🐐 String indexing and slicing

README.md

@@ -70,6 +70,21 @@ Any expression can appear inside `{}`. Values are coerced to strings the same wa
 
 Single-quoted strings do **not** interpolate — `'{nomen}'` is the literal text `{nomen}`. Use `{{` and `}}` for literal braces in double-quoted strings: `"use {{braces}}"` → `use {braces}`.
 
+#### String Indexing and Slicing
+
+Strings support the same indexing and slicing syntax as arrays. Indexing is 1-based and returns a single-character string:
+
+```
+"SALVTE"[I]             @> "S"
+"SALVTE"[III]           @> "L"
+```
+
+Slicing uses `VSQVE` with inclusive bounds, returning a substring:
+
+```
+"SALVTE"[II VSQVE IV]   @> "ALV"
+```
+
 Integer modulo is `RELIQVVM`: `VII RELIQVVM III` evaluates to `I`. Under the `FRACTIO` module it returns a fraction, so `IIIS RELIQVVM IS` is `S` (i.e. 1/2).
 
 ### Integers

centvrion/ast_nodes.py

@@ -833,6 +833,17 @@ class ArrayIndex(Node):
       if k not in d:
         raise CentvrionError(f"Key not found in dict")
       return vtable, d[k]
+    if isinstance(array, ValStr):
+      if isinstance(index, ValInt):
+        i = index.value()
+      elif isinstance(index, ValFrac) and index.value().denominator == 1:
+        i = index.value().numerator
+      else:
+        raise CentvrionError("String index must be a number")
+      s = array.value()
+      if i < 1 or i > len(s):
+        raise CentvrionError(f"Index {i} out of range for string of length {len(s)}")
+      return vtable, ValStr(s[i - 1])
     if not isinstance(array, ValList):
       raise CentvrionError("Cannot index a non-array value")
     if isinstance(index, ValInt):
@@ -877,6 +888,16 @@ class ArraySlice(Node):
     vtable, array = self.array.eval(vtable)
     vtable, from_val = self.from_index.eval(vtable)
     vtable, to_val = self.to_index.eval(vtable)
+    if isinstance(array, ValStr):
+      from_int = _to_index_int(from_val)
+      to_int = _to_index_int(to_val)
+      s = array.value()
+      if from_int < 1 or to_int > len(s) or from_int > to_int:
+        raise CentvrionError(
+          f"Slice [{from_int} VSQVE {to_int}] out of range"
+          f" for string of length {len(s)}"
+        )
+      return vtable, ValStr(s[from_int - 1 : to_int])
     if not isinstance(array, ValList):
       raise CentvrionError("Cannot slice a non-array value")
     from_int = _to_index_int(from_val)

centvrion/compiler/runtime/cent_runtime.c

@@ -721,6 +721,22 @@ void cent_list_push(CentValue *lst, CentValue v) {
 CentValue cent_list_index(CentValue lst, CentValue idx) {
   if (lst.type == CENT_DICT)
     return cent_dict_get(lst, idx);
+  if (lst.type == CENT_STR) {
+    long i;
+    if (idx.type == CENT_INT)
+      i = idx.ival;
+    else if (idx.type == CENT_FRAC && idx.fval.den == 1)
+      i = idx.fval.num;
+    else
+      cent_type_error("string index must be an integer");
+    long slen = (long)strlen(lst.sval);
+    if (i < 1 || i > slen)
+      cent_runtime_error("string index out of range");
+    char *ch = cent_arena_alloc(cent_arena, 2);
+    ch[0] = lst.sval[i - 1];
+    ch[1] = '\0';
+    return cent_str(ch);
+  }
   if (lst.type != CENT_LIST)
     cent_type_error("index requires a list or dict");
   long i;
@@ -736,6 +752,20 @@ CentValue cent_list_index(CentValue lst, CentValue idx) {
 }
 
 CentValue cent_list_slice(CentValue lst, CentValue lo, CentValue hi) {
+  if (lst.type == CENT_STR) {
+    if (lo.type != CENT_INT || hi.type != CENT_INT)
+      cent_type_error("slice indices must be integers");
+    long from = lo.ival;
+    long to = hi.ival;
+    long slen = (long)strlen(lst.sval);
+    if (from < 1 || to > slen || from > to)
+      cent_runtime_error("string slice out of range");
+    int len = (int)(to - from + 1);
+    char *buf = cent_arena_alloc(cent_arena, len + 1);
+    memcpy(buf, lst.sval + from - 1, len);
+    buf[len] = '\0';
+    return cent_str(buf);
+  }
   if (lst.type != CENT_LIST)
     cent_type_error("slice requires a list");
   if (lo.type != CENT_INT || hi.type != CENT_INT)

