{smcl}
{bf:Mata course:  Using views}
{hline}

{title:A statistical application}

{p 4 4 2}
Calculate 

		{it:y} = ({it:X}'{it:X})^(-1)*{it:X}'{it:y} 

{p 4 4 2}
for 

		{it:y} = (mpg)

		{it:X} = (weight, foreign, 1)

{p 4 4 2}
Solution:

	{com}. sysuse auto, clear
	{txt}(1978 Automobile Data)

	{com}. gen one = 1
	{txt}
	{com}. mata
	{txt}{hline 49} mata (type {cmd:end} to exit) {hline}
	{com}: st_view(X=., ., ("weight", "foreign", "one"))
	{res}
	{com}: st_view(y=., ., "mpg")
	{res}
	{com}: invsym(X'X)*X'y
	{res}       {txt}           1
	    {c TLC}{hline 16}{c TRC}
	  1 {c |}  {res}-.0065878864{txt}  {c |}
	  2 {c |}  {res}-1.650029106{txt}  {c |}
	  3 {c |}  {res} 41.67970233{txt}  {c |}
	    {c BLC}{hline 16}{c BRC}

	{com}: end
	{txt}{hline}

{p 4 4 2}
Notes:

{p 8 12 2}
1.
Using lots of views is not only okay, it is encouraged.

{p 8 12 2}
2.
What is bad about the code above is the calculation of {cmd:X'}.  
It would be better to use {cmd:cross()}.

{p 12 12 2}
Rather than code 

		{cmd:invsym(X'X)*X'y}

{p 12 12 2}
code

		{cmd:invsym(cross(X,X))*cross(X,y)}

{p 12 12 2}
That not only uses less memory, it is faster.  

{p 12 12 2}
See {bf:{help mf_cross:[M-5] cross()}}.


{title:A data-management application}

{p 4 4 2}
You have a dataset containing variables 

		stat72, stat73, ..., stat99.

{p 4 4 2}
They record a patient's status in 1972, 1973, ..., 1999.

{p 4 4 2}
You wish to add new variable 

		firstyear 

{p 4 4 2}
recording the first year 
in which a status variable takes on the value 1.

{p 4 4 2}
Solution:


	{com}. use ds1, clear 
	{txt}
	{com}. gen byte firstyear = .
	{txt}(500 missing values generated)

	{com}. 
	. mata:
	{txt}{hline 49} mata (type {cmd:end} to exit) {hline}
	{com}: names =    "s72 s73 s74 s75 s76 s77 s78 s79 
                   s80 s81 s82 s83 s84 s85 s86 s87 s88 s89
                   s90 s91 s92 s93 s94 s95 s96 s97 s98 s99"
	{res}
	{com}: st_view(s=., ., tokens(names))
	{res}
	{com}: st_view(first=., ., "firstyear")
	{res}
	{com}: 
	: for (i=1; i<=rows(s); i++) {c -(}
	>         for (j=1; j<=cols(s); j++) {c -(}
	>                 if (s[i,j]==1) {c -(}
	>                         first[i] = j+71
	>                         break
	>                 {c )-}
	>         {c )-}
	> {c )-}
	{res}
	{com}: end
	{txt}{hline}

        {bf:{stata do ds1}}

{title:Another example}

{p 4 4 2}
See 
{bf:{help mf_panelsetup:panelsetup()}}.


{hline}
{bf:{view talk.smcl:Top}}