Improving Pseudo-Code CS16: Introduction to Data Structures & - - PowerPoint PPT Presentation

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Improving Pseudo-Code CS16: Introduction to Data Structures & - - PowerPoint PPT Presentation

Jun 26, 2023 181 likes •622 views

Improving Pseudo-Code CS16: Introduction to Data Structures & Algorithms Spring 2020 CleanYour Code! Errors per line is approximately constant fewer lines fewer errors overall Fewer lines are easier to grade more likely to

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SLIDE 1

Improving Pseudo-Code

CS16: Introduction to Data Structures & Algorithms Spring 2020

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SLIDE 2

CleanYour Code!

  • Errors per line is approximately constant
  • fewer lines → fewer errors overall
  • Fewer lines are easier to grade
  • more likely to receive credit
  • Clean code reflects clean thinking
  • and a better understanding of material
  • Let’s see some examples
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SLIDE 3

Lowest Common Ancestor

  • Given two nodes u and v
  • determine deepest node that is ancestor of both
3 A B D C G H E F I J K u v LCA(u, v) C D A J E B G J A G C C 3
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SLIDE 4

Lowest Common Ancestor

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3 min

Activity #1

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SLIDE 5

Lowest Common Ancestor

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3 min

Activity #1

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SLIDE 6

Lowest Common Ancestor

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2 min

Activity #1

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SLIDE 7

Lowest Common Ancestor

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1 min

Activity #1

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SLIDE 8

Lowest Common Ancestor

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0 min

Activity #1

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SLIDE 9

Ways to Improve Pseudo-Code

  • Clarify inputs and outputs with comments
  • good habit and makes methods easier to understand
  • Make sure all necessary arguments are included

as parameters

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SLIDE 10

Lowest Common Ancestor

10 function LCA(u, v): lca = null udepth = T.depth(u) vdepth = T.depth(v) if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if udepth > vdepth then u = T.parent(u) udepth = udepth – 1 else if vdepth > udepth v = T.parent(v) vdepth = vdepth – 1 else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1 return lca

Inputs &

  • utputs ?
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SLIDE 11

Lowest Common Ancestor

11 function LCA(u, v): // Input: two nodes u, v // Output: the lowest common ancestor of u and v lca = null udepth = T.depth(u) vdepth = T.depth(v) if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if udepth > vdepth then u = T.parent(u) udepth = udepth – 1 else if vdepth > udepth v = T.parent(v) vdepth = vdepth – 1 else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1 return lca
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SLIDE 12

Lowest Common Ancestor

12 function LCA(u, v): // Input: two nodes u, v // Output: the lowest common ancestor of u and v lca = null udepth = T.depth(u) vdepth = T.depth(v) if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if udepth > vdepth then u = T.parent(u) udepth = udepth – 1 else if vdepth > udepth v = T.parent(v) vdepth = vdepth – 1 else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1 return lca

Where does T come from?

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SLIDE 13

Lowest Common Ancestor

13 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null udepth = T.depth(u) vdepth = T.depth(v) if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if udepth > vdepth then u = T.parent(u) udepth = udepth – 1 else if vdepth > udepth v = T.parent(v) vdepth = vdepth – 1 else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1 return lca
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SLIDE 14

Ways to Improve Pseudo-Code

  • Get rid of unnecessary variables
  • Using vars for information that is elsewhere…
  • …leads to careless errors
  • In example, no need for udepth and vdepth
  • since Tree keeps track of node’s depth
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SLIDE 15

Lowest Common Ancestor

15 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null udepth = T.depth(u) vdepth = T.depth(v) if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if udepth > vdepth then u = T.parent(u) udepth = udepth – 1 else if vdepth > udepth v = T.parent(v) vdepth = vdepth – 1 else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1 return lca

〉

Needlessly complex

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SLIDE 16

Lowest Common Ancestor

16 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null udepth = T.depth(u) vdepth = T.depth(v) if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1 return lca

Now irrelevant

〉

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SLIDE 17

Lowest Common Ancestor

17 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca
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SLIDE 18

Ways to Improve Pseudo-Code

  • If method returns boolean
  • no need to check if returned value ==true
  • Logical operators can be used on boolean

returned valued

  • !T.isroot(u) is same as

T.isroot(u)==false

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SLIDE 19

Lowest Common Ancestor

19 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca

Redundant equality checks

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SLIDE 20

Lowest Common Ancestor

20 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca
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SLIDE 21

Lowest Common Ancestor

21 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca

Just removed whitespace

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SLIDE 22

Ways to Improve Pseudo-Code

  • As soon as you found answer, return it
  • This avoids going through unnecessary code
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SLIDE 23

Lowest Common Ancestor

23 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca

It’s the answer. Return it!

