# [R] gridding values in a data frame

jdeisenberg catcode at catcode.com
Fri May 1 00:06:16 CEST 2009

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dxc13 wrote:
>
> Hi all,
> I have a data frame that looks like such:
> LATITUDE   LONGITUDE   TEMPERATURE   TIME
> 36.73         -176.43        58.32               1
>
> and this goes on for a A LOT more records, until time=1200
>
> I want to create a 5 degree by 5 degree grid of this data, with the value
> of temperature in the appropriate grid cell.  I want one grid for each
> time value.
>
> dxc13
>

The following appears to work, but is most definitely *not* in the "spirit
of R" -- it's more like a C program written in R.  Undoubtedly someone will
come up with a much more clever method. (This is always my problem; I come
up with a workable solution, but it's not elegant.)

Note: the grid has to have 37 rows, since latitude -90 to +90 (inclusive)
takes 37 5-degree increments; since -180 and +180 longitude are the same,
you would never have them as separate numbers.

for (i in 1:1200)
{
x <- d[d\$TIME==i,]
if (length(x\$TIME) > 0)
{
print(sprintf("# of elements for time %d:  %d", i,
length(x\$TIME)))
grid <- matrix(NA, nrow=37, ncol=72)
for (j in 1:length(x\$TIME))
{
lat <- 1 + trunc((90 + x\$LATITUDE[j]) / 5)
long <-  1 + trunc((180 + x\$LONGITUDE[j]) / 5)
grid[lat,long] <- x\$TEMPERATURE[j]
}
write(t(grid), file=sprintf("time%d.csv", i), ncolumns=72, sep=",")
}
}

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