String

Definition

Minimal yet powerful variable-length string type with modern Fortran features. This module implements a lightweight string derived type that behaves like a true variable-length character string while remaining fully compatible with intrinsic Fortran character operations.

Features:

• Automatic memory management via allocatable character(:)
• Overloaded assignment (=) between string and character(*)
• Overloaded operators: // (concatenation), == (equality), .contains. (membership)
• Generic interfaces for len, len_trim, trim
• Full support for formatted I/O (write, print)
• Helper routines for parsing Fortran source (line continuation, upper/lower case conversion, etc.)

The design is intentionally minimal — it provides only what's necessary for robust string handling in scientific and preprocessing applications, avoiding the bloat of larger string libraries while remaining fast and standards-compliant.

Note

All procedures are pure or elemental when possible for maximum performance and usability in array contexts.

Examples

Basic Usage

type(string) :: s, t
character(:), allocatable :: line
 
s = 'Hello'              ! Assignment from literal
t = s // ' World!'       ! Concatenation
print *, t%chars         ! Output: Hello World!
 
if (s == 'Hello') then
     print *, 'Equal'
else
     print *, 'Case sensitive'
endif
 
print *, len(t)          ! -> 12
print *, len_trim(t)     ! -> 12
...

Array and Container Support

type(string) :: words(3)
logical      :: found
 
words = [string('apple'), string('banana'), string('cherry')]
found = words .contains. 'banana'     ! --> .true.
found = words .contains. string('date') ! --> .false.
...

Advanced: Source Code Processing

character(len=:), allocatable :: code_line
code_line = uppercase('program hello_world  ! comment')  ! --> 'PROGRAM HELLO_WORLD  ! comment'
...

Data Types
type	string
	Represents text as a sequence of ASCII code units. The derived type wraps an allocatable character array. More...
interface	len
	Return the length of a string. More...
interface	len_trim
	Return the trimmed length of a string. More...
interface	trim
	Return the trimmed string. More...
interface	operator(//)
	Concatenation operator. More...
interface	operator(.contains.)
	Check whether a string belongs to a list or not. More...
interface	index
	Index operator. More...

Methods

concat()

character(:) function, allocatable, public concat	(	character(*), intent(in)	str1,
		character(*), intent(in)	str2 )

Smart concatenation that removes continuation markers (&) and handles line-continuation rules.

Parameters

[in]	str1	first line
[in]	str2	second line

Returns

Concatenated string with proper continuation handling

Remarks

Definition at line 548 of file string.f90.

head()

character function, public head

(

character(*), intent(in)

str

)

Returns the first non-blank character of a string.

Parameters

[in]

str

input string

Returns

First character (space if empty)

Remarks

Definition at line 515 of file string.f90.

lowercase()

pure character(len_trim(str)) function, public lowercase

(

character(*), intent(in)

str

)

Convert string to lower case (respects contents of quotes).

Parameters

[in]

str

input string

Returns

Lower-case version of the string

Examples

character(*), parameter :: input = 'TEST'
character(:), allocatable :: output
output = lowercase(input)
if (output == 'test') print*, 'OK'

Remarks

Definition at line 639 of file string.f90.

previous()

character(1) function, public previous	(	character(*), intent(in)	line,
		integer, intent(inout)	pos )

Returns the previous non-blank character before position pos (updates pos).

Parameters

[in]	line	input line
[in,out]	pos	current position (moved backward)

Returns

Previous non-blank character

Remarks

Definition at line 698 of file string.f90.

starts_with()

logical function, public starts_with	(	character(*), intent(in)	str,
		character(*), intent(in)	arg1,
		integer, intent(out), optional	idx )

Checks if a string starts with a given prefix Returns .true. if the string str (after trimming leading/trailing whitespace) begins exactly with the substring arg1. The function uses index() after trimming both strings with trim(adjustl()).

Parameters

[in]	str	The string to be tested
[in]	arg1	The prefix to look for at the beginning of str
[out]	idx	(optional) If present, receives the starting position of arg1 in the trimmed string (will be 1 if the function returns .true., otherwise >1 or 0)

Returns

.true. if str starts with arg1 (after trimming), .false. otherwise

Note

• Leading and trailing whitespace of both str and arg1 is ignored
• Comparison is case-sensitive
• Empty arg1 will always return .true. (any string starts with empty string)

Warning The returned index (when requested) is the position after trimming of the input string, not in the original untrimmed string.

Examples

character(80) :: line = '   hello world  '
logical :: ok
integer :: pos
 
ok = starts_with(line, 'hello')               ! → .true.
ok = starts_with(line, 'hello', pos)          ! → .true. and pos = 1
ok = starts_with(line, 'world')               ! → .false.
ok = starts_with('  test123  ', 'test')       ! → .true.
...

Remarks

Definition at line 497 of file string.f90.

tail()

character function, public tail

(

character(*), intent(in)

str

)

Returns the last non-blank character of a string.

Parameters

[in]

str

input string

Returns

Last character (space if empty)

Remarks

Definition at line 530 of file string.f90.

uppercase()

pure character(len_trim(str)) function, public uppercase

(

character(*), intent(in)

str

)

Convert string to upper case (respects contents of quotes).

Parameters

[in]

str

input string

Returns

Upper-case version of the string

Examples

character(*), parameter :: input = 'test'
character(:), allocatable :: output
output = uppercase(input)
if (output == 'TEST') print*, 'OK'

Remarks

Definition at line 595 of file string.f90.

writechk()

subroutine, public writechk	(	integer, intent(in)	unit,
		character(*), intent(in)	str )

Write a long line split into chunks of size CHKSIZE with continuation (&).

Parameters

[in]	unit	logical unit
[in]	str	string to write

Remarks

Definition at line 675 of file string.f90.