# 038. Use unformatted stream of Fortran 2003 for large-scale I/O topic: {ref}`input-and-output` Consider using unformatted stream I/O introduced in Fortran 2003 for large data sets. Formatted I/O converts numerical data to and from ASCII and is much slower than unformatted. This program times the writing and reading of $10^6$ double precision floats[^size] using formatted and unformatted stream I/O. [^size]: float64; $10^6 \times 64\,\text{bits} = 8\,\text{MB}$ ```{literalinclude} ../../src/stream-io.f90 :language: fortran :caption: stream-io.f90 | | ₀ |

``` ```{code-block} text :caption: Output[^gfortran-version] n = 1000000 irate = 1000000000 5.5511151231257827E-017 4.0471064854941119E-007 0.99999926540348483 4.0471064850000000E-007 0.99999926540348483 0.0000000000000000 4.0471064854941119E-007 0.99999926540348483 4.0471064854941119E-007 0.99999926540348483 formatted formatted unformatted unformatted write read write read 0.585 0.445 0.936E-02 0.137E-02 enhancement enhancement write read 62.5 325. ``` [^gfortran-version]: Compiled using `GNU Fortran (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0` with no flags The degree of enhancement depends on $n$ and your hardware, but it should be considerable. ```{note} The Tweet, using $n=10^7$, reported that unformatted stream write and read were, respectively, 25 and 260 times faster than formatted. ``` ```{note} Using 64-bit integer arguments when calling `system_clock` allows for greater timing precision[^sys-clock]. For 32-bit integers (default) or smaller, we only get millisecond precision, which was consistently not sufficient to measure the unformatted write time on my laptop. ``` [^sys-clock]: ---

Use unformatted stream of Fortran 2003 for large-scale I/O. pic.twitter.com/I2PHa3YiaA
— FortranTip (@fortrantip) December 21, 2021