What does -= and += mean in the function?

Question

au FileType php call PHPFuncList()    
function PHPFuncList()    
    set dictionary-=/etc/vim/php_funclist.txt dictionary+=/etc/vim/php_funclist.txt    
    set complete-=k complete+=k    
endfunction

For the PHPFunctionList above, two lines in function body confused me.

set dictionary-=/etc/vim/php_funclist.txt dictionary+=/etc/vim/php_funclist.txt    
set complete-=k complete+=k

For the two lines ,first -= to remove it and then += to add it again.

Why can't write the two lines just += in them?

set dictionary+=/etc/vim/php_funclist.txt    
set complete+=k

Are there any differences between them?
:h set-= point out that When the option is a list of flags, {value} must be exactly as they appear in the option. Remove flags one by one to avoid problems.
To avoid which problem?

I would guess the developer did not understand how `+=` works, and was using `-=` to ensure things were not added twice. — Dan Lowe, Jul 11 '17 at 15:33

score 6 · Answer 1 · answered Jul 11 '17 at 11:39

6

In vim doc :

:h :set+=

and

:h :set-=

has explained the usage clearly. basically the += will append an element to the option, and -= will substract the value from the option.

answered Jul 11 '17 at 11:39

Kent

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To add it with += directly has the same effect,in my opinion. – showkey Jul 11 '17 at 13:32

What does -= and += mean in the function?

1 Answers1