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# dim_sum_n

Computes the arithmetic sum of a variable's given dimension(s) at all other dimensions.

## Prototype

```	function dim_sum_n (
x        : numeric,
dims [*] : integer
)

return_val  :  float or double
```

## Arguments

x

A variable of numeric type and any dimensionality.

dims

The dimension(s) of x on which to calculate the sum. Must be consecutive and monotonically increasing.

## Return value

The output will be double if x is double, and float otherwise.

The output dimensionality will be the same as all but dims's dimensions of the input variable. The dimension rank of the input variable will be reduced by the rank of dims.

## Description

The dim_sum_n function computes the sum of all elements of the dimensions indicated by dims' for each index of the remaining dimensions. Missing values are ignored.

Use the dim_sum_n_Wrap function if metadata retention is desired. The interface is identical.

## Examples

Example 1

Create a variable, q, of size (3,5,10) array. Then calculate the sum of the rightmost dimension.

```    q   = random_uniform(-20,100,(/3,5,10/))
qav = dim_sum_n(q,2)   ;==>  qav(3,5)

; Use dim_sum_n_Wrap if metadata retention is desired
; qav = dim_sum_n_Wrap(q,2)   ;==>  qav(3,5)
```
Example 2

Let x be of size (ntim,nlat,mlon) and with named dimensions "time", "lat" and "lon", respectively. Then, for each time and latitude, the zonal sum (i.e., sum of all non-missing longitudes) is:

```    xSumLon = dim_sum_n(x,2)    ; ==> xSumLon(ntim,nlat)

; Use dim_sum_n_Wrap if metadata retention is desired
; xSumLon = dim_sum_n_Wrap(x,2)    ; ==> xSumLon(time,lat)
```
Example 3

Let x be defined as in Example 2: x(time,lat,lon). Compute the sum over time at each latitude/longitude grid point.

```    xSumTime = dim_sum_n(x, 0)    ; ==> xSumTime(nlat,nlon)

; Use dim_sum_n_Wrap if metadata retention is desired
; xSumTime = dim_sum_n_Wrap(x, 0)    ; ==> xSumTime(lat,lon)
```
Example 4

Let x be defined as x(time,lev,lat,lon). Compute the sum over time and level at each latitude/longitude grid point.

```    xSum = dim_sum_n(x, (/0,1/))    ; ==> xSum(nlat,nlon)

; Use dim_sum_n_Wrap if metadata retention is desired
; xSum = dim_sum_n_Wrap(x, (/0,1/))    ; ==> xSum(nlat,nlon)
```
To compute the sum over lat and lon at each time/lev grid point:
```    xSum = dim_sum_n(x,(/2,3/))    ; ==> xSum(nlev,ntim)

; Use dim_sum_n_Wrap if metadata retention is desired
; xSum = dim_sum_n_Wrap(x,(/2,3/))    ; ==> xSum(nlev,ntim)
```
Example 5

Let p(time,lat,lon) contain hourly (eg, 0Z, 1Z, ... 23Z) accumulated precipitation totals. Create a variable containing accumulated 6-hour (0Z, 6Z, 12Z, 18Z) totals. Note: The subscripting below assigns the 0Z-to-5Z total to the 0 subscript (0Z time); the 6Z-to-11Z total to subscript 1 (6Z time); etc. If different assignments are desired the user should make the appropriate adjustments.

```   dimp   = dimsizes( p )
ntim   = dimp(0)                 ; number of times in array
nlat   = dimp(1)
mlon   = dimp(2)

nhr    = 6                       ; 24 for daily total; 12 for twelve hour total

nhrdim = ntim/nhr                ; number of nhr-hour segments

ptot   =  new ( (/nhrdim,nlat,mlon/), typeof(p), getFillValue(p) )
ntStrt = 0
ntLast = nhr-1

do nt=0,ntim-1,nhr
ptot(nt/nhr,:,:) = dim_sum_n( p(ntStrt:ntLast,:,:) , 0)

ntStrt = ntStrt + nhr
ntLast = ntLast + nhr
end do
; optional meta data assignment
; unnecessary if using dim_sum_n_Wrap
copy_VarMeta(p(::nhr,:,:), ptot)          ; meta data;  ::nhr makes time assignment
ptot@long_name = nhr+"-hr accumulated ..."

printVarSummary( ptot )
printMinMax( ptot, True )
```