use warnings;
use strict;

# 例 x^2 定積分の近似値
my $a = 0;
my $b = 1;
my $N = 100;
my $Integrand = '$x*$x';
	
my $Rectangle = &RECTANGLEAPPROXIMATIONMETHOD($a, $b, $N, $Integrand);
print "$Rectangle\n";

# 長方形近似 Rectangle Approximation Method
# 引数 左端a 右端b 分割数 被積分関数 ($a, $b, $N, $Integrand)
# 戻り値 長方形近似 ($RectangleApproximationMethod)
sub RECTANGLEAPPROXIMATIONMETHOD{
	my ($a, $b, $N, $Integrand) = @_;
	  ($a, $b) = ($b, $a) if($a > $b);
	my $RectangleApproximationMethod = 0;
	my $h = ($b - $a) / $N;
	my $Sum = 0;
	
	# 分割数の確認
	if($N < 2){
		return 0;
	}
	
	# 計算
	for(my $i = 0; $i < $N; $i++){
		my $x = $a + ($h * $i);
		my $tmp = &INTEGRAND($x, $Integrand);
		
		$Sum += $tmp;
	}
	
	# 長方形近似 Rectangle Approximation Method
	$RectangleApproximationMethod = $Sum * $h;
	
	return $RectangleApproximationMethod;
}

# 被積分関数 Integrand
# 引数 積分変数 被積分関数 (\@Integrand)
# 戻り値 被積分関数 ($Integrand)
sub INTEGRAND{
	my ($x, $Integrand) = @_;
	my $tmp = eval($Integrand);

	return $tmp;
}