两个变量不被断言识别为相同

Question

我想详细了解以下代码的行为：

clear
set obs 10000
set seed 98034

* I generate three variables
generate double u1 = runiform()
generate double u2=u1

*check
assert u2==u1
***

generate double var1=runiform()

* I generate some ids
generate byte id_=0

forvalues i=1(1)`=10000/100'{
    replace id_=`i' if _n>`=(`i'-1)*`=10000/100''
}

*I sum by id_ u1 and u2
bysort id_: egen double u11= total(u1)
bysort id_: egen double u21= total(u2)

*check
assert u11==u21
***

*I drop duplicates
bysort id_: drop if _n>1

*I generate a new variable which should be equal to var1 (I am adding and 
*subtracting the same quantities)

generate double var2= var1 - u11 + u21

*(1) 
assert var2==var1

特别是，我无法理解为什么 assert (1) 会失败，我已经生成了我以相同方式求和的每个变量，因此 var1 和 var2 应该是相同的。

有趣的是，如果我对总和进行不同排序，assert 有效：

drop var2
generate double var2= - u11 + u21 + var1 

*(2)
assert var2==var1

Answer 1

这两个变量不相同。要查看此内容，请更改他们的 format:

format var1 %20.15f
format var2 %20.15f

list var1 var2 in 1/10

     +---------------------------------------+
     |              var1                var2 |
     |---------------------------------------|
  1. | 0.498376312204773   0.498376312204776 |
  2. | 0.394671386281136   0.394671386281132 |
  3. | 0.515152901323075   0.515152901323077 |
  4. | 0.789668809822002   0.789668809822004 |
  5. | 0.931897887273974   0.931897887273976 |
     |---------------------------------------|
  6. | 0.947614996238336   0.947614996238336 |
  7. | 0.207296218919878   0.207296218919879 |
  8. | 0.368812285027951   0.368812285027950 |
  9. | 0.565084085641873   0.565084085641871 |
 10. | 0.331114583239097   0.331114583239099 |
     +---------------------------------------+

ordering of mathematical operations 确实重要，从左到右发生：

generate double var2= var1 - u11 + u21
format var2 %20.15f

generate double v2a = - u11 + u21
generate double v2b = var1 + v2a
format v2b %20.15f

generate double v2c = var1 - u11
generate double v2d = v2c + u21
format v2d %20.15f

list var2 v2b v2d in 1/10

     +-----------------------------------------------------------+
     |              var2                 v2b                 v2d |
     |-----------------------------------------------------------|
  1. | 0.498376312204776   0.498376312204773   0.498376312204776 |
  2. | 0.394671386281132   0.394671386281136   0.394671386281132 |
  3. | 0.515152901323077   0.515152901323075   0.515152901323077 |
  4. | 0.789668809822004   0.789668809822002   0.789668809822004 |
  5. | 0.931897887273976   0.931897887273974   0.931897887273976 |
     |-----------------------------------------------------------|
  6. | 0.947614996238336   0.947614996238336   0.947614996238336 |
  7. | 0.207296218919879   0.207296218919878   0.207296218919879 |
  8. | 0.368812285027950   0.368812285027951   0.368812285027950 |
  9. | 0.565084085641871   0.565084085641873   0.565084085641871 |
 10. | 0.331114583239099   0.331114583239097   0.331114583239099 |
     +-----------------------------------------------------------+

在这种情况下，由于差异的大小，也可能涉及精度问题。有关详细信息，请在 Stata 的提示符中键入 help precision。

两个变量不被断言识别为相同

Two variables are not recognized as identical by assert

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sum

stata