m2_op_conditional.hlp

是一个经济学管理应用软件 很难找的 但是经济学学生又必须用到

{smcl}
{* 07apr2005}{...}
{cmd:help m2 op_conditional}
{hline}
{* index operators}{...}
{* index conditional operator}{...}

{title:Title}

{p 4 4 2}
{hi:[M-2] op_conditional -- Conditional operator}


{title:Syntax}

	{it:a} {cmd:?} {it:b} : {it:c}


{p 4 4 2}
where {it:a} must evaluate to a real scalar, and {it:b} and {it:c} may 
be of any type whatsoever.


{title:Description}

{p 4 4 2}
The conditional operator returns {it:b} if {it:a} is true ({it:a} is 
not equal to 0) and {it:c} otherwise.


{title:Remarks}

{p 4 4 2}
Conditional operators

	{cmd:dof = (k==0 ? n-1 : n-k)}

{p 4 4 2}
are more compact than the {cmd:if}-{cmd:else} alternative

	{cmd:if (k==0) dof = n-1}
	{cmd:else      dof = n-k}

{p 4 4 2} and they can be used as parts of expressions:

	{cmd:mse = ess/(k==0 ? n-1 : n-k)}


{title:Conformability}

    {it:a} {cmd:?} {it:b} : {it:c}:
		{it:a}:  1 {it:x} 1
		{it:b}:  {it:r1 x c1}
		{it:c}:  {it:r2 x c2}
	   {it:result}:  {it:r1 x c1}  or  {it:r2 x c2}
	

{title:Diagnostics}

{p 4 4 2}
In {it:a} {cmd:?} {it:b} : {it:c}, only the necessary parts are evaluated:
{it:a} and {it:b} if {it:a} is true, or {it:a} and {it:c} if {it:a} is false.
However, the {cmd:++} and {cmd:--} operators are always evaluated:

	{cmd:(}{it:k}{cmd:==0 ?} {it:i}{cmd:++ :} {it:j}{cmd:++)}

{p 4 4 2}
increments both {it:i} and {it:j}, regardless of the value of {it:k}.


{title:Also see}

{p 4 13 2}
Manual:  {hi:[M-2] op_conditional}

{p 4 13 2}
Online:  help for 
{bf:{help m2_exp:[M-2] exp}};
{bf:{help m2_intro:[M-2] intro}}
{p_end}