pic_num = 1;#放在绘图循环外面

#放在绘图循环里面
    drawnow
    F=getframe(gcf);
    I=frame2im(F);
    [I,map]=rgb2ind(I,256);
    if pic_num == 1
        imwrite(I,map,'test.gif','gif', 'Loopcount',inf,'DelayTime',0.2);
    else
        imwrite(I,map,'test.gif','gif','WriteMode','append','DelayTime',0.2);
    end
    pic_num = pic_num + 1;