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# NAG Toolbox: nag_sort_realvec_rank_rearrange (m01ea)

## Purpose

nag_sort_realvec_rank_rearrange (m01ea) rearranges a vector of real numbers into the order specified by a vector of ranks.

## Syntax

[rv, irank, ifail] = m01ea(rv, m1, irank, 'm2', m2)
[rv, irank, ifail] = nag_sort_realvec_rank_rearrange(rv, m1, irank, 'm2', m2)

## Description

nag_sort_realvec_rank_rearrange (m01ea) is designed to be used typically in conjunction with the M01D ranking functions. After one of the M01D functions has been called to determine a vector of ranks, nag_sort_realvec_rank_rearrange (m01ea) can be called to rearrange a vector of real numbers into the rank order. If the vector of ranks has been generated in some other way, then nag_sort_permute_check (m01zb) should be called to check its validity before nag_sort_realvec_rank_rearrange (m01ea) is called.

None.

## Parameters

### Compulsory Input Parameters

1:     $\mathrm{rv}\left({\mathbf{m2}}\right)$ – double array
Elements m1 to m2 of rv must contain real values to be rearranged.
2:     $\mathrm{m1}$int64int32nag_int scalar
m1 and m2 must specify the range of the ranks supplied in irank and the elements of rv to be rearranged.
Constraint: $0<{\mathbf{m1}}\le {\mathbf{m2}}$.
3:     $\mathrm{irank}\left({\mathbf{m2}}\right)$int64int32nag_int array
Elements m1 to m2 of irank must contain a permutation of the integers m1 to m2, which are interpreted as a vector of ranks.

### Optional Input Parameters

1:     $\mathrm{m2}$int64int32nag_int scalar
Default: the dimension of the arrays irank, rv. (An error is raised if these dimensions are not equal.)
m1 and m2 must specify the range of the ranks supplied in irank and the elements of rv to be rearranged.
Constraint: $0<{\mathbf{m1}}\le {\mathbf{m2}}$.

### Output Parameters

1:     $\mathrm{rv}\left({\mathbf{m2}}\right)$ – double array
These values are rearranged into rank order. For example, if ${\mathbf{irank}}\left(i\right)={\mathbf{m1}}$, then the initial value of ${\mathbf{rv}}\left(i\right)$ is moved to ${\mathbf{rv}}\left({\mathbf{m1}}\right)$.
2:     $\mathrm{irank}\left({\mathbf{m2}}\right)$int64int32nag_int array
Used as internal workspace prior to being restored and hence is unchanged.
3:     $\mathrm{ifail}$int64int32nag_int scalar
${\mathbf{ifail}}={\mathbf{0}}$ unless the function detects an error (see Error Indicators and Warnings).

## Error Indicators and Warnings

Errors or warnings detected by the function:
${\mathbf{ifail}}=1$
 On entry, ${\mathbf{m2}}<1$, or ${\mathbf{m1}}<1$, or ${\mathbf{m1}}>{\mathbf{m2}}$.
${\mathbf{ifail}}=2$
Elements m1 to m2 of irank contain a value outside the range m1 to m2.
${\mathbf{ifail}}=3$
Elements m1 to m2 of irank contain a repeated value.
${\mathbf{ifail}}=-99$
An unexpected error has been triggered by this routine. Please contact NAG.
${\mathbf{ifail}}=-399$
Your licence key may have expired or may not have been installed correctly.
${\mathbf{ifail}}=-999$
Dynamic memory allocation failed.
If ${\mathbf{ifail}}={\mathbf{2}}$ or ${\mathbf{3}}$, elements m1 to m2 of irank do not contain a permutation of the integers m1 to m2. On exit, the contents of rv may be corrupted. To check the validity of irank without the risk of corrupting rv, use nag_sort_permute_check (m01zb).

Not applicable.

## Further Comments

The average time taken by the function is approximately proportional to $n$, where $n={\mathbf{m2}}-{\mathbf{m1}}+1$.

## Example

This example reads a matrix of real numbers and rearranges its rows so that the elements of the $k$th column are in ascending order. To do this, the program first calls nag_sort_realvec_rank (m01da) to rank the elements of the $k$th column, and then calls nag_sort_realvec_rank_rearrange (m01ea) to rearrange each column into the order specified by the ranks. The value of $k$ is read from the datafile.
```function m01ea_example

fprintf('m01ea example results\n\n');

k  = int64(1);

rm = [6 5 4;
5 2 1;
2 4 9;
4 9 6;
4 9 5;
4 1 2;
3 4 1;
2 4 6;
1 6 4;
9 3 2;
6 2 5;
4 9 6];

m1 = int64(1);
m2 = int64(size(rm,1));
n  = int64(size(rm,2));

% Rank by column k
order = 'Ascending';
[irank, ifail] = m01da(rm(:,k), m1, order);

% Sort columns by ranking
for j = 1:n
[rm(:,j), irank, ifail] = m01ea(rm(:,j), m1, irank);
end

fprintf('Matrix sorted on column %2d\n', k);
for i = m1:m2
fprintf('%7.1f',rm(i,:));
fprintf('\n');
end

```
```m01ea example results

Matrix sorted on column  1
1.0    6.0    4.0
2.0    4.0    9.0
2.0    4.0    6.0
3.0    4.0    1.0
4.0    9.0    6.0
4.0    9.0    5.0
4.0    1.0    2.0
4.0    9.0    6.0
5.0    2.0    1.0
6.0    5.0    4.0
6.0    2.0    5.0
9.0    3.0    2.0
```

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