st: what does _N mean under by varlist in the -egen- and -gen-?

[gjhxmu@sina.com]

Dear statalists,

I know that under by varlist: _N is interpreted within each group of observations, not for the whole dataset. 

However, I found the meaning _N in the -egen- is a little different.

Take the following for example,
clear
input family   sex  
         1     1  
         1     1  
         1     1  
         2     0  
         2     1  
         2     0  
         2     1  
         2     0  
         3     0  
         3     1  
         3     0  
         3     1  
         3     1  
         3     1 
  
end
bys family: g x=sum(sex) if sex[_N]==0
bys family: egen xx=total(sex) if sex[_N]==0
bys family: g xxx=sex[_N]
bys family: egen xxxx=total(sex[_N]==0)
l,sepby(family) noobs

  +------------------------------------+
  | family   sex   x   xx   xxx   xxxx |
  |------------------------------------|
  |      1     1   .    .     1      0 |
  |      1     1   .    .     1      0 |
  |      1     1   .    .     1      0 |
  |------------------------------------|
  |      2     0   0    .     0      5 |
  |      2     1   1    .     0      5 |
  |      2     0   1    .     0      5 |
  |      2     1   2    .     0      5 |
  |      2     0   2    .     0      5 |
  |------------------------------------|
  |      3     0   .    .     1      0 |
  |      3     1   .    .     1      0 |
  |      3     0   .    .     1      0 |
  |      3     1   .    .     1      0 |
  |      3     1   .    .     1      0 |
  |      3     1   .    .     1      0 |
  +------------------------------------+

Based on the results, Obviously the meaning of _N in the first two is different,
while the meaning of _N in the last two is the same.

Could anyone tell me why?

Thank you very much.

Best regards,
Rose.

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