(double x, double h = 0.0000001)
{
    return (f(x + h) - f(x - h)) / (2 * h);
}

double dsinc(double x)
{
    return -sin(x) / (x * x) + (1 / x) * cos(x);
}

int main()
{
    for(double x = -13; x <= 13; x += 0.1)
        cout << x << "\t" << f(x) << "\t" << df(x, 1) << "\t" << dsinc(x) << endl;

    return 0;
}

makefile

TARGET = dsinc

all: $(TARGET)

$(TARGET): $(TARGET).cpp
	g++ -Wall -Wextra -O2 $(TARGET).cpp -o $(TARGET)

.PHONY: tar
tar: 
	@tar -cvJf $(TARGET).tar.xz makefile $(TARGET).cpp

plot.plt

#!/usr/bin/gnuplot

set xtics pi
set format x '%.0Pπ'
set xrange [-5*pi:5*pi]
set xlabel "{/:Italic x}"
set grid

plot "sinc.dat" using 1:2 w lp title "sinc({/:Italic x})", "sinc.dat" using 1:3 w p title "numerical derivative, {/:Italic  h} = 1", "sinc.dat" using 1:4 w l title "analytical derivative"

pause -1