Use the optval function in the string routines

Each routine using the optional 'back' argument could be shortend to a
single line of code.

Author: Carl Burkert

src/stdlib_string_type.fypp

@@ -16,6 +16,7 @@ module stdlib_string_type
     use stdlib_ascii, only: to_lower_ => to_lower, to_upper_ => to_upper, &
        & to_title_ => to_title, to_sentence_ => to_sentence, reverse_ => reverse
     use stdlib_kinds, only : int8, int16, int32, int64, lk, c_bool
+    use stdlib_optval, only: optval
     implicit none
     private
 
@@ -569,11 +570,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = index(maybe(string), maybe(substring), back)
-        else
-          pos = index(maybe(string), maybe(substring), .false.)
-        end if
+        pos = index(maybe(string), maybe(substring), optval(back, .false.))
 
     end function index_string_string
 
@@ -585,11 +582,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = index(maybe(string), substring, back)
-        else
-          pos = index(maybe(string), substring, .false.)
-        end if
+        pos = index(maybe(string), substring, optval(back, .false.))
 
     end function index_string_char
 
@@ -601,11 +594,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = index(string, maybe(substring), back)
-        else
-          pos = index(string, maybe(substring), .false.)
-        end if
+        pos = index(string, maybe(substring), optval(back, .false.))
 
     end function index_char_string
 
@@ -619,11 +608,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = scan(maybe(string), maybe(set), back)
-        else
-          pos = scan(maybe(string), maybe(set), .false.)
-        end if
+        pos = scan(maybe(string), maybe(set), optval(back, .false.))
 
     end function scan_string_string
 
@@ -635,11 +620,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = scan(maybe(string), set, back)
-        else
-          pos = scan(maybe(string), set, .false.)
-        end if
+        pos = scan(maybe(string), set, optval(back, .false.))
 
     end function scan_string_char
 
@@ -651,11 +632,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = scan(string, maybe(set), back)
-        else
-          pos = scan(string, maybe(set), .false.)
-        end if
+        pos = scan(string, maybe(set), optval(back, .false.))
 
     end function scan_char_string
 
@@ -669,11 +646,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = verify(maybe(string), maybe(set), back)
-        else
-          pos = verify(maybe(string), maybe(set), .false.)
-        end if
+        pos = verify(maybe(string), maybe(set), optval(back, .false.))
 
     end function verify_string_string
 
@@ -686,11 +659,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = verify(maybe(string), set, back)
-        else
-          pos = verify(maybe(string), set, .false.)
-        end if
+        pos = verify(maybe(string), set, optval(back, .false.))
 
     end function verify_string_char
 
@@ -703,11 +672,7 @@ contains
         logical, intent(in), optional :: back
         integer :: pos
 
-        if (present(back)) then
-          pos = verify(string, maybe(set), back)
-        else
-          pos = verify(string, maybe(set), .false.)
-        end if
+        pos = verify(string, maybe(set), optval(back, .false.))
 
     end function verify_char_string