Generate a bianry code from an integer array?

You are assigning a bit vector to a single bit which isn't possible, a simple issue of VHDL syntax.

You want to write something like

a(u+3 downto u)<= std_logic_vector(to_unsigned(x,4));

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And what I want to do is, for example, introducing 2314 to X and to generate "0010 0011 0001 0100", so I have tryied this:

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2314 isn't an array of integer literal, it's either a decimal value or a string respectively character array.