Prefix and Suffix Functions

Both classes of functions are related, Prefix functions go from left to right along an array. The difference between PRODUCT_PREFIX and PRODUCT is that the latter returns a single value whereas the former returns an array result. This array contains the value obtained by multiplying the current element by all the elements to the left of it. In the case of a 2D (or more) example, the array is traversed in array element order.

Suffix functions are exactly the same except that the array is traversed from right to left (or for multi-dimensional arrays in reverse array element order).

• Prefix functions scan left to right along an array. Each element of the result depends upon the preceding elements (in array element order). For example,

  PRODUCT_PREFIX((/1,2,3,4/)) = (/1,2,6,24/)
  PRODUCT_PREFIX((/1,4,7/), = ((/1, 24,  5040/),
                 (/2,5,8/), =  (/2,120, 40320/),
                 (/3,6,9/)) =  (/6,720,362880/))

• suffix functions do the same but scan backwards.

  PRODUCT_SUFFIX((/1,2,3,4/)) = (/24,24,12,4/)
  PRODUCT_SUFFIX((/1,4,7/), = ((/362880,60480,504/),
                 (/2,5,8/), =  (/362880,15120, 72/),
                 (/3,6,9/)) =  (/181440, 3024,  9/))

 

There are prefix and suffix functions corresponding to all the Fortran 90 and new HPF reduction functions. They all work in the same way and have more or less the same set of arguments. These include:

• SUM and PRODUCT: for example,

  SUM_PREFIX, PRODUCT_SUFFIX,

• MAXVAL and MINVAL: for example,

  MAXVAL_SUFFIX, MINVAL_PREFIX,

• ALL, ANY and COUNT: for example,

  ALL_SUFFIX, ANY_PREFIX,

plus HPF defined intrinsics: IALL, IANY, IPARITY and PARITY.

 

All these functions take (more or less) the same arguments, for example:

 MINVAL_PREFIX(ARRAY[,DIM][,MASK][,SEGMENT][,EXCLUSIVE])

• a MASK argument works as in Fortran 90,
• COPY_ ... doesn't have MASK and EXCLUSIVE,
• ALL_ ..., ANY_ ..., COUNT_ ... and PARITY_ ... do not have MASK as ARRAY is LOGICAL.

MASK and SEGMENT are LOGICAL, MASK conforms to ARRAY, SEGMENT has same shape as ARRAY.

Example of the MASK argument,

PRODUCT_PREFIX((/1,2,3,4/), MASK=(/T,F,T,F/)) = (/1,1,3,3/)

Only the elements corresponding to .TRUE. positions of the mask contribute to the results, the positions of the source array which correspond to the .FALSE. mask elements are ignored.

 

The SEGMENT argument is a LOGICAL array and allows the function to be applied to sections of the source array, for example,

  S       =  (/T,T,T, F,F, T,T, F, T,T/)
  !            -----  ---  ---  -  ---
  SUM_PREFIX((/1,2,3, 4,5, 6,1, 2, 3,4/),SEGMENT=S) =
             (/1,3,6, 4,9, 6,7, 2, 3,7/)

The EXCLUSIVE argument is a LOGICAL valued scalar. If it is .FALSE. (the default) then each element takes part in operation for its position, otherwise it does not and the first scanned element has identity value. For example,

  PRODUCT_PREFIX((/1,2,3,4/), EXCLUSIVE=.TRUE.) =
                 (/1,1,2,6/)
  SUM_PREFIX((/1,2,3,4/), EXCLUSIVE=.TRUE.) =
             (/0,1,3,6/)

When both a MASK and a SEGMENT argument are supplied the result is partitioned and the function only applied to the .TRUE. elements of each partition.

Now try this question

Return to corresponding overview page