74 lines
1.3 KiB
C
74 lines
1.3 KiB
C
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/*! @file qselect.c
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* \brief Quickselect: returns the k-th (zero-based) largest value in A[].
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*
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* <pre>
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* -- SuperLU routine (version 4.1) --
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* Lawrence Berkeley National Laboratory.
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* November, 2010
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* </pre>
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*/
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#include "slu_ddefs.h"
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double dqselect(int n, double A[], int k)
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{
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register int i, j, p;
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register double val;
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k = SUPERLU_MAX(k, 0);
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k = SUPERLU_MIN(k, n - 1);
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while (n > 1)
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{
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i = 0; j = n-1;
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p = j; val = A[p];
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while (i < j)
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{
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for (; A[i] >= val && i < p; i++);
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if (A[i] < val) { A[p] = A[i]; p = i; }
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for (; A[j] <= val && j > p; j--);
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if (A[j] > val) { A[p] = A[j]; p = j; }
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}
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A[p] = val;
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if (p == k) return val;
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else if (p > k) n = p;
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else
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{
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p++;
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n -= p; A += p; k -= p;
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}
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}
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return A[0];
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}
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float sqselect(int n, float A[], int k)
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{
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register int i, j, p;
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register float val;
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k = SUPERLU_MAX(k, 0);
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k = SUPERLU_MIN(k, n - 1);
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while (n > 1)
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{
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i = 0; j = n-1;
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p = j; val = A[p];
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while (i < j)
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{
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for (; A[i] >= val && i < p; i++);
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if (A[i] < val) { A[p] = A[i]; p = i; }
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for (; A[j] <= val && j > p; j--);
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if (A[j] > val) { A[p] = A[j]; p = j; }
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}
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A[p] = val;
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if (p == k) return val;
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else if (p > k) n = p;
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else
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{
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p++;
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n -= p; A += p; k -= p;
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}
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}
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return A[0];
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}
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