generating a mex file problem with sum command

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Hi I am having a problem converting my MATLAB code to a mex file. I belive the error lies with the sum command , within matlab the sum command sums along one dimension and then removes the dimension but during the mex file generation the summation occurs and but the extra dimension is still there. does anyone know how i could rewrite the lines to get them working within the mex file? in particular the following line: BPvol_seg(:,:,:,seg)=sum(BP,4);

Here is the matlab function code:

function BPVOL=SAFT3DTDRECON(fullcscan,dx,dy,sampleRate,f,c,ReconRatio)
%input Recon parameters
x=0:dx:(size(fullcscan,1)-1)*dx;
y=0:dy:(size(fullcscan,2)-1)*dy;
z=0:(c/sampleRate):(size(fullcscan,3)-1)*(c/sampleRate);
dt=1/sampleRate;
dpx=x;
dpy=y;
%solve the 3D recon in depth segments given by the ReconRatio
ReconDepthSegment=(size(fullcscan,3))/ReconRatio;
section=1:ReconDepthSegment:(size(fullcscan,3));
%generate x,y, dp and f matrices to avoid for nested loop
y_mat=repmat(y,size(x,2),1, ReconDepthSegment,size(dpx,2),size(dpy,2));
x_mat=repmat(x',1,size(y,2),ReconDepthSegment,size(dpx,2),size(dpy,2));
dpx_mat=permute(x_mat,[4,2,3,1,5]);
dpy_mat=permute(y_mat,[5,2,3,4,1]);
delpx_mat= (x_mat-dpx_mat).^2;
delpy_mat= (y_mat-dpy_mat).^2;
%clear y_mat x_mat dpx_mat dpy_mat
%preallocate
Td_d=zeros(size(x,2),size(y,2),ReconDepthSegment,size(dpx,2),size(dpy,2));
BP=zeros(size(x,2),size(y,2),ReconDepthSegment,size(dpx,2),size(dpy,2));
BPvol_seg=zeros(size(x,2),size(y,2),ReconDepthSegment,ReconRatio);
for seg=1:ReconRatio
    seg
    z_seg=z(section(seg):(ReconDepthSegment+section(seg)-1));
    z_mat=repmat(z_seg,size(x,2),1,size(y,2),size(dpx,2),size(dpy,2));
    z_mat=permute(z_mat,[1,3,2,4,5]);
    Td_d= round((f+sqrt((z_mat-f).^2+delpy_mat+delpx_mat)/c)/dt)*dt;   %discrete Time delay matrix
    for i=1:1:(size(fullcscan,1))
        for j=1:1:(size(fullcscan,2))
            Ascan=fullcscan(i,j,:);
            index=round(Td_d(:,:,:,i,j)/dt+1);
            for m=1:1:(size(index,1))
                for n=1:1:(size(index,2))
                    for o=1:1:(size(index,3))
                        if index(m,n,o)<=length(Ascan)
                            BP(m,n,o,i,j)=Ascan(index(i,j,o));
                        else 
                            BP(m,n,o,i,j)=0;
                        end
                    end
                end
            end
        end
    end
    %sum all BP frames to generate Backprojection volume
    
    BP=(sum(BP,5));
    BPvol_seg(:,:,:,seg)=sum(BP,4);
end
BPVOL=reshape(BPvol_seg,size(BPvol_seg,1),size(BPvol_seg,2),size(BPvol_seg,3)*size(BPvol_seg,4));