请教高手,不太明白里面的循环,请指教
谢谢!!!

求:圆球的体积
条件:
r=0, r=2
phi=pi/4, phi=pi/2
theta=pi/4, theta=pi/2 
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clc;
clear;

r_lower = 0;
phi_lower = pi/4;
theta_lower = pi/4;
V=0;

r_upper = 2;
phi_upper = pi/2;
theta_upper = pi/2;

number_of_r_steps = 100;
number_of_theta_steps = 100;
number_of_phi_steps = 100;

dr = (r_upper - r_lower) / number_of_r_steps;
dphi = (phi_upper - phi_lower) / number_of_phi_steps;
dtheta = (theta_upper - theta_lower) / number_of_theta_steps;

for k=1: number_of_phi_steps
    for j=1: number_of_theta_steps
        for i=1: number_of_r_steps
            V=V+r_lower^2*sin(theta_lower)*dr*dtheta*dphi;
            r_lower=r_lower+dr;
        end; 
        theta_lower = theta_lower + dtheta;
    end;
    r_lower=0;
    theta_lower = pi/4;
end
   
V