*Stata Programming by Example
*Tutorial 5
**Looking at the incrementing, _n, and _N 


*--------------Start Example  1 ----------------*

sysuse auto, clear
local z=1

forvalues i=1/10 {

display mpg[`z']
display mpg[`i']
display 
local ++z

}

*--------------End Example----------------------*


*decrementing
*--------------Start Example  2 ----------------*

sysuse auto, clear
local z=10

forvalues i=1/10 {

display mpg[11-`z']
display mpg[`i']
display 
local --z

}

*--------------End Example----------------------*
*Stata 10 users guide p204



*lags and leads
*--------------Start Example  3 ----------------*

clear
set obs 10     //see stata 10 Data management manual p411
generate a=_n   //_n is the observation number
list a

*--------------End Example----------------------*


*--------------Start Example  4 ----------------*

clear
set obs 10     //see stata 10 Data management manual p411
generate a=_N   //_N is the last observation number
list a

*--------------End Example----------------------*


*--------------Start Example  5 ----------------*

clear
set obs 10     //see stata 10 Data management manual p411
generate a=_N-_n   //_N is the last observation number
list a

*--------------End Example----------------------*


*using _n to get a group number
*--------------Start Example  6 ----------------*

clear
sysuse auto
sort mpg
list mpg in 1/10 
generate a=1 if mpg[_n]!=mpg[_n-1]   //_N is the last observation number
list mpg a in 1/10
replace a=sum(a)
list mpg a in 1/10

*--------------End Example----------------------*


*using _N and _n  to reverse the order of mpg
*--------------Start Example  7 ----------------*

clear
sysuse auto
keep in 1/15
keep mpg
generate a=mpg[_N-_n+1]
list mpg a 
*--------------End Example----------------------*



*End tutorial 5 - programming Stata