find the zero crossings in a signal in matlab

%############################################
%# Function to find position of the positive going zero    #
%# crossing of supplied data                #
%############################################
function offset = zero_cross(volts,offset)

swt             = 0;

while ( swt == 0 )
    
    if (offset+1 >= size(volts,1))
        swt = 1;
    else
        offset = offset + 1;
    end
    
    if (offset+1 >size(volts,1) )
        swt = 1;
    end
    
    if( volts(offset)<=0)
        if( volts(offset+1)>=0)
            swt = 1;
        end;
    end
    
end