language/main.tex

@@ -98,7 +98,7 @@
   \item \textbf{module-name}: \\ Modules are flags given to the interpreter/compiler, to let it know you want to be using certain rules, functions, or features. Available modules: \texttt{FORS} (randomness), \texttt{FRACTIO} (fractions), \texttt{MAGNVM} (large integers), \texttt{SCRIPTA} (file I/O: \texttt{LEGE}, \texttt{SCRIBE}, \texttt{ADIVNGE}), \texttt{SVBNVLLA} (negative literals).
   \item \textbf{id}: \\ Variable. Can only consist of lowercase characters and underscores, but not the letters j, u, or w.
   \item \textbf{builtin}: \\ Builtin functions are uppercase latin words.
-  \item \textbf{string}: \\ Any text encased in \texttt{"} or \texttt{'} characters. Single-quoted strings are always literal.
+  \item \textbf{string}: \\ Any text encased in \texttt{"} or \texttt{'} characters. Single-quoted strings are always literal. Strings support 1-based indexing (\texttt{string[I]}) and inclusive slicing (\texttt{string[I VSQVE III]}), returning single-character strings and substrings respectively.
   \item \textbf{interpolated-string}: \\ A double-quoted string containing \texttt{\{}\textit{expression}\texttt{\}} segments. Each expression is evaluated and coerced to a string. Use \texttt{\{\{} and \texttt{\}\}} for literal braces.
   \item \textbf{numeral}: \\ Roman numerals consisting of the uppercase characters I, V, X, L, C, D, and M. Can also include underscore if the module MAGNVM.
   \item \textbf{bool}: \\ VERITAS or FALSITAS.

tests.py

@@ -639,6 +639,10 @@ error_tests = [
   ("I * \"hello\"", CentvrionError),                                   # multiplication with string
   ("\"hello\" MINVS \"world\"", CentvrionError),                       # comparison with strings
   ("I[I]", CentvrionError),                                           # indexing a non-array
+  ('"SALVTE"[VII]', CentvrionError),                                   # string index out of range
+  ('"SALVTE"[NVLLVS]', CentvrionError),                                # string index with non-integer
+  ('"SALVTE"[II VSQVE VII]', CentvrionError),                          # string slice out of range
+  ('"SALVTE"[III VSQVE II]', CentvrionError),                          # string slice from > to
   ("DESIGNA x VT I\nDESIGNA x[I] VT II", CentvrionError),             # index-assign to non-array
   ("SEMEN(I)", CentvrionError),                                        # requires FORS module
   ('CVM FORS\nSEMEN("abc")', CentvrionError),                          # SEMEN requires integer seed
@@ -1583,6 +1587,88 @@ class TestArraySlice(unittest.TestCase):
     run_test(self, source, nodes, value)
 
 
+# --- String indexing ---
+
+string_index_tests = [
+  # first character
+  ('"SALVTE"[I]',
+    Program([], [ExpressionStatement(ArrayIndex(String("SALVTE"), Numeral("I")))]),
+    ValStr("S")),
+  # last character
+  ('"SALVTE"[VI]',
+    Program([], [ExpressionStatement(ArrayIndex(String("SALVTE"), Numeral("VI")))]),
+    ValStr("E")),
+  # middle character
+  ('"SALVTE"[III]',
+    Program([], [ExpressionStatement(ArrayIndex(String("SALVTE"), Numeral("III")))]),
+    ValStr("L")),
+  # string index via variable
+  ('DESIGNA s VT "SALVTE"\ns[II]',
+    Program([], [
+      Designa(ID("s"), String("SALVTE")),
+      ExpressionStatement(ArrayIndex(ID("s"), Numeral("II"))),
+    ]),
+    ValStr("A")),
+  # expression as index
+  ('"SALVTE"[I + II]',
+    Program([], [ExpressionStatement(ArrayIndex(
+      String("SALVTE"),
+      BinOp(Numeral("I"), Numeral("II"), "SYMBOL_PLUS")))]),
+    ValStr("L")),
+]
+
+class TestStringIndex(unittest.TestCase):
+  @parameterized.expand(string_index_tests)
+  def test_string_index(self, source, nodes, value):
+    run_test(self, source, nodes, value)
+
+
+# --- String slicing ---
+
+string_slice_tests = [
+  # substring from middle
+  ('"SALVTE"[II VSQVE IV]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"), Numeral("II"), Numeral("IV")))]),
+    ValStr("ALV")),
+  # full string slice
+  ('"SALVTE"[I VSQVE VI]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"), Numeral("I"), Numeral("VI")))]),
+    ValStr("SALVTE")),
+  # single-char slice
+  ('"SALVTE"[III VSQVE III]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"), Numeral("III"), Numeral("III")))]),
+    ValStr("L")),
+  # slice on variable
+  ('DESIGNA s VT "SALVTE"\ns[II VSQVE IV]',
+    Program([], [
+      Designa(ID("s"), String("SALVTE")),
+      ExpressionStatement(ArraySlice(ID("s"), Numeral("II"), Numeral("IV"))),
+    ]),
+    ValStr("ALV")),
+  # chaining: slice then index
+  ('"SALVTE"[I VSQVE III][II]',
+    Program([], [ExpressionStatement(ArrayIndex(
+      ArraySlice(String("SALVTE"), Numeral("I"), Numeral("III")),
+      Numeral("II")))]),
+    ValStr("A")),
+  # expression as slice bounds
+  ('"SALVTE"[I + I VSQVE II + II]',
+    Program([], [ExpressionStatement(ArraySlice(
+      String("SALVTE"),
+      BinOp(Numeral("I"), Numeral("I"), "SYMBOL_PLUS"),
+      BinOp(Numeral("II"), Numeral("II"), "SYMBOL_PLUS")))]),
+    ValStr("ALV")),
+]
+
+class TestStringSlice(unittest.TestCase):
+  @parameterized.expand(string_slice_tests)
+  def test_string_slice(self, source, nodes, value):
+    run_test(self, source, nodes, value)
+
+
 # --- Comments ---
 
 comment_tests = [