Algorithms

const GROUPS_COUNTER = 10;
const MIN_ASCII = 65;
const MAX_ASCII = 69;

/*
 * generate children pairs.
 */
function generateGroups($n) {
    $matrix = [];
    $exist = [];
    echo "************************\n";

    for ($i = 0; $i < $n; $i++) {
        $a = chr(rand(MIN_ASCII, MAX_ASCII));
        $b = chr(rand(MIN_ASCII, MAX_ASCII));

        if ($a == $b || isset($exist[$a][$b]) || isset($exist[$b][$a])) {
            $i--;
            continue;
        }

        $matrix[$i][0] = $a;
        $matrix[$i][1] = $b;
        $exist[$a][$b] = true;

        echo $a . " " . $b . "\n";
    }

    echo "************************\n";

    return $matrix;
}

/*
 * find all children which are not like by parent.
 */
function findGroups($groups) {
    $dislikeList = [];
    $checked = [];

    foreach ($groups as $groupParent) {
        if (isset($checked[$groupParent[0]])) {
            continue;
        }

        $groupChildList = [];

        foreach ($groups as $groupChild) {
            if ($groupParent[0] == $groupChild[0]) {
                array_push($groupChildList, $groupChild[1]);
            }
        }

        echo $groupParent[0] . " => " . implode(", ", $groupChildList) . "\n";
        $dislikeList[$groupParent[0]] = $groupChildList;
        $checked[$groupParent[0]] = true;
    }

    echo "************************\n";

    return $dislikeList;
}

/*
 * phpTarjanEntry() iterates through the graph array rows, executing phpTarjan().
 */
function phpTarjanEntry($groups) {
    global $cycles;
    global $marked;
    global $markedStack;
    global $pointStack;

    $cycles = array();
    $marked = array();
    $markedStack = array();
    $pointStack = array();

    $marked = array();

    foreach ($groups as $key => $value) {
        $marked[$key] = FALSE;
    }

    foreach ($groups as $key => $value) {
        phpTarjan($key, $key);

        while (!empty($markedStack)) {
            $marked[array_pop($markedStack)] = FALSE;
        }
    }

    $cycles = array_keys($cycles);

    return $cycles;
}

/*
 * Recursive function to detect strongly connected components (cycles, loops).
 */
function phpTarjan($s, $v) {
    global $G;
    global $cycles;
    global $marked;
    global $markedStack;
    global $pointStack;

    $f = FALSE;
    $pointStack[] = $v;
    $marked[$v] = TRUE;
    $markedStack[] = $v;

    foreach ($G[$v] as $w) {
        if ($w < $s) {
            $G[$w] = array();
        } else if ($w == $s) {
            $cycles[implode('|', $pointStack)] = TRUE;
            $f = TRUE;
        } else if (isset($marked[$w]) && $marked[$w] === FALSE) {
            $g = phpTarjan($s, $w);

            if (!empty($f) OR ! empty($g)) {
                $f = TRUE;
            }
        }
    }

    if ($f === TRUE) {
        while (end($markedStack) != $v) {
            $marked[array_pop($markedStack)] = FALSE;
        }

        array_pop($markedStack);
        $marked[$v] = FALSE;
    }

    array_pop($pointStack);
    return $f;
}

$mixGroups = generateGroups(GROUPS_COUNTER);
$res = findGroups($mixGroups);
$G = $res;
$cycles = phpTarjanEntry($res);

echo '<p>Cycles found: ' . count($cycles);
print_r($cycles);

from numpy import *
import sys
import numpy as np
x=1
epsilon=sys.float_info.epsilon/2.0
# machine epsilon
h = np.linspace(1,epsilon,1000)
e1= (((sin(x)- sin(x-h))/h-cos(x))/cos(x))                      #realtive error in first order
e2= (((sin(x-2*h)-4*sin(x-h)+3*sin(x))/(2*h)-cos(x))/cos(x))    #realtive error in second order
e3= (((2*sin(x+h)+3*sin(x)-6*sin(x-h)+sin(x-2*h))/(6*h)-cos(x))/cos(x))#realtive error in third order
e4 = (((-sin(x-3*h)+6*sin(x-2*h)-18*sin(x-h)+10*sin(x)+3*sin(x+h))/(12*h)-cos(x))/cos(x))#realtive error in fourth order
print(h)
import matplotlib.pyplot as plt
plt.plot(h,e1,'-b',lw=3)
plt.plot(h,e2,'-g',lw=3)
plt.plot(h,e3,'-m',lw=3)
plt.plot(h,e4,'-r',lw=3)
plt.xscale('log')
plt.yscale('log')
plt.xlabel('h')
plt.ylabel('Relative error')
plt.legend(['first order','second order','third order','fourth order'])
plt.title('Effect of order of accuracy on the numerical differentiation')
plt.show()