Scilab Function

corr - correlation, covariance

### Calling Sequence

  `[cov,Mean]=corr(x,[y],nlags)`
  `[cov,Mean]=corr('fft',xmacro,[ymacro],n,sect)`
  ``
  `[w,xu]=corr('updt',x1,[y1],w0)`
  `[w,xu]=corr('updt',x2,[y2],w,xu)`
  `...`
  `[wk]=corr('updt',xk,[yk],w,xu)`

### Parameters

- `x` : a real vector
- `y` : a real vector, default value x.
- `nlags` : integer, number of correlation coefficients desired.
- `xmacro` : a scilab external (see below).
- `ymacro` : a scilab external (see below), default value xmacro
- `n` : an integer, total size of the sequence (see below).
- `sect` : size of sections of the sequence (see below).
- `xi` : a real vector
- `yi` : a real vector,default value xi.
- `cov` : real vector, the correlation coefficients
- `Mean` : real number or vector, the mean of x and if given y

### Description

Computes

                    n - m
                     ====
                     \                                                 1
            cov(m) =  >        (x(k)  - xmean) (y(m+k)      - ymean) * ---
                     /                                                  n
                     ====
                     k = 1

for m=0,..,`nlag-1` and two vectors `x=[x(1),..,x(n)]` `y=[y(1),..,y(n)]`

Note that if x and y sequences are differents corr(x,y,...) is different with corr(y,x,...)

Short sequences:

returns the first nlags correlation coefficients and Mean = `mean(x)` (mean of `[x,y]` if `y` is an argument). The sequence `x` (resp. `y`) is assumed real, and `x` and `y` are of same dimension n.

Long sequences:

Here `xmacro` is either

- - a function of type `[xx]=xmacro(sect,istart)` which returns a vector `xx` of dimension `nsect` containing the part of the sequence with indices from `istart` to `istart+sect-1`.
- - a fortran subroutine which performs the same calculation. (See the source code of `dgetx` for an example). `n` = total size of the sequence. `sect` = size of sections of the sequence. `sect` must be a power of 2. `cov` has dimension `sect`. Calculation is performed by FFT.

"Updating method":

    [w,xu]=corr('updt',x1,[y1],w0)
    [w,xu]=corr('updt',x2,[y2],w,xu)
     ...
    wk=corr('updt',xk,[yk],w,xu)

With this calling sequence the calculation is updated at each call to `corr`.

    w0 = 0*ones(1,2*nlags);
    nlags = power of 2.

`x1,x2,...` are parts of `x` such that `x=[x1,x2,...]` and sizes of `xi` a power of 2. To get `nlags` coefficients a final fft must be performed `c=fft(w,1)/n`; `cov=c(1nlags)` (`n` is the size of `x (y)`). Caution: this calling sequence assumes that `xmean = ymean = 0`.

### Examples

    x=%pi/10:%pi/10:102.4*%pi;
    rand('seed');rand('normal');
    y=[.8*sin(x)+.8*sin(2*x)+rand(x);.8*sin(x)+.8*sin(1.99*x)+rand(x)];
    c=[];
    for j=1:2,for k=1:2,c=[c;corr(y(k,:),y(j,:),64)];end;end;
    c=matrix(c,2,128);cov=[];
    for j=1:64,cov=[cov;c(:,(j-1)*2+1:2*j)];end;
    rand('unif')
    //
    rand('normal');x=rand(1,256);y=-x;
    deff('[z]=xx(inc,is)','z=x(is:is+inc-1)');
    deff('[z]=yy(inc,is)','z=y(is:is+inc-1)');
    [c,mxy]=corr(x,y,32);
    x=x-mxy(1)*ones(x);y=y-mxy(2)*ones(y);  //centring
    c1=corr(x,y,32);c2=corr(x,32);
    norm(c1+c2,1)
    [c3,m3]=corr('fft',xx,yy,256,32);
    norm(c1-c3,1)
    [c4,m4]=corr('fft',xx,256,32);
    norm(m3,1),norm(m4,1)
    norm(c3-c1,1),norm(c4-c2,1)
    x1=x(1:128);x2=x(129:256);
    y1=y(1:128);y2=y(129:256);
    w0=0*ones(1:64);   //32 coeffs
    [w1,xu]=corr('u',x1,y1,w0);w2=corr('u',x2,y2,w1,xu);
    zz=real(fft(w2,1))/256;c5=zz(1:32);
    norm(c5-c1,1)
    [w1,xu]=corr('u',x1,w0);w2=corr('u',x2,w1,xu);
    zz=real(fft(w2,1))/256;c6=zz(1:32);
    norm(c6-c2,1)
    rand('unif')
    // test for Fortran or C external
    //
    deff('[y]=xmacro(sec,ist)','y=sin(ist:(ist+sec-1))');
    x=xmacro(100,1);
    [cc1,mm1]=corr(x,2^3);
    [cc,mm]=corr('fft',xmacro,100,2^3);
    [cc2,mm2]=corr('fft','corexx',100,2^3);
    [maxi(abs(cc-cc1)),maxi(abs(mm-mm1)),maxi(abs(cc-cc2)),maxi(abs(mm-mm2))]

    deff('[y]=ymacro(sec,ist)','y=cos(ist:(ist+sec-1))');
    y=ymacro(100,1);
    [cc1,mm1]=corr(x,y,2^3);
    [cc,mm]=corr('fft',xmacro,ymacro,100,2^3);
    [cc2,mm2]=corr('fft','corexx','corexy',100,2^3);
    [maxi(abs(cc-cc1)),maxi(abs(mm-mm1)),maxi(abs(cc-cc2)),maxi(abs(mm-mm2))]

### See Also

`fft`