Feature: loadtxt skiprows and max_rows (#652)

* stdlib_io: add skiprows to loadtxt

* Add test for skiprows in loadtxt

* stdlib_io: add check for skiprows >= 0

* stdlib_io: add max_rows to loadtxt

* Update src/stdlib_io.fypp

Co-authored-by: Ivan Pribec <[email protected]>

* loadtxt: Add test for max_rows

* loadtxt: treat negative skiprows as 0 (numpy like)

* stdlib_io: cut off skiprows

* stdlib_io: docs for cut off skiprows

* rename test in src/tests/io/test_loadtxt.f90

Co-authored-by: Ian Giestas Pauli <[email protected]>

* update specs docs of loadtxt

* Update doc/specs/stdlib_io.md

Co-authored-by: Milan Curcic <[email protected]>

* Update src/stdlib_io.fypp

Co-authored-by: Milan Curcic <[email protected]>

* loadtxt: determine number of columns from first line to be read

* number_of_columns: bugfix for missing int

Co-authored-by: Ivan Pribec <[email protected]>
Co-authored-by: Ian Giestas Pauli <[email protected]>
Co-authored-by: Milan Curcic <[email protected]>

Author: 4 people

doc/specs/stdlib_io.md

@@ -17,14 +17,18 @@ Loads a rank-2 `array` from a text file.
 
 ### Syntax
 
-`call [[stdlib_io(module):loadtxt(interface)]](filename, array)`
+`call [[stdlib_io(module):loadtxt(interface)]](filename, array [, skiprows] [, max_rows])`
 
 ### Arguments
 
 `filename`: Shall be  a character expression containing the file name from which to load the rank-2 `array`.
 
 `array`: Shall be an allocatable rank-2 array of type `real`, `complex` or `integer`.
 
+`skiprows` (optional): Skip the first `skiprows` lines. If skipping more rows than present, a 0-sized array will be returned. The default is 0.
+
+`max_rows` (optional): Read `max_rows` lines of content after `skiprows` lines. A negative value results in reading all lines. A value of zero results in no lines to be read. The default value is -1.
+
 ### Return value
 
 Returns an allocated rank-2 `array` with the content of `filename`.
@@ -314,4 +318,4 @@ program demo_fmt_constants
     print FMT_COMPLEX_DP, c64               ! outputs: 1.0000000000000000E+002  0.0000000000000000E+000
 
 end program demo_fmt_constants
-```
+```

src/stdlib_io.fypp

@@ -81,7 +81,7 @@ module stdlib_io
 contains
 
   #:for k1, t1 in KINDS_TYPES
-    subroutine  loadtxt_${t1[0]}$${k1}$(filename, d)
+    subroutine  loadtxt_${t1[0]}$${k1}$(filename, d, skiprows, max_rows)
       !! version: experimental
       !!
       !! Loads a 2D array from a text file.
@@ -93,6 +93,13 @@ contains
       character(len=*), intent(in) :: filename
       !! The array 'd' will be automatically allocated with the correct dimensions
       ${t1}$, allocatable, intent(out) :: d(:,:)
+      !! Skip the first `skiprows` lines. If skipping more rows than present, a 0-sized array will be returned. The default is 0.
+      integer, intent(in), optional :: skiprows
+      !! Read `max_rows` lines of content after `skiprows` lines.
+      !! A negative value results in reading all lines.
+      !! A value of zero results in no lines to be read.
+      !! The default value is -1.
+      integer, intent(in), optional :: max_rows
       !!
       !! Example
       !! -------
@@ -111,21 +118,32 @@ contains
       !!     ...
       !!
       integer :: s
-      integer :: nrow, ncol, i
+      integer :: nrow, ncol, i, skiprows_, max_rows_
+
+      skiprows_ = max(optval(skiprows, 0), 0)
+      max_rows_ = optval(max_rows, -1)
 
       s = open(filename)
 
+      ! determine number or rows
+      nrow = number_of_rows(s)
+      skiprows_ = min(skiprows_, nrow)
+      if ( max_rows_ < 0 .or. max_rows_ > (nrow - skiprows_) ) max_rows_ = nrow - skiprows_
+
       ! determine number of columns
-      ncol = number_of_columns(s)
+      ncol = 0
+      if ( skiprows_ < nrow ) ncol = number_of_columns(s, skiprows=skiprows_)
       #:if 'complex' in t1
       ncol = ncol / 2
       #:endif
 
-      ! determine number or rows
-      nrow = number_of_rows(s)
+      allocate(d(max_rows_, ncol))
 
-      allocate(d(nrow, ncol))
-      do i = 1, nrow
+      do i = 1, skiprows_
+        read(s, *)
+      end do
+
+      do i = 1, max_rows_
         #:if 'real' in t1
           read(s, "(*"//FMT_REAL_${k1}$(1:len(FMT_REAL_${k1}$)-1)//",1x))") d(i, :)
         #:elif 'complex' in t1
@@ -179,17 +197,25 @@ contains
   #:endfor
 
 
-  integer function number_of_columns(s)
+  integer function number_of_columns(s, skiprows)
     !! version: experimental
     !!
     !! determine number of columns
     integer,intent(in) :: s
+    integer, intent(in), optional :: skiprows
 
-    integer :: ios
+    integer :: ios, skiprows_, i
     character :: c
     logical :: lastblank
 
+    skiprows_ = optval(skiprows, 0)
+
     rewind(s)
+
+    do i = 1, skiprows_
+      read(s, *)
+    end do
+
     number_of_columns = 0
     lastblank = .true.
     do

src/tests/io/test_loadtxt.f90

@@ -19,6 +19,7 @@ subroutine collect_loadtxt(testsuite)
             new_unittest("loadtxt_sp_huge", test_loadtxt_sp_huge), &
             new_unittest("loadtxt_sp_tiny", test_loadtxt_sp_tiny), &
             new_unittest("loadtxt_dp", test_loadtxt_dp), &
+            new_unittest("loadtxt_dp_max_skip", test_loadtxt_dp_max_skip), &
             new_unittest("loadtxt_dp_huge", test_loadtxt_dp_huge), &
             new_unittest("loadtxt_dp_tiny", test_loadtxt_dp_tiny), &
             new_unittest("loadtxt_complex", test_loadtxt_complex) &
@@ -134,6 +135,29 @@ subroutine test_loadtxt_dp(error)
     end subroutine test_loadtxt_dp
 
 
+    subroutine test_loadtxt_dp_max_skip(error)
+        !> Error handling
+        type(error_type), allocatable, intent(out) :: error
+        real(dp), allocatable :: input(:,:), expected(:,:)
+        integer :: n, m
+
+        allocate(input(10,10))
+
+        do m = 0, 5
+            do n = 1, 11
+                call random_number(input)
+                input = input - 0.5
+                call savetxt('test_dp_max_skip.txt', input)
+                call loadtxt('test_dp_max_skip.txt', expected, skiprows=m, max_rows=n)
+                call check(error, all(input(m+1:min(n+m,10),:) == expected))
+                deallocate(expected)
+                if (allocated(error)) return
+            end do
+        end do
+
+    end subroutine test_loadtxt_dp_max_skip
+
+
     subroutine test_loadtxt_dp_huge(error)
         !> Error handling
         type(error_type), allocatable, intent(out) :: error