removed redundant outer loop, improved documentation of slice function

Author: aman-godara

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