removed redundant outer loop, improved documentation of slice function

doc/specs/stdlib_strings.md

@@ -201,8 +201,17 @@ end program demo
 
 Extracts the characters from the defined region of the input string. 
 Argument `first` and `last` defines the region for the function `slice` to operate. 
-Extraction starts from the index `first` and takes stride of length `stride`. 
-Argument `stride` cannot take the value 0.
+If the defined region is invalid (user provides atleast one invalid index), `first` and 
+`last` indexes are converted to first and last valid indexes in this defined region respectively, 
+if no valid index exists in this region an empty string is returned. 
+
+Extraction starts from `first` index and takes stride of length `stride`. 
+Extraction is active until `last` index is crossed. 
+Extraction starts only if `last` index is crossable from `first` index taking 
+stride of length `stride`. 
+`stride` can attain both negative or positive values but when the invalid value 
+0 is given, it is converted to 1. 
+Function automatically deduces the arguments which are not provided by the user.
 
 #### Syntax
 
@@ -248,8 +257,10 @@ program demo_slice
  ! char <-- "abcdefghij"
 
  print'(a)', slice("abcdefghij", 2, 6, 2) ! "bdf"
- print'(a)', slice(string, 2, 6, 2) ! "bdf"
  print'(a)', slice(char, 2, 6, 2) ! "bdf"
 
+ string = slice(string, 2, 6, 2)
+ ! string <-- "bdf"
+
 end program demo_slice
 ```

src/stdlib_strings.f90

@@ -320,55 +320,51 @@ pure function slice_char(string, first, last, stride) result(sliced_string)
  integer, intent(in), optional :: first, last, stride
  integer :: first_index, last_index, stride_vector, strides_taken, length_string, i, j
  character(len=:), allocatable :: sliced_string
-
  length_string = len(string)
- if (length_string > 0) then
- first_index = 1
- last_index = length_string
- stride_vector = 1
-
- if (present(stride)) then
- if (stride /= 0) then
- if (stride < 0) then
- first_index = length_string
- last_index = 1
- end if
- stride_vector = stride
+
+ first_index = 1
+ last_index = length_string
+ stride_vector = 1
+
+ if (present(stride)) then
+ if (stride /= 0) then
+ if (stride < 0) then
+ first_index = length_string
+ last_index = 1
  end if
- else
- if (present(first) .and. present(last)) then
- if (last < first) then
- stride_vector = -1
- end if
+ stride_vector = stride
+ end if
+ else
+ if (present(first) .and. present(last)) then
+ if (last < first) then
+ stride_vector = -1
  end if
  end if
+ end if
 
- if (present(first)) then
- first_index = first
- end if
- if (present(last)) then
- last_index = last
- end if
-
- if (stride_vector > 0) then
- first_index = max(first_index, 1)
- last_index = min(last_index, length_string)
- else
- first_index = min(first_index, length_string)
- last_index = max(last_index, 1)
- end if
-
- strides_taken = floor( real(last_index - first_index)/real(stride_vector) )
- allocate(character(len=max(0, strides_taken + 1)) :: sliced_string)
-
- j = 1
- do i = first_index, last_index, stride_vector
- sliced_string(j:j) = string(i:i)
- j = j + 1
- end do
+ if (present(first)) then
+ first_index = first
+ end if
+ if (present(last)) then
+ last_index = last
+ end if
+
+ if (stride_vector > 0) then
+ first_index = max(first_index, 1)
+ last_index = min(last_index, length_string)
  else
- sliced_string = ""
+ first_index = min(first_index, length_string)
+ last_index = max(last_index, 1)
  end if
+
+ strides_taken = floor( real(last_index - first_index)/real(stride_vector) )
+ allocate(character(len=max(0, strides_taken + 1)) :: sliced_string)
+
+ j = 1
+ do i = first_index, last_index, stride_vector
+ sliced_string(j:j) = string(i:i)
+ j = j + 1
+ end do
  end function slice_char