Question

I have two large data frames, a and b for which identical(a,b) is TRUE, as is all.equal(a,b), but identical(digest(a),digest(b)) is FALSE. What could cause this?

What’s more, I tried to dig in deeper, by applying digest to bunches of rows. Incredibly, at least to me, there is agreement in the digest values on sub-frames all the way to the last row of the data frames.

Here is a sequence of comparisons:

> identical(a, b)
[1] TRUE
> all.equal(a, b)
[1] TRUE
> digest(a)
[1] "cac56b06078733b6fb520442e5482684"
> digest(b)
[1] "fdd5ab78ca961982d195f800e3cf60af"
> digest(a[1:nrow(a),])
[1] "e44f906723405756509a6b17b5949d1a"
> digest(b[1:nrow(b),])
[1] "e44f906723405756509a6b17b5949d1a"

Every method I can think of indicates these two objects are identical, but their digest values are different. Is there something else about data frames that can produce such discrepancies?

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For further details: the objects are about 10M rows x 12 columns. Here’s the output of str():

'data.frame':   10056987 obs. of  12 variables:
 $ V1 : num  1 11 21 31 41 61 71 81 91 101 ...
 $ V2 : num  1 1 1 1 1 1 1 1 1 1 ...
 $ V3 : num  2 3 2 3 4 5 2 4 2 4 ...
 $ V4 : num  1 1 1 1 1 1 1 1 1 1 ...
 $ V5 : num  1.8 2.29 1.94 2.81 3.06 ...
 $ V6 : num  0.0653 0.0476 0.0324 0.034 0.0257 ...
 $ V7 : num  0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 ...
 $ V8 : num  0.00653 0.00476 0.00324 0.0034 0.00257 ...
 $ V9 : num  1.8 2.3 1.94 2.81 3.06 ...
 $ V10: num  0.1957 0.7021 0.0604 0.1866 0.9371 ...
 $ V11: num  1704 1554 1409 1059 1003 ...
 $ V12: num  23309 23309 23309 23309 23309 ...

> print(object.size(a), units = "Mb")
920.7 Mb

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Update 1: On a whim, I converted these to matrices. The digests are the same.

> aM = as.matrix(a)
> bM= as.matrix(b)
> identical(aM,bM)
[1] TRUE
> digest(aM)
[1] "c5147d459ba385ca8f30dcd43760fc90"
> digest(bM)
[1] "c5147d459ba385ca8f30dcd43760fc90"

I then tried converting back to a data frame, and the digest values are equal (and equal to the previous value for a).

> aMF = as.data.frame(aM)
> bMF = as.data.frame(bM)
> digest(aMF)
[1] "cac56b06078733b6fb520442e5482684"
> digest(bMF)
[1] "cac56b06078733b6fb520442e5482684"

So, b looks like the bad boy, and it has a colorful past. b came from a much bigger data frame, say B. I took only the columns of B that appeared in a and checked to see if they were equal. Well, they were equal, but had different digests. I converted the column names (from “InformativeColumnName1” to “V1”, etc.), just to avoid any issues that might arise – though all.equal and identical tend to point out when column names differ.

Since I am working on two different programs and don’t have simultaneous access to a and b, it is easiest for me to use the digest values to check the calculations. However, something seems to be odd in how I extract columns from a data frame and then apply digest() to it.

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ANSWER:
It turns out, to my astonishment (dismay, horror, embarrassment, you name it), identical is very forgiving about attributes. I had assumed that only all.equal was forgiving about attributes.

This was discovered via Tommy’s suggestion identical(d1, d2, attrib.as.set=FALSE). Running attributes(a) is a bad, bad idea: the deluge of row names took awhile before Ctrl-C could interrupt it. Here is the output of names(attributes()):

> names(attributes(a))
[1] "names"     "row.names" "class"    
> names(attributes(b))
[1] "names"     "class"     "row.names"

They’re in different orders! Kudos to digest() for being straight with me.

UPDATE

To aid others with this problem, it seems that simply rearranging the attributes will be adequate to get identical hash values. Since tinkering with attribute orders is new to me, this may break something, but it works in my case. Note that it is a little time consuming if the objects are big; I’m not aware of a faster method for doing this. (I’m also looking to move to using matrices or data tables instead of data frames, and this may be another incentive to avoid data frames.)

tmpA0   = attributes(a)
tmpA1   = tmpA0[sort(names(tmpA0))]
a2      = a
attributes(a2) = tmpA1

tmpB0   = attributes(b)
tmpB1   = tmpB0[sort(names(tmpB0))]
b2      = b
attributes(b2) = tmpB1

digest(a2)  # e04e624692d82353479efbd713ec03f6
digest(b2)  # e04e624692d82353479efbd713ec03f6

identical(b,b2, attrib.as.set = FALSE) # FALSE
identical(b,b2, attrib.as.set = TRUE) # TRUE
identical(a2,b2, attrib.as.set = FALSE) # TRUE

Answer 1

Without having the actual data.frames it is of course hard to know, but one difference could be the order of the attributes. identical ignores that by default, but setting attrib.as.set=FALSE can change that:

d1 <- structure(1, foo=1, bar=2)
d2 <- structure(1, bar=2, foo=1)

identical(d1, d2) # TRUE
identical(d1, d2, attrib.as.set=FALSE) # FALSE