From 58149b5f65dc120050f3b3f8dce7dac3627290c9 Mon Sep 17 00:00:00 2001
From: NikolajDanger <nikolaj@ingemanngade.net>
Date: Tue, 31 Mar 2026 22:10:55 +0200
Subject: [PATCH] :goat: Array indexing

---
 README.md              |  9 +++++++++
 centvrion/ast_nodes.py | 18 ++++++++++++++++++
 centvrion/parser.py    |  5 +++++
 tests.py               | 16 +++++++++-------
 4 files changed, 41 insertions(+), 7 deletions(-)

diff --git a/README.md b/README.md
index 621b331..4d23f5f 100644
--- a/README.md
+++ b/README.md
@@ -102,6 +102,15 @@ Arrays are defined using square brackets (`[]`) and commas (`,`). An array of in
 DESIGNA x VT [I VSQVE X]
 ```
 
+Individual elements can be accessed by index using square brackets. Indexing is 1-based, so `I` refers to the first element:
+
+```
+DESIGNA x VT [(I, II, III)]
+DICE(x[I])
+
+> I
+```
+
 ## Conditionals
 ### SI/TVNC
 If-then statements are denoted with the keywords `SI` (if) and `TVNC` (then). Thus, the code
diff --git a/centvrion/ast_nodes.py b/centvrion/ast_nodes.py
index ad0f461..98138ae 100644
--- a/centvrion/ast_nodes.py
+++ b/centvrion/ast_nodes.py
@@ -342,6 +342,24 @@ class UnaryMinus(Node):
     return vtable, ValInt(-val.value())
 
 
+class ArrayIndex(Node):
+  def __init__(self, array, index) -> None:
+    self.array = array
+    self.index = index
+
+  def __repr__(self) -> str:
+    return f"ArrayIndex({self.array!r}, {self.index!r})"
+
+  def _eval(self, vtable):
+    vtable, array = self.array.eval(vtable)
+    vtable, index = self.index.eval(vtable)
+    i = index.value()
+    lst = array.value()
+    if i < 1 or i > len(lst):
+      raise IndexError(f"Index {i} out of range for array of length {len(lst)}")
+    return vtable, lst[i - 1]
+
+
 class SiStatement(Node):
   def __init__(self, test, statements, else_part) -> None:
     self.test = test
diff --git a/centvrion/parser.py b/centvrion/parser.py
index 6d9a4ad..2d4acad 100644
--- a/centvrion/parser.py
+++ b/centvrion/parser.py
@@ -16,6 +16,7 @@ class Parser():
         ('left', ["SYMBOL_PLUS", "SYMBOL_MINUS"]),
         ('left', ["SYMBOL_TIMES", "SYMBOL_DIVIDE"]),
         ('right', ["UMINUS"]),
+        ('left', ["INDEX"]),
       ]
     )
 
@@ -194,6 +195,10 @@ class Parser():
     def range_array(tokens):
       return ast_nodes.DataRangeArray(tokens[1], tokens[3])
 
+    @self.pg.production('expression : expression SYMBOL_LBRACKET expression SYMBOL_RBRACKET', precedence='INDEX')
+    def array_index(tokens):
+      return ast_nodes.ArrayIndex(tokens[0], tokens[2])
+
     # ids
     @self.pg.production('ids : SYMBOL_LPARENS id_list')
     def ids(tokens):
diff --git a/tests.py b/tests.py
index a8d2b66..4a386f9 100644
--- a/tests.py
+++ b/tests.py
@@ -5,7 +5,7 @@ from unittest.mock import patch
 from parameterized import parameterized
 
 from centvrion.ast_nodes import (
-  Bool, BinOp, BuiltIn, DataArray, DataRangeArray, Defini,
+  ArrayIndex, Bool, BinOp, BuiltIn, DataArray, DataRangeArray, Defini,
   Designa, DumStatement, Erumpe, ExpressionStatement, ID,
   Invoca, ModuleCall, Nullus, Numeral, PerStatement,
   Program, Redi, SiStatement, String, UnaryMinus,
@@ -231,6 +231,8 @@ error_tests = [
   ("DEFINI f (x, y) VT { REDI(x) }\nINVOCA f (I)", TypeError),   # too few args
   ("DEFINI f () VT { REDI(I) }\nINVOCA f (I)", TypeError),        # args to zero-param function
   ("SI NVLLVS TVNC { DESIGNA r VT I }", TypeError),               # NVLLVS cannot be used as boolean
+  ("[(I, II)][III]", IndexError),                                  # index too high
+  ("[(I, II)][-I]", IndexError),                                   # negative index
 ]
 
 class TestErrors(unittest.TestCase):
@@ -604,17 +606,17 @@ class TestEtAvt(unittest.TestCase):
 
 
 # --- Array indexing ---
-# Indexing is 0-based; NVLLVS serves as index 0
+# Indexing is 1-based; I is the first element
 
 array_index_tests = [
   # basic indexing
-  ("[(I, II, III)][NVLLVS]", None, ValInt(1)),   # index 0 → first element
-  ("[(I, II, III)][I]", None, ValInt(2)),          # index 1 → second element
-  ("[(I, II, III)][II]", None, ValInt(3)),          # index 2 → third element
+  ("[(I, II, III)][I]", None, ValInt(1)),    # first element
+  ("[(I, II, III)][II]", None, ValInt(2)),   # second element
+  ("[(I, II, III)][III]", None, ValInt(3)),  # third element
   # index into a variable
-  ("DESIGNA a VT [(X, XX, XXX)]\na[I]", None, ValInt(20)),
+  ("DESIGNA a VT [(X, XX, XXX)]\na[II]", None, ValInt(20)),  # second element
   # index into range array
-  ("[I VSQVE V][II]", None, ValInt(3)),
+  ("[I VSQVE V][II]", None, ValInt(2)),      # second element of [1,2,3,4]
 ]
 
 class TestArrayIndex(unittest.TestCase):