Ones counter using numeric_std inside process

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out_buf1(i) is a single bit. You need to make it into an array. something like:

one_buf1 := one_buf1 + ("" & out_buf(i));

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Thanks for the answer, Tricky.

I tried and then there are some other errors.

Error (10327): VHDL error at mess_extractor.vhd(278): can't determine definition of operator ""&"" -- found 4 possible definitions

Error (10647): VHDL type inferencing error at mess_extractor.vhd(278): type of expression is ambiguous - "std_ulogic_vector" or "std_logic_vector" are two possible matches

Can you comment on this too? Thanks!