Lesson 4 of 21 · 48 min · UG / professional
del and editing a list in place
The third UPI row was a duplicate. You may replace it by index or drop it. del is the laptop spelling; here you rebuild keepers so the source file still exists as evidence.
Market shopping list
A list holds many items together.
You will be able to
- ✓ Replace a bad row by index when the slot must stay
- ✓ Drop a row by rebuilding a keepers list (del on a laptop)
- ✓ Leave the source evidence intact when an audit is likely
The third UPI row was a duplicate
Someone pasted the canteen txn twice. You may overwrite that slot by index if the file layout is fixed — rows[2] = the correction. Or you may drop it. On a laptop, del rows[2] removes that slot and shifts the tail. This sandbox practises the two honest twins: assign a new dict at an index, or walk the source and append keepers to a new list.
Deleting while you walk the same list is how internships skip a row by accident. Rebuild keepers. The source file on disk still exists as evidence until a laptop write replaces it.
In-place versus a new table
In-place means the same list object changes: a replacement at an index, or a laptop del. A new table means keepers.append. Prefer a new table when the principal may still ask “what did the export actually contain at 09:00?” Prefer in-place when you are editing a scratch list you just built and nobody else holds a reference.
Never “delete” by writing 0 into amount and hoping reports skip zeros. That is how PF lines turn into fake rupees.
Laptop: del drops a slot
rows = ["UPI-A", "UPI-B", "UPI-A"] del rows[2] print(rows) rows[0] = "UPI-C" print(rows)
Words that matter
- del
- Laptop spelling that removes a slot (or a name).
- In-place
- Mutate this list object — replace by index, or del.
- Keepers
- A new list of rows you still want.
Common mistakes
Avoid: Removing items from a list while a for-loop is walking that same list.
Do this: Build a keepers list, or replace one index you already computed.
Run it step by step
Each box is a real program. Press Run, change a number, Run again — the output must follow your code.
1. Step 1 — replace by index
Slot 2 was a duplicate A. Overwrite with C.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
2. Step 2 — drop by rebuilding keepers
Keep unique UPI ids. Second A is skipped. Keepers length is 2.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
Example program — Correct a PF line, keep the sheet length
Index 1 was 0 by mistake. Replace. Three rows remain.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
Your turn — Fix slot 1
rows is A, A, B. Replace index 1 with C. Print so C appears.
Edit the example, press Run, then Build if you want a compile check.
build
Press Build to compile.
Self-assessment
Check your understanding before you mark the lesson complete.
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