首先將陣列排序

如果元素數量是奇數，取最中間那個元素；

如果元素數量是偶數，取最中間兩個元素的算術平均值。

>>editmedian

functiony=median(x,dim)

%MEDIANMedianvalue.

%Forvectors,MEDIAN(X)isthemedianvalueoftheelementsinX.

%Formatrices,MEDIAN(X)isarowvectorcontainingthemedian

%valueofeachcolumn.ForN-Darrays,MEDIAN(X)isthemedian

%valueoftheelementsalongthefirstnon-singletondimension

%ofX.

%

%MEDIAN(X,DIM)takesthemedianalongthedimensionDIMofX.

%

%Example:IfX=[012

%345]

%

%thenmedian(X,1)is[1.52.53.5]andmedian(X,2)is[1

%4]

%

%ClasssupportforinputX:

%float:double,single

%

%SeealsoMEAN,STD,MIN,MAX,VAR,COV,MODE.

%Copyright1984-2005TheMathWorks,Inc.

%$Revision:5.15.4.6$$Date:2005/05/3116:30:47$

%Calculationmethodforevenlists:b-(b-a)/2.

%Thismethodreducesthelikelihoodofroundingerrors.

nInputs=nargin;

ifisempty(x)

ifnInputs==1

%Theoutputsizefor[]isaspecialcasewhenDIMisnotgiven.

ifisequal(x,[]),y=nan(1,class(x));return;end

%Determinefirstnonsingletondimension

dim=find(size(x)~=1,1);

end

s=size(x);

ifdim<=length(s)

s(dim)=1;%Setsizeto1alongdimension

end

y=nan(s,class(x));

elseifnInputs==1&&isvector(x)

%Ifinputisavector,calculatesinglevalueofoutput.

x=sort(x);

nCompare=numel(x);

half=floor(nCompare/2);

y=x(half+1);

if2*half==nCompare%Averageifevennumberofelements

y=meanof(x(half),y);

end

ifisnan(x(nCompare))%ChecklastindexforNaN

y=nan(class(x));

end

else

ifnInputs==1%Determinefirstnonsingletondimension

dim=find(size(x)~=1,1);

end

s=size(x);

ifdim>length(s)%Ifdimensionistoohigh,justreturninput.

y=x;

return

end

%Sortalonggivendimension

x=sort(x,dim);

nCompare=s(dim);%Numberofelementsusedtogenerateamedian

half=floor(nCompare/2);%Midwaypoint,usedformediancalculation

ifdim==1

%Ifcalculatingalongcolumns,usevectorizedmethodwithcolumn

%indexing.Reshapeatendtoappropriatedimension.

y=x(half+1,:);

if2*half==nCompare

y=meanof(x(half,:),y);

end

y(isnan(x(nCompare,:)))=NaN;%ChecklastindexforNaN

elseifdim==2&&length(s)==2

%Ifcalculatingalongrows,usevectorizedmethodonlywhenpossible.

%Thisrequirestheinputtobe2-dimensional.Reshapeatend.

y=x(:,half+1);

if2*half==nCompare

y=meanof(x(:,half),y);

end

y(isnan(x(:,nCompare)))=NaN;%ChecklastindexforNaN

else

%Inallothercases,uselinearindexingtodetermineexactlocation

%ofmedians.Uselinearindicestoextractmedians,thenreshapeat

%endtoappropriatesize.

cumSize=cumprod(s);

total=cumSize(end);%EquivalenttoNUMEL(x)

numMedians=total/nCompare;

numConseq=cumSize(dim-1);%Numberofconsecutiveindices

increment=cumSize(dim);%Gapbetweenrunsofindices

ixMedians=1;

y=repmat(x(1),numMedians,1);%Preallocateappropriatetype

%NestedFORlooptracksdownmediansbytheirindices.

forseqIndex=1:increment:total

forconsIndex=half*numConseq:(half+1)*numConseq-1

absIndex=seqIndex+consIndex;

y(ixMedians)=x(absIndex);

ixMedians=ixMedians+1;

end

end

%Averageinsecondvalueifniseven

if2*half==nCompare

ixMedians=1;

forseqIndex=1:increment:total

forconsIndex=(half-1)*numConseq:half*numConseq-1

absIndex=seqIndex+consIndex;

y(ixMedians)=meanof(x(absIndex),y(ixMedians));

ixMedians=ixMedians+1;

end

end

end

%ChecklastindicesforNaN

ixMedians=1;

forseqIndex=1:increment:total

forconsIndex=(nCompare-1)*numConseq:nCompare*numConseq-1

absIndex=seqIndex+consIndex;

ifisnan(x(absIndex))

y(ixMedians)=NaN;

end

ixMedians=ixMedians+1;

end

end

end

%Nowreshapeoutput.

s(dim)=1;

y=reshape(y,s);

end

%============================

functionc=meanof(a,b)

%MEANOFthemeanofAandBwithB>A

%MEANOFcalculatesthemeanofAandB.Itusesdifferentformula

%inordertoavoidoverflowinfloatingpointarithmetic.

c=a+(b-a)/2;

k=(sign(a)~=sign(b))|isinf(a)|isinf(b);

c(k)=(a(k)+b(k))/2;