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SLIDE 24

Lowest Common Ancestor

24 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root return lca while (lca == null) do if (u == v) then lca = u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca

It’s the answer. Return it!

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SLIDE 25

Lowest Common Ancestor

25 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root return lca while (lca == null) do if (u == v) then lca = u return lca else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca
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SLIDE 26

Ways to Improve Pseudo-Code

  • If variable is only used to return something
  • simply return it
  • Avoids keeping track of unnecessary variables
  • and makes code shorter and cleaner
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SLIDE 27

Lowest Common Ancestor

27 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root return lca while (lca == null) do if (u == v) then lca = u return lca else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v) return lca

Condition is irrelevant

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SLIDE 28

Lowest Common Ancestor

28 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v lca = null if T.isroot(u) or T.isroot(v) then lca = T.root return lca repeat if (u == v) then lca = u return lca else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v)

lca is no longer used

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SLIDE 29

Lowest Common Ancestor

29 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v if T.isroot(u) or T.isroot(v) then return T.root repeat if (u == v) then return u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v)
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SLIDE 30

Lowest Common Ancestor

30 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v if T.isroot(u) or T.isroot(v) then return T.root repeat if (u == v) then return u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u) v = T.parent(v)
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SLIDE 31

Ways to Improve Pseudo-Code

  • If you never enter a conditional in a while loop,
  • try to use two while loops to simplify
  • If u is at lower depth than v, algorithm will not

allow u to get to a higher depth than v

  • so unnecessary to check both within one

while loop

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SLIDE 32

Lowest Common Ancestor

32 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v if T.isroot(u) or T.isroot(v) then return T.root repeat if (u == v) then return u else if T.depth(u) > T.depth(v) then u = T.parent(u) else if T.depth(v) > T.depth(u) v = T.parent(v) else u = T.parent(u); udepth = udepth - 1 v = T.parent(v); vdepth = vdepth – 1

Only one of these conditionals will ever be true

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SLIDE 33

Lowest Common Ancestor

33 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) if T.isroot(u) or T.isroot(v) then return T.root repeat if (u == v) then return u else u = T.parent(u) v = T.parent(v)
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SLIDE 34

Ways to Improve Pseudo-Code

  • Avoid unnecessary conditional checks
  • In example, after the two while loops,
  • u and v have same depth
  • if either is root, they both are the same root
  • Try to be as concise as possible!
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SLIDE 35

Lowest Common Ancestor

35 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) if T.isroot(u) or T.isroot(v) then return T.root repeat if (u == v) then return u else u = T.parent(u) v = T.parent(v)
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SLIDE 36

Lowest Common Ancestor

36 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) if T.isroot(u) or T.isroot(v) or u == v then return u repeat if (u == v) then return u else u = T.parent(u) v = T.parent(v)
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SLIDE 37

Ways to Improve Pseudo-Code

  • After the two while loops,
  • u and v have same depth
  • if either is root, then they both are root
  • Try to be as concise as possible!
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SLIDE 38

Lowest Common Ancestor

38 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) if T.isroot(u) or T.isroot(v) or u == v then return u repeat if (u == v) then return u else u = T.parent(u) v = T.parent(v)

Can be simplified

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SLIDE 39

Lowest Common Ancestor

39 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) if u == v then return u repeat if (u == v) then return u else u = T.parent(u) v = T.parent(v)
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SLIDE 40

Lowest Common Ancestor

40 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) if u == v then return u repeat if (u == v) then return u else u = T.parent(u) v = T.parent(v)

Condense into a single loop

〉

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SLIDE 41

Lowest Common Ancestor

41 function LCA(u, v, T): // Input: two nodes u, v in a tree T // Output: the lowest common ancestor of u and v while T.depth(u) > T.depth(v) u = T.parent(u) while T.depth(v) > T.depth(u) v = T.parent(v) while u != v then u = T.parent(u) v = T.parent(v) return u

From clunky 19 lines to elegant 8 lines!

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SLIDE 42

Improve Pseudo-Code

  • Now that you have seen how easily pseudocode

can be simplified…

  • …you are expected to make similar improvements to

your pseudo-code

  • Good pseudo-code is both accurate & concise
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