I can't add to what I said earlier, or wrote much earlier as cited,
except to emphasise that this function is here on a knife-edge:
. di round(8.755, 0.01)
8.76
. di round(3.86 + 4.895, 0.01)
8.75
If you want exact decimal calculations, you need to do all your workings
in integers, and convert only when obliged to.
Nick
[email protected]
[email protected]
Hello Nick,
thanks for quick answering.
Quoting "Nick Cox" <[email protected]>:
3. Not your question, but -egen, sum()- is a poor way to do a sum.
but in the numerical example I provided egen,sum() turns out to
provide the exact answer (8.76), which I can't get using collapse (sum).
My problem is that I never use "egen, sum" when working with real data
while I often use "collapse (sum)", that seems to be not very
appropriate.
I think the problem is in the way the "collapse" command and the "round"
function are related: I verified that if the line "gen double
r2=round(a*100)/100" (see after the "collapse" command) is separated
in 2 parts, like:
collapse (sum) a
gen double temp=a*100
gen double r2=round(temp)/100
The resulting r2 is correct (but I don't know why).
Quoting "Nick Cox" <[email protected]>:
I have three comments here.
1. -egen- by default will generate -float- variables (unless you have
-set type double-). So, you shouldn't be surprised to lose a little
precision there. Functions like -round()- that may make knife-edge
decisions are likely to show this up. The first FAQ cited below is a
similar case.
2. This is all part of a larger issue: Stata works in binary. Stata
does
not do decimal arithmetic!
See for example
FAQ . . . . . . . . . . . . . . . . . . . Results of the mod(x,y)
function
. . . . . . . . . . . . . . . . . . . . . N. J. Cox and T. J.
Steichen
2/03 Why does the mod(x,y) function sometimes give
puzzling results?
Why is mod(0.3,0.1) not equal to 0?
http://www.stata.com/support/faqs/data/mod.html
FAQ . . . . . . . . . . . . . . . . . The accuracy of the float
data type
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
W.
Gould
5/01 How many significant digits are there in a float?
http://www.stata.com/support/faqs/data/prec.html
FAQ . . . . . . . . . Comparing floating-point values (the float
function)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
J.
Wernow
3/01 Why can't I compare two values that I know are equal?
http://www.stata.com/support/faqs/data/float.html
FAQ . . . . . . . . . Why am I losing precision with large whole
numbers?
. . . . . . . . . . . . . . . . . . UCLA Academic Technology
Services
7/08 http://www.ats.ucla.edu/stat/stata/faq/longid.htm
SJ-8-2 pr0038 Mata Matters: Overflow, underflow & IEEE
floating-point
format
. . . . . . . . . . . . . . . . . . . . . . . . . . . . J. M.
Linhart
Q2/08 SJ 8(2):255--268 (no
commands)
focuses on underflow and overflow and details of how
floating-point numbers are stored in the IEEE 754
floating-point standard
SJ-6-4 pr0025 . . . . . . . . . . . . . . . . . . . Mata matters:
Precision
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
W.
Gould
Q4/06 SJ 6(4):550--560 (no
commands)
looks at programming implications of the floating-point,
base-2 encoding that modern computers use
SJ-6-2 dm0022 . Tip 33: Sweet sixteen: Hexadec. formats & precision
problems
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
N.
J. Cox
Q2/06 SJ 6(2):282--283 (no
commands)
tip for using hexadecimal formats to understand precision
problems in Stata
3. Not your question, but -egen, sum()- is a poor way to do a sum. A
better way is -summarize, meanonly-. However, I guess that your real
problem is understanding what -egen- does with some real data, but
nevertheless note using -summarize- directly and picking up r(sum) is
always better for a single sum.
Nick
[email protected]
Carlo
Here is my code:
clear
version 9.2
set obs 1
gen double a=3.86
save data1,replace
clear
set obs 1
gen double a=4.895
save data2,replace
use data1,clear
append using data2
egen sum=sum(a)
gen double r1=round(sum*100)/100
list
collapse (sum) a
gen double r2=round(a*100)/100
list
Why are r1 and r2 not equal ?
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