NAG Library Function Document

nag_search_vector (m01fsc)


    1  Purpose
    7  Accuracy


nag_search_vector (m01fsc) searches a vector of arbitrary type data objects for the first or last match to a given value.


#include <nag.h>
#include <nagm01.h>
Nag_Boolean  nag_search_vector (Pointer key, const Pointer vec, size_t n, ptrdiff_t stride,
Integer (*compare)(const Nag_Pointer a, const Nag_Pointer b),
Nag_SortOrder order, Nag_SearchMatch final, Pointer *match, NagError *fail)


nag_search_vector (m01fsc) searches a sorted vector of n  arbitrary type data objects, which are stored in the elements of an array at intervals of length stride. vec must have previously been sorted into the specified order.
The function searches for the first or last match depending on the value of final. It returns Nag_TRUE if an exact match is found and match is set to point at that object. If there is no exact match then Nag_FALSE is returned and match is set to point to either the next later element, if final=Nag_First, or the next earlier element, if final=Nag_Last.




1:     key Pointer Input
On entry: the object to search for.
2:     vec[n] const Pointer Input
On entry: the array of objects to be searched.
3:     n size_tInput
On entry: the number n  of objects to be searched.
Constraint: 0nMAX_LENGTH, where MAX_LENGTH is an implementation-dependent value for the maximum size of an array.
4:     stride ptrdiff_tInput
On entry: the increment between data items in vec to be searched.
Note: if stride is positive, vec should point at the first data object; otherwise vec should point at the last data object.
Constraint: 0<stridep, where p is an implementation-dependent value for the maximum size_t size on the system, divided by n if n is positive.
5:     compare function, supplied by the userExternal Function
nag_search_vector (m01fsc) compares two data objects. If its arguments are pointers to a structure, this function must allow for the offset of the data field in the structure (if it is not the first).
The function must return:
-1 if the first data field is less than the second,
-0 if the first data field is equal to the second,
-1 if the first data field is greater than the second.
The specification of compare is:
Integer  compare (const Nag_Pointer a, const Nag_Pointer b)
1:     a const Nag_Pointer Input
On entry: the first data field.
2:     b const Nag_Pointer Input
On entry: the second data field.
6:     order Nag_SortOrderInput
On entry: specifies whether the array will be sorted into ascending or descending order.
Constraint: order=Nag_Ascending or Nag_Descending.
7:     final Nag_SearchMatchInput
On entry: specifies whether to search for the first or last match. This also determines the pointer returned if an exact match cannot be found.
Constraint: final=Nag_First or Nag_Last.
8:     match Pointer *Output
On exit: if an exact match is found this is a pointer to a pointer to the matching data object. If an exact match is not found this is set to point to the nearest object. If final=Nag_First this is the next later element, otherwise the next earlier element.
9:     fail NagError *Input/Output
The NAG error argument (see Section 3.7 in How to Use the NAG Library and its Documentation).

Error Indicators and Warnings

On entry, argument final had an illegal value.
On entry, argument order had an illegal value.
On entry, stride=value.
Constraint: stride0.
On entry, n=value.
Constraint: nvalue, an implementation-dependent size that is printed in the error message.
On entry, stride=value.
Constraint: stridevalue, an implementation-dependent size that is printed in the error message.
On entry, n=value.
Constraint: n0.


Not applicable.

Parallelism and Performance

nag_search_vector (m01fsc) is not threaded in any implementation.

Further Comments

The maximum time taken by the function is approximately proportional to log2n .


The example program reads a key and a list of real numbers, which have been sorted into ascending order. It then searches the list for the first number which matches the key.

Program Text

Program Text (m01fsce.c)

Program Data

Program Data (m01fsce.d)

Program Results

Program Results (m01fsce.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017