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Fix string negative indexing #323

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Merged
softmattertheory merged 1 commit into from
Jul 7, 2026
Merged

Fix string negative indexing #323

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17 changes: 16 additions & 1 deletion src/classes/strng.c
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Original file line number Diff line number Diff line change
Expand Up @@ -176,6 +176,7 @@ int string_countchars(objectstring *s) {

/** Get a pointer to the i'th character of a string */
char *string_index(objectstring *s, int i) {
if (i<0) return NULL;
int n=0;
for (char *c = s->string; *c!='\0'; ) {
if (i==n) return (char *) c;
Expand Down Expand Up @@ -219,6 +220,20 @@ value String_clone(vm *v, int nargs, value *args) {
return out;
}

/** Gets a specified character from a string */
value String_getindex(vm *v, int nargs, value *args) {
objectstring *slf = MORPHO_GETSTRING(MORPHO_SELF(args));
value out=MORPHO_NIL;
int n=MORPHO_GETINTEGERVALUE(MORPHO_GETARG(args, 0));

char *c = string_index(slf, n);
if (c) {
out=object_stringfromcstring(c, morpho_utf8numberofbytes(c));
morpho_bindobjects(v, 1, &out);
} else morpho_runtimeerror(v, VM_OUTOFBOUNDS);
return out;
}

/** Enumerate members of a string */
value String_enumerate(vm *v, int nargs, value *args) {
objectstring *slf = MORPHO_GETSTRING(MORPHO_SELF(args));
Expand Down Expand Up @@ -292,7 +307,7 @@ MORPHO_BEGINCLASS(String)
MORPHO_METHOD_SIGNATURE(MORPHO_COUNT_METHOD, "Int ()", String_count, MORPHO_FN_PUREFN),
MORPHO_METHOD_SIGNATURE(MORPHO_PRINT_METHOD, "String ()", String_print, MORPHO_FN_IO),
MORPHO_METHOD_SIGNATURE(MORPHO_CLONE_METHOD, "String ()", String_clone, MORPHO_FN_PUREFN|MORPHO_FN_ALLOCATES),
MORPHO_METHOD_SIGNATURE(MORPHO_GETINDEX_METHOD, "(Int)", String_enumerate, MORPHO_FN_THROWS),
MORPHO_METHOD_SIGNATURE(MORPHO_GETINDEX_METHOD, "String (Int)", String_getindex, MORPHO_FN_THROWS),
MORPHO_METHOD_SIGNATURE(MORPHO_GETINDEX_METHOD, "Nil (...)", String_enumerate__err, MORPHO_FN_THROWS),
MORPHO_METHOD_SIGNATURE(MORPHO_ENUMERATE_METHOD, "(Int)", String_enumerate, MORPHO_FN_THROWS),
MORPHO_METHOD_SIGNATURE(MORPHO_ENUMERATE_METHOD, "Nil (...)", String_enumerate__err, MORPHO_FN_THROWS),
Expand Down
5 changes: 5 additions & 0 deletions test/string/index_neg.morpho
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@@ -0,0 +1,5 @@
var s = "Hello"

print s[-1]
// expect Error 'IndxBnds'

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