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Good morning everyone, you have helped me before and I am very grateful for that. Now I come with another doubt, this time it has to do with the code:

```
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
use ieee.numeric_std.all;
entity alu is
port (
s:in std_logic_vector (2 downto 0);
a:in std_logic_vector (3 downto 0);
b:in std_logic_vector (3 downto 0);
f:out std_logic_vector (3 downto 0)
);
end alu;
architecture comportamiento of alu is
begin
with s select
f<= a+b when "000",
a-b when "001",
a or b when "011",
a xor b when "100",
a and b when "101",
0 when others;
end comportamiento;
```

The error it throws is:

10316 VHDL error at alu.vhd(25): character "0" used but not declared for type "std_logic_vector"

I don't understand why if it is declared "std_logic_vector".

I appreciate your help.

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Assuming line 25 is "0 when others", since f is a std_logic_vector, this should be:

"000" when others;

Single bit values would be enclosed by ' (like '0') but multi-bit should be double-quotes ("000").

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Assuming line 25 is "0 when others", since f is a std_logic_vector, this should be:

"000" when others;

Single bit values would be enclosed by ' (like '0') but multi-bit should be double-quotes ("000").

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