program mainimplicit noneinteger(8)::n,i,kreal(8),external::ff,fbreal(8)::h,x(100),y(100),ya(100),yb(100),yc(100),ka(100),kb(100),kc(100),kd(100)h=0.05n=40open(unit=99,file="xable.csv")open(unit=100,file="R-K2.csv")open(unit=101,file="R-K3.csv")open(unit=102,file="R-K4.csv")open(unit=103,file="exact.csv")do i=1,n+1,1x(i)=(i-1)*hwrite(99,*)x(i)end doclose(99)
!This is second-order Runge-Kutta algorithmya(1)=1write(100,*)ya(1)do i=1,n,1ka(i)=h*ff(x(i),ya(i))kb(i)=h*ff(x(i)+h,ya(i)+ka(i))ya(i+1)=ya(i)+0.5*ka(i)+0.5*kb(i)write(100,*)ya(i+1)end doclose(100)
!This is third-order Runge-Kutta algorithmyb(1)=1write(101,*)yb(1)do i=1,n,1ka(i)=h*ff(x(i),yb(i))kb(i)=h*ff(x(i)+0.5*h,yb(i)+0.5*ka(i))kc(i)=h*ff(x(i)+h,yb(i)-ka(i)+2*kb(i))yb(i+1)=yb(i)+(ka(i)+4.0*kb(i)+kc(i))/6.0write(101,*)yb(i+1)end doclose(101)
!This is fourth-order Runge-Kutta algorithmyc(1)=1write(102,*)yc(1)do i=1,n,1ka(i)=h*ff(x(i),yc(i))kb(i)=h*ff(x(i)+0.5*h,yc(i)+0.5*ka(i))kc(i)=h*ff(x(i)+0.5*h,yc(i)+0.5*kb(i))kd(i)=h*ff(x(i)+h,yc(i)+kc(i))yc(i+1)=yc(i)+(ka(i)+2.0*kb(i)+2.0*kc(i)+kd(i))/6.0write(102,*)yc(i+1)end doclose(102)
!This is the exact solutiondo i=1,n+1,1y(i)=fb(x(i))write(103,*)y(i)end doclose(103)
end program main!!exact solution 的function函数
function fb(x)implicit nonereal(8)::xreal(8)::fbfb=dexp(-x*x/2)return
end
!!-x*y的function函数
function ff(x,y)implicit nonereal(8)::x,yreal(8)::ffff=-x*yreturn
end

绘制图像

遇到问题：1.A specification statement cannot appear in the executable section.为将主程序关闭

2.存储文件空白，数组定义过小