fix length based example selector (#862)

Author: hwchase17

langchain/prompts/example_selector/length_based.py

@@ -51,7 +51,7 @@ def select_examples(self, input_variables: Dict[str, str]) -> List[dict]:
         examples = []
         while remaining_length > 0 and i < len(self.examples):
             new_length = remaining_length - self.example_text_lengths[i]
-            if i < 0:
+            if new_length < 0:
                 break
             else:
                 examples.append(self.examples[i])

tests/unit_tests/prompts/test_length_based_example_selector.py

@@ -17,7 +17,7 @@ def selector() -> LengthBasedExampleSelector:
     selector = LengthBasedExampleSelector(
         examples=EXAMPLES,
         example_prompt=prompts,
-        max_length=25,
+        max_length=30,
     )
     return selector