How to change the output file name with the same input file name but with a different extension ?

In line 1, I have read all files with .nc extension. And in the last fourth line, I want to save the output file with the same name but with different extension. For eg. If the input file name is 'BOB_LEHAR_CS_2013112718_TRMM.nc' then i want to save the output file as 'BOB_LEHAR_CS_2013112718_TRMM.grd' How to do this. Please help me.

   File= dir('*.nc');
    for j =1:2
        dx= 10
    radius= 600
    lon=ncread(File(j).name,'lon');
    lat=ncread(File(j).name,'lat') ;
    rain=ncread(File(j).name,'rf');
    rain=rain(:,:,1);
    for i =1: radius/dx
        i=i*dx
        A=cyclonic(0,0,1,i,lat,lon,rain);
        B(:,i/dx)=A(:,4);
    end
    B1=B(1:360,:);

    fpo=fopen(sprintf('file%i.grd',j),'wb');
    fwrite(fpo,B1,'float32');
    fclose(fpo);

    end

Answer 1

[Stephen]

Use fileparts to get the filename without the extension:

>> old = 'BOB_LEHAR_CS_2013112718_TRMM.nc';
>> [~,name] = fileparts(old);
>> new = sprintf('%s.grd',name)
new = BOB_LEHAR_CS_2013112718_TRMM.grd

Answer 2

[José-Luis]

 old = 'BOB_LEHAR_CS_2013112718_TRMM.nc';
 new = regexprep(old, '\.[^.]+$', '.grd')