# Re: [R] inverse cumsum

From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>
Date: Wed, 18 Jun 2008 12:53:20 +0100

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
ECRC, UCL Geography,          [f] +44 (0)20 7679 0565
Pearson Building,             [e] gavin.simpsonATNOSPAMucl.ac.uk
Gower Street, London          [w] http://www.ucl.ac.uk/~ucfagls/
UK. WC1E 6BT.                 [w] http://www.freshwaters.org.uk
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