Data commit

Task/Determine-if-a-string-is-numeric/Liberty-BASIC/determine-if-a-string-is-numeric.basic

@@ -0,0 +1,56 @@
+DATA "PI", "0123", "-0123", "12.30", "-12.30", "123!", "0"
+DATA "0.0", ".123", "-.123", "12E3", "12E-3", "12+3", "end"
+
+
+while n$ <> "end"
+    read n$
+    print n$, IsNumber(n$)
+wend
+end
+
+function IsNumber(string$)
+    on error goto [NotNumber]
+    string$ = trim$(string$)
+    'check for float overflow
+    n = val(string$)
+
+    'assume it is number and try to prove wrong
+    IsNumber = 1
+    for i = 1 to len(string$)
+        select case mid$(string$, i, 1)
+            case "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
+                HasNumeric = 1 'to check if there are any digits
+            case "e", "E"
+                '"e" must not occur more than once
+                'must not occur before digits
+
+                if HasE > 0 or HasNumeric = 0 then
+                    IsNumber = 0
+                    exit for
+                end if
+                HasE = i 'store position of "e"
+                HasNumeric = 0 'needs numbers after "e"
+            case "-", "+"
+                'must be either first character or immediately after "e"
+                '(HasE = 0 if no occurrences yet)
+                if HasE <> i-1 then
+                    IsNumber = 0
+                    exit for
+                end if
+            case "."
+                'must not have previous points and must not come after "e"
+                if HasE <> 0 or HasPoint <> 0 then
+                    IsNumber = 0
+                    exit for
+                end if
+                HasPoint = 1
+            case else
+                'no other characters allowed
+                IsNumber = 0
+                exit for
+        end select
+    next i
+    'must have digits
+    if HasNumeric = 0 then IsNumber = 0
+    [NotNumber]
+end function