by default, if a number has decimal . after it, then Mathematica will do computation using machine Precision, which on my PC (intel hardware) running windows 7 64 bit is double Precision.

I’d like to get the computation also but using single and quad precision, to match the following small Fortran program output.

Is there a recommend way to do this? Here is the fortran program and its output

PROGRAM foo IMPLICIT NONE INTEGER:: i  INTEGER, PARAMETER ::                              &     sp = kind(1.0),                                &     dp = selected_real_kind(2*precision(1.0_sp)),  &     qp = selected_real_kind(2*precision(1.0_dp))  REAL(kind=sp) ::  sum1,x1 REAL(kind=dp) ::  sum2,x2 REAL(kind=qp) ::  sum3,x3  sum1=0.0_sp; sum2=0.0_dp; sum3=0.0_qp x1 = 0.00001_sp x2 = 0.00001_dp x3 = 0.00001_qp  DO i=1,10**5    sum1 = sum1 + x1    sum2 = sum2 + x2    sum3 = sum3 + x3 END DO  PRINT *,sum1; PRINT *,sum2; PRINT *,sum3; END 

Compiled using gfortran foo.f90 gives

ex1>./a.out     1.00099015      ---> SINGLE   0.99999999999808376      --> DOUBLE   0.999999999999999999999999999998395508  ----> QUAD 

The first line is single, the second is double (which Mathematica matches exactly) and the third is quad.

Here is the Mathematica basic code which gives output that matches the double Precision:

x1=0.00001; sum1=0.0; Do[sum1=sum1+x1,{i,1,10^5}] InputForm[sum1] 

Which gives

 0.9999999999980838 

I tried doing

 x1=0.00001`32;  sum1=0.0`32; 

To see if will something close to the quad result, but it had not effect on final output. I still get 0.9999999999980838

How to change the above Mathematica code to make it give the single and quad Precision as shown by Fortran?