MINLOC and MAXLOC

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## MINLOC and MAXLOC

minimum and location maximum and location

The operator MPI_MINLOC is used to compute MPI_MINLOC MPI_MAXLOC a global minimum and also an index attached to the minimum value. MPI_MAXLOC similarly computes a global maximum and index. One application of these is to compute a global minimum (maximum) and the rank of the process containing this value.

The operation that defines MPI_MAXLOC is: MPI_MAXLOC

Both operations are associative and commutative. Note that if MPI_MAXLOC MPI_MAXLOC is applied to reduce a sequence of pairs , then the value returned is (u,r), where and r is the index of the first global maximum in the sequence. Thus, if each process supplies a value and its rank within the group, then a reduce operation with op = MPI_MAXLOC will return the maximum value and the rank of the first process with that value. Similarly, MPI_MINLOC can be used to return a minimum and its index. More generally, MPI_MINLOC computes a lexicographic MPI_MINLOC minimum, where elements are ordered according to the first component of each pair, and ties are resolved according to the second component.

The reduce operation is defined to operate on arguments that consist of a pair: value and index. In order to use MPI_MINLOC and MPI_MAXLOC in a MPI_MINLOC MPI_MAXLOC reduce operation, one must provide a datatype argument that represents a pair (value and index). MPI provides nine such predefined datatypes. In C, the index is an int and the value can be a short or long int, a float, or a double. The potentially mixed-type nature of such arguments is a problem in Fortran. The problem is circumvented, for Fortran, by having the MPI-provided type consist of a pair of the same type as value, and coercing the index to this type also.

The operations MPI_MAXLOC and MPI_MINLOC can be used with each of the following datatypes.

MPI_2REAL MPI_2DOUBLE_PRECISION MPI_2INTEGER MPI_FLOAT_INT MPI_DOUBLE_INT MPI_LONG_INT MPI_2INT MPI_SHORT_INT MPI_LONG_DOUBLE_INT

The datatype MPI_2REAL is as if defined by the following MPI_2REAL (see Section ).

`MPI_TYPE_CONTIGUOUS(2, MPI_REAL, MPI_2REAL)`

Similar statements apply for MPI_2INTEGER, MPI_2DOUBLE_PRECISION, and MPI_2INT.

The datatype MPI_FLOAT_INT is as if defined by the MPI_FLOAT_INT following sequence of instructions.

```type[0] = MPI_FLOAT
type[1] = MPI_INT
disp[0] = 0
disp[1] = sizeof(float)
block[0] = 1
block[1] = 1
MPI_TYPE_STRUCT(2, block, disp, type, MPI_FLOAT_INT)```
Similar statements apply for the other mixed types in C.

Next: All Reduce Up: Global Reduction Operations Previous: Predefined Reduce Operations

Jack Dongarra
Fri Sep 1 06:16:55 EDT 1995