问Swift中积分结果的计算

EN

import Foundation

var sum: Double = 0.0 //The integral result
var step_size: Double = 0.0000025
var a: Double = 0.0
var R: Double = 5.0
var next_point: Double = 0.0
var midpoint: Double = 0.0
var height: Double = 0.0
var r_val: Double = 0.0

func psi_func(r_val: Double) -> Double {

    return exp(-r_val)
}

//Integration

while next_point < R {

    next_point = a + step_size
    midpoint = a + step_size/2
    height = psi_func(r_val: midpoint)
    sum += psi_func(r_val: midpoint)*step_size

    a = a + step_size

}

print("S = ", 2*3.14159*3.14159*sum) // This is a 3-D orbital, so I multiply by 2*pi*pi