{S,D,C,Z}SUM (n, x, incx)
sum real*4 | real*8 | complex*8 | complex*16 The total of the values of the elements in the vector x. If n<=0, sum returns the value 0.0.
n integer*4 On entry, the number of elements in the vector x. On exit, n is unchanged. x real*4 | real*8 | complex*8 | complex*16 On entry, a one-dimensional array X of length at least (1+(n-1)*|incx|), containing the n elements of the vector x. On exit, x is unchanged. incx integer*4 On entry, the increment for the array X. If incx > 0, vector x is stored forward in the array, so that x(i) is stored in location X(1+(i-1)*incx). If incx < 0, vector x is stored backward in the array, so that x(i) is stored in location X(1+(n-i)*|incx|). If incx = 0, only the first element is accessed. On exit, incx is unchanged.
The _SUM subprograms compute the total value of the elements of a vector, performing the following operation: SUM(i=1...n,x(i)) Because of efficient coding, rounding errors can cause the final result to differ from the result computed by a sequential evaluation of the sum of the elements of the vector. If incx < 0, the result is identical to using |incx|. If incx = 0, the computation is a time-consuming way of setting sum = nx(1).
INTEGER*4 N, INCX REAL*4 X(200), SUM INCX = 2 N = 50 SUM = SSUM(N,X,INCX) This FORTRAN example shows how to compute the sum of the values of the elements of the vector x.