io: use a default delimiter for loadtxt and savetxt functions

Author: zoziha

src/stdlib_io.fypp

@@ -143,13 +143,14 @@ contains
       !!     11 12 13
       !!     ...
       !!
+      character(len=1), parameter :: delimiter_default = " "
       integer :: s
       integer :: nrow, ncol, i, ios, skiprows_, max_rows_
       character(len=1024) :: iomsg, msgout
 
       skiprows_ = max(optval(skiprows, 0), 0)
       max_rows_ = optval(max_rows, -1)
-      delimiter_ = optval(delimiter, " ")
+      delimiter_ = optval(delimiter, delimiter_default)
 
       s = open(filename)
 
@@ -240,14 +241,14 @@ contains
       !! call savetxt("log.txt", data)
       !!```
       !!
-      
+      character(len=1), parameter :: delimiter_default = " "
       integer :: s, i, ios
       character(len=1) :: delimiter_
       character(len=3) :: delim_str
       character(len=:), allocatable :: fmt_
       character(len=1024) :: iomsg, msgout
 
-      delimiter_ = optval(delimiter, " ")
+      delimiter_ = optval(delimiter, delimiter_default)
       delim_str = "'"//delimiter_//"'"
       #:if 'real' in t1
         fmt_ = "(*"//FMT_REAL_${k1}$(1:len(FMT_REAL_${k1}$)-1)//",:,"//delim_str//"))"
@@ -288,14 +289,15 @@ contains
     integer, intent(in), optional :: skiprows
     character(len=1), intent(in), optional :: delimiter
 
+    character(len=1), parameter :: delimiter_default = " "
     integer :: ios, skiprows_, i
     character :: c
     character(len=:), allocatable :: line
     character(len=1) :: delimiter_
     logical :: last_delim
 
     skiprows_ = optval(skiprows, 0)
-    delimiter_ = optval(delimiter, " ")
+    delimiter_ = optval(delimiter, delimiter_default)
 
     rewind(s)
 
@@ -309,7 +311,7 @@ contains
     if (ios/=0 .or. .not.allocated(line)) return
 
     last_delim = .true.
-    if (delimiter_ == " ") then
+    if (delimiter_ == delimiter_default) then
       do i = 1,len(line)
         c = line(i:i)
         if (last_delim .and. .not. is_blank(c)) number_of_columns = number_of_columns + 1