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{bf:Mata course:  Functions}
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{title:Example}


	{cmd}real matrix id(real scalar n)
	{c -(}
	        real scalar     i
	        real matrix     res

	        res = J(n, n, 0)
	        for (i=1; i<=n; i++) {c -(}
	                res[i,i] = 1
	        {c )-}
	        return(res)
	{c )-}{txt}



{title:Syntax}

{p 4 8 2}
    1.  They are used in Mata programs


{p 4 8 2}
    2.  They follow the classic definition 

{p 12 16 2}
             f({it:a},{it:b},{it:c},...) returns a value based on {it:a}, {it:b}, {it:c}, ...

{p 8 8 2}
        or
{p_end}

{p 12 16 2}
             f({it:a},{it:b},{it:c},...) returns multiple values by modifying its arguments

{p 8 8 2}
        or
{p_end}

{p 12 16 2}
             f({it:a},{it:b},{it:c},...) does something but returns nothing


{p 4 8 2}
    3.  The words function, subroutine, and program are used interchangeably;
        all mean the same thing


{title:Flavors of functions}

{p 4 8 2}
    1.  Built-in
{p_end}

{p 12 16 2}
	     They are part of Mata itself, and are written in C

{p 4 8 2}
    2.  Library
{p_end}

{p 12 16 2}
             They are part of Mata, and are written in Mata


{p 4 8 2}
    3.  User written
{p_end}

{p 12 16 2}
             All other, including the functions (programs, subroutines) 
             you write


{p 4 4 2}
Sometimes the word built-in is used to refer to (2).  


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