Overlap and chunk boundaries in LangChain - Step-by-Step Execution

Concept Flow - Overlap and chunk boundaries

Start with large text

↓

Split text into chunks

↓

Apply chunk size limit

↓

Add overlap between chunks

↓

Output chunks with boundaries

↓

End

This flow shows how large text is split into smaller chunks with overlaps to keep context between chunks.

Execution Sample

LangChain

text = "The quick brown fox jumps over the lazy dog"
chunk_size = 10
overlap = 3
chunks = []
start = 0
while start < len(text):
  end = start + chunk_size
  chunk = text[start:end]
  chunks.append(chunk)
  start += chunk_size - overlap

This code splits a sentence into chunks of 10 characters with 3 characters overlapping between chunks.

Execution Table

Step	start	end	chunk extracted	Action	Next start
1	0	10	"The quick"	Extract chunk from 0 to 10	7
2	7	17	"ck brown f"	Extract chunk from 7 to 17	14
3	14	24	"own fox ju"	Extract chunk from 14 to 24	21
4	21	31	"x jumps ov"	Extract chunk from 21 to 31	28
5	28	38	"ps over th"	Extract chunk from 28 to 38	35
6	35	43	"e lazy dog"	Extract chunk from 35 to 43 (end of text)	42
7	42	52	"g"	Extract chunk from 42 to 43 (end of text)	49
8	49	59	""	Start 49 beyond text length, stop loop	-

💡 start index 49 is beyond text length 43, loop ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
start	0	7	14	21	28	35	42	49	49
end	10	17	24	31	38	43	52	59	59
chunk	"The quick"	"ck brown f"	"own fox ju"	"x jumps ov"	"ps over th"	"e lazy dog"	"g"	""	-
chunks.length	0	1	2	3	4	5	6	7	7

Key Moments - 3 Insights

Why does the start index increase by chunk_size - overlap instead of chunk_size?

Because we want chunks to overlap, so we move start forward by less than chunk_size to keep some text from the previous chunk. See execution_table rows where start moves 0->7->14 instead of 0->10->20.

What happens when the end index goes beyond the text length?

The chunk extracts only up to the text end without error. In the last chunk, end is beyond text length, so chunk extracts till the end. See execution_table rows 6 and 7.

Why does the loop stop when start is beyond text length?

Because no more chunks can be extracted. The condition start < len(text) fails at step 8 when start is 49 and text length is 43. See exit_note.

Visual Quiz - 3 Questions

Look at the execution_table, what is the chunk extracted at step 3?

A "x jumps ov"

B "own fox ju"

C "ck brown f"

D "ps over th"

💡 Hint

Check the 'chunk extracted' column at step 3 in the execution_table.

At which step does the start index first become greater than or equal to the text length?

A Step 8

B Step 7

C Step 6

D Step 5

💡 Hint

Look at the 'start' column in variable_tracker and execution_table rows.

If overlap was set to 0, how would the start index change between steps?

A start would increase by chunk_size - 1 each step

B start would stay the same

C start would increase by chunk_size each step

D start would increase by overlap each step

💡 Hint

Overlap controls how much start moves back; zero overlap means no overlap, so start moves full chunk_size.

Concept Snapshot

Overlap and chunk boundaries split large text into smaller pieces.
Chunks have fixed size with some overlap to keep context.
Start index moves by chunk_size minus overlap.
Last chunk may be shorter if text ends.
Overlap helps in tasks like text processing to avoid losing info.

Full Transcript

This visual execution shows how a long text is split into smaller chunks with overlaps. We start at index 0 and extract a chunk of fixed size. Then we move the start index forward by chunk_size minus overlap to create the next chunk. This overlap keeps some text from the previous chunk to maintain context. The process repeats until the start index goes beyond the text length. The last chunk may be shorter if the end index exceeds text length. This method is useful in text processing frameworks like Langchain to handle large documents in manageable pieces while preserving continuity.