Merge pull request #1099 from Thirumalai-Shaktivel/neg_index

Implement negative indexing in Lists

Author: certik

integration_tests/test_list_01.py

@@ -42,8 +42,30 @@ def test_list_02():
     for j in range(len(y)):
         assert x[j] == y[j]
 
+# Negative Indexing
+def test_list_03():
+    x: list[f64] = []
+
+    i: i32
+    for i in range(2):
+        x.append(float(i))
+
+    assert x[1] == x[-1]
+    assert x[0] == x[-2]
+    assert x[-1] == 1.0
+    assert x[-2] == 0.0
+
+    size: i32 = 2
+    for i in range(100):
+        x.append((i * size)/2)
+        size = len(x)
+
+    for i in range(size):
+        assert x[i] == x[((i-len(x)) + size) % size]
+
 def tests():
     test_list_01()
     test_list_02()
+    test_list_03()
 
 tests()

src/lpython/semantics/python_ast_to_asr.cpp

@@ -2515,6 +2515,19 @@ class CommonVisitor : public AST::BaseVisitor<Derived> {
 
             } else if (ASR::is_a<ASR::List_t>(*type)) {
                 index = ASRUtils::EXPR(tmp);
+                ASR::expr_t* val = ASRUtils::expr_value(index);
+                if (val && ASR::is_a<ASR::IntegerConstant_t>(*val)) {
+                    if (ASR::down_cast<ASR::IntegerConstant_t>(val)->m_n < 0) {
+                        // Replace `x[-1]` to `x[len(x)+(-1)]`
+                        ASR::ttype_t *int_type = ASRUtils::TYPE(ASR::make_Integer_t(
+                                                        al, loc, 4, nullptr, 0));
+                        ASR::expr_t *list_len = ASRUtils::EXPR(ASR::make_ListLen_t(
+                                    al, loc, value, int_type, nullptr));
+                        ASR::expr_t *neg_idx = ASRUtils::expr_value(index);
+                        index = ASRUtils::EXPR(ASR::make_IntegerBinOp_t(al, loc,
+                            list_len, ASR::binopType::Add, neg_idx, int_type, nullptr));
+                    }
+                }
                 tmp = make_ListItem_t(al, loc, value, index,
                                       ASR::down_cast<ASR::List_t>(type)->m_type, nullptr);
                 return false;