求助，
最近自学彭国伦的F95，边学边在电脑上操作，发现彭的书上第45页，有个例子我照抄后运行不了，希望有高手帮忙看下是什么原因小弟不胜感激。原程序如下：
  program      ex0413
  character(len=20)  string
string="Good moring."
write(*,*)string
string(6)="evening."!重新设置从第6个字符之后的字符串
write(*,*)string
end
运行之后报错
Compiling Fortran...
D:\fortran\sdf\sdfa.f90
D:\fortran\sdf\sdfa.f90(9) : Error: This name has not been declared as an array.   [STRING]
string(6)="evening."!重新设置从第6个字符之后的字符串
^ Error executing df.exe.
sdf.exe - 1 error(s), 0 warning(s)
诚心求帮助，谢谢