Formatted I/O

Analogy categoryAnalogies

Edit Descriptors and the PRINT/WRITE Statement

Fortran's formatted I/O uses edit descriptors packed inside a format string to control exactly how each value is rendered: I5 prints an integer right-justified in a 5-character field, F8.2 prints a real number in fixed-point notation with 8 total characters and 2 decimal places, and A10 prints a character string in a 10-character field. A statement like WRITE(*, '(A, I5, F8.2)') 'Result: ', count, total applies each descriptor to the corresponding output item in order, giving column-aligned output that list-directed WRITE(*,*) cannot guarantee.

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Cricket analogy: A scoreboard that always prints the batsman's score in a fixed 3-digit field so single, double, and triple-digit scores all align in a column mirrors I3 forcing consistent field width regardless of the value's size.

Common Numeric Descriptors

Beyond I and F, Fortran offers E for scientific notation (E12.4 gives something like 0.1234E+05), ES for scientific notation with the mantissa normalized to a single leading nonzero digit, G as a general descriptor that picks a sensible representation automatically, and X for inserting blank spaces between fields, as in WRITE(*, '(I3, 2X, F6.2)'). Repeat counts and grouping are also supported: 3(I4, 1X) repeats that pair of descriptors three times, and the slash / descriptor forces output to continue on a new line within a single WRITE statement.

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Cricket analogy: Printing a bowling figure like 4-32 with two spaces of padding before the economy rate, using I2,2X,F5.2, mirrors the X descriptor inserting deliberate blank space between output fields.

Reading Formatted Input

The same edit descriptors that control output also control input parsing with READ, so READ(unit, '(I4, F6.2)') code, price expects exactly four columns for an integer followed by six columns for a fixed-point real, regardless of embedded spaces. This fixed-column reading is essential for parsing legacy data files where fields are positioned by column rather than delimited by commas or spaces, but it means a single misaligned column in the input file silently reads garbage or triggers a runtime format error rather than gracefully skipping whitespace the way list-directed READ(*,*) would.

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Cricket analogy: Reading a paper scoresheet where the over number always occupies columns 1-3 and the runs always occupy columns 4-6 mirrors fixed-column formatted READ expecting data at exact character positions.

Reusable FORMAT Statements

Instead of inlining a format string in every WRITE, a labeled FORMAT statement can be defined once and referenced by its label: 100 FORMAT(A10, I5, F8.2) followed elsewhere by WRITE(*, 100) name, id, salary. Modern style favors character-parameter format strings instead of numeric labels, such as CHARACTER(LEN=*), PARAMETER :: FMT1 = '(A10, I5, F8.2)' and WRITE(*, FMT1) ..., which avoids scattering unlabeled magic numbers through a program and lets one format definition be reused consistently across many WRITE statements that must line up in the same table.

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Cricket analogy: Defining one reusable scoreboard template used for every innings display, rather than redesigning the layout each time, mirrors a labeled FORMAT statement reused across multiple WRITE statements.

fortran

program formatted_io_demo
  implicit none
  character(len=*), parameter :: hdr_fmt = '(A10, A8, A10)'
  character(len=*), parameter :: row_fmt = '(A10, I8, F10.2)'
  character(len=10) :: name
  integer :: id
  real :: salary

  write(*, hdr_fmt) 'Name', 'ID', 'Salary'

  name = 'Alice'
  id = 1042
  salary = 68500.5
  write(*, row_fmt) name, id, salary

  name = 'Bob'
  id = 1108
  salary = 5321.25
  write(*, row_fmt) name, id, salary

  ! Reading fixed-column data: columns 1-4 integer, 5-10 real
  block
    integer :: code
    real :: price
    character(len=10) :: line = '1042 68.50'
    read(line, '(I4, 1X, F5.2)') code, price
    print *, 'parsed code =', code, ' price =', price
  end block
end program formatted_io_demo

If a numeric value is too large to fit its field width (for example printing 123456 with an I5 descriptor), Fortran does not silently truncate -- it fills the entire field with asterisks (*****) to signal an overflow. Always size numeric fields generously enough for the largest expected value.

Edit descriptors (I, F, E, ES, A, X) precisely control field width and representation for each output item.
F8.2 means a fixed-point field 8 characters wide with 2 digits after the decimal point.
Repeat counts like 3(I4,1X) and the slash (/) descriptor control repetition and line breaks within one WRITE.
READ with a format string parses input by fixed column position, unlike flexible list-directed READ(*,*).
Reusable FORMAT labels or character-parameter format strings avoid duplicating format definitions.
Overflowing a numeric field produces a row of asterisks rather than silent truncation or wraparound.
Formatted I/O is essential for producing aligned tables and parsing fixed-width legacy data files.

Practice what you learned

1. What does the edit descriptor F8.2 specify?

2. What happens if a value is too large to fit in its specified field width?

3. How does formatted READ differ from list-directed READ(*,*)?

4. What does the X edit descriptor do?

5. Why prefer a character-parameter format string over a numeric FORMAT label in modern code?

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#Programming#FortranStudyNotes#FormattedIO#Formatted#Edit#Descriptors#PRINT#StudyNotes#SkillVeris#ExamPrep

Edit Descriptors and the PRINT/WRITE Statement

Common Numeric Descriptors

Reading Formatted Input

Reusable FORMAT Statements

Practice what you learned

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