# [R] inverse cumsum

Gavin Simpson gavin.simpson at ucl.ac.uk
Wed Jun 18 13:53:20 CEST 2008

```On Wed, 2008-06-18 at 12:37 +0200, Wacek Kusnierczyk wrote:
> Alfredo Alessandrini wrote:
> > I've a matrix like this:
> >
> > 1985     1.38     1.27     1.84     2.10     0.59     3.47
> > 1986     1.05     1.13     1.21     1.54     0.21     2.14
> > 1987     1.33     1.21     1.77     1.44     0.27     2.85
> > 1988     1.86     1.06     2.33     2.14     0.55     1.40
> > 1989     2.10     0.65     2.74     2.43     1.19     1.45
> > 1990     1.55     0.00     1.59     1.94     0.99     2.14
> > 1991     0.92     0.72     0.50     1.29     0.54     1.22
> > 1992     2.15     1.28     1.23     2.26     1.22     3.17
> > 1993     1.50     0.87     1.68     1.97     0.83     2.55
> > 1994     0.69     0.00     0.76     1.89     0.60     0.87
> > 1995     1.13     1.04     1.19     1.52     1.13     1.78
> >
> > Can I utilise a cumsum inverse? from 1995 to 1985?
> >
> i guess you mean columnwise cumsums computed starting from the bottom up?
> then this should do (given that your data is in matrix m, and years are
> row labels, not data):
>
> rc = nrow(m)
> cumsums = apply(m[rc:1,], 2, cumsum)[rc:1,]

Alternatively, just use rev() instead of doing it by hand:

> dat <- as.data.frame(matrix(scan(), ncol = 7, byrow = TRUE))
1: 1985     1.38     1.27     1.84     2.10     0.59     3.47
8: 1986     1.05     1.13     1.21     1.54     0.21     2.14
15: 1987     1.33     1.21     1.77     1.44     0.27     2.85
22: 1988     1.86     1.06     2.33     2.14     0.55     1.40
29: 1989     2.10     0.65     2.74     2.43     1.19     1.45
36: 1990     1.55     0.00     1.59     1.94     0.99     2.14
43: 1991     0.92     0.72     0.50     1.29     0.54     1.22
50: 1992     2.15     1.28     1.23     2.26     1.22     3.17
57: 1993     1.50     0.87     1.68     1.97     0.83     2.55
64: 1994     0.69     0.00     0.76     1.89     0.60     0.87
71: 1995     1.13     1.04     1.19     1.52     1.13     1.78
78:
> rownames(dat) <- as.character(dat[,1])
> dat <- dat[,-1]
> rc <- nrow(dat)
> cs1 <- apply(dat[rc:1,], 2, cumsum)[rc:1,]
> cs1
V2   V3    V4    V5   V6    V7
1985 15.66 9.23 16.84 20.52 8.12 23.04
1986 14.28 7.96 15.00 18.42 7.53 19.57
1987 13.23 6.83 13.79 16.88 7.32 17.43
1988 11.90 5.62 12.02 15.44 7.05 14.58
1989 10.04 4.56  9.69 13.30 6.50 13.18
1990  7.94 3.91  6.95 10.87 5.31 11.73
1991  6.39 3.91  5.36  8.93 4.32  9.59
1992  5.47 3.19  4.86  7.64 3.78  8.37
1993  3.32 1.91  3.63  5.38 2.56  5.20
1994  1.82 1.04  1.95  3.41 1.73  2.65
1995  1.13 1.04  1.19  1.52 1.13  1.78
> cs2 <- apply(dat, 2, function(x) {rev(cumsum(rev(x)))})
> cs2
V2   V3    V4    V5   V6    V7
1985 15.66 9.23 16.84 20.52 8.12 23.04
1986 14.28 7.96 15.00 18.42 7.53 19.57
1987 13.23 6.83 13.79 16.88 7.32 17.43
1988 11.90 5.62 12.02 15.44 7.05 14.58
1989 10.04 4.56  9.69 13.30 6.50 13.18
1990  7.94 3.91  6.95 10.87 5.31 11.73
1991  6.39 3.91  5.36  8.93 4.32  9.59
1992  5.47 3.19  4.86  7.64 3.78  8.37
1993  3.32 1.91  3.63  5.38 2.56  5.20
1994  1.82 1.04  1.95  3.41 1.73  2.65
1995  1.13 1.04  1.19  1.52 1.13  1.78
> all.equal(cs1, cs2)
[1] TRUE

HTH

G
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Dr. Gavin Simpson             [t] +44 (0)20 7679 0522
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