fortran学习中遇到的问题

找不到fortran代码中存在的问题

    implicit none
    integer :: i
    character(len = 20) ::str1
    character(len = 20) ::str1a
    character(len = 20) ::str2
    character(len = 20) ::str2a
    write(*,*)"enter the first string to compare:"
    read(*,*)str1
    write(*,*)"enter the second string to compare:"
    read(*,*)str2
    str1a = str1
    str2a = str2
    do i = 1,len(str1a)
        if(str1a(i:i) >= 'a '.and. str1a(i:i) <= 'z')then
            str1a(i:i) = achar(iachar(str1a(i:i)) - 32)
        end if
    end do
    do i = 1,len(str2a)
        if(str2a(i:i) >= 'a '.and. str2a(i:i) <= 'z')then
            str2a(i:i) = achar(iachar(str2a(i:i)) - 32)
        end if
    end do
    if(str1a == str2a)then
        write(*,*)"'",str1,"'"," = ",str2,"ignoring case."
    else
        write(*,*)"'",str1,"'","/ = ",str2,"ignoring case."
    end if
    end


代码没问题。