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Episode 2 of 22 · DSA, end to end

Loops and pattern printing: the discipline before the algorithms

Chapters

1. Three decisions0:00
2. Where it starts0:15
3. When it stops0:33
4. What changes each pass0:49
5. The three, on one line1:06
6. The same three, scattered1:21
7. Posts and gaps1:37
8. One index past the end1:52
9. Half-open on purpose2:10
10. One sentence, three times2:28
11. An invariant you can say2:44
12. The invariant names the end3:02
13. Down the page, along a line3:17
14. I and j are a coordinate3:32
15. Row-major order3:47
16. Nesting multiplies4:03
17. The inner bound can depend on i4:20
18. Two questions to ask4:35
19. Squared paper4:49
20. The solid square5:03
21. The triangle, as an inequality5:20
22. Flip it, flip the inequality5:35
23. Spaces are what centre it5:51
24. A border is four ors6:09
25. Stop drawing, start counting6:24
26. The count, per row6:41
27. The padding formula6:58
28. Add the rows up7:17
29. Half the cells, same shape7:33
30. A formula runs backwards7:50
31. Why this earns a class8:07
32. Binary search, said out loud8:22
33. Two pointers, ruled out8:38
34. The sorted prefix grows8:54
35. Something has to shrink9:12
36. Where the bug is9:29
37. The whole discipline9:45

Loops and pattern printing: the discipline before the algorithms. A loop is the first thing you write and the last thing you stop getting wrong. Strip it down and it is exactly three decisions. Where it starts. When it stops. And what changes on

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