IMPROVING PSEUDOCODE CS16: Introduction to Data Structures & Algorithms Thursday, February 26, 2015 1
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1
IMPROVING PSEUDOCODE
CS16: Introduction to Data Structures & Algorithms
Thursday, February 26, 2015
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Clean Your Code
• Errors per line ~ constant• Fewer lines fewer errors overall
• Fewer lines are easier to grade• More likely to receive credit
• Cleaner code reflects cleaner thinking and a better understanding of the material
• Let’s work through an example…
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Lowest Common Ancestor (LCA)
Problem: Given two nodes u and v, determine the deepest node that’s an ancestor of both.
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A
B DC
G HE F
I J K
u v LCA(u, v)
C D A
J E B
G J A
G C C
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function LCA(u, v):
lca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif udepth > vdepth then
u = T.parent(u)udepth = udepth – 1
else if vdepth > udepthv = T.parent(v)vdepth = vdepth – 1
elseu = T.parent(u); udepth = udepth - 1v = T.parent(v); vdepth = vdepth – 1
return lca
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function LCA(u, v):
lca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif udepth > vdepth then
u = T.parent(u)udepth = udepth – 1
else if vdepth > udepthv = T.parent(v)vdepth = vdepth – 1
elseu = T.parent(u); udepth = udepth - 1v = T.parent(v); vdepth = vdepth – 1
return lca
What are the inputs? The output?
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function LCA(u, v):// Input: two nodes u, v// Output: the lowest common ancestor of u and vlca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif udepth > vdepth then
u = T.parent(u)udepth = udepth – 1
else if vdepth > udepthv = T.parent(v)vdepth = vdepth – 1
elseu = T.parent(u); udepth = udepth - 1v = T.parent(v); vdepth = vdepth – 1
return lca
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function LCA(u, v):// Input: two nodes u, v// Output: the lowest common ancestor of u and vlca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif udepth > vdepth then
u = T.parent(u)udepth = udepth – 1
else if vdepth > udepthv = T.parent(v)vdepth = vdepth – 1
elseu = T.parent(u); udepth = udepth - 1v = T.parent(v); vdepth = vdepth – 1
return lca
Where does T come from?
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif udepth > vdepth then
u = T.parent(u)udepth = udepth – 1
else if vdepth > udepthv = T.parent(v)vdepth = vdepth – 1
elseu = T.parent(u); udepth = udepth - 1v = T.parent(v); vdepth = vdepth – 1
return lca
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Ways to improve psuedocode
• Clarify your input and output with comments
• This is a good habit to get into and makes methods easily understandable for somebody who has never seen them before
• Make sure all necessary arguments are included as parameters
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif udepth > vdepth then
u = T.parent(u)udepth = udepth – 1
else if vdepth > udepthv = T.parent(v)vdepth = vdepth – 1
elseu = T.parent(u); udepth = udepth - 1v = T.parent(v); vdepth = vdepth – 1
return lca
Needlessly complex
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nulludepth = T.depth(u)vdepth = T.depth(v)if (T.isroot(u) == true) or (T.isroot(v) == true) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif T.depth(u) > T.depth(v) then
u = T.parent(u)
else if T.depth(v) > T.depth(u)v = T.parent(v)
elseu = T.parent(u)v = T.parent(v)
return lca
Now irrelevant
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Ways to improve psuedocode
• Get rid of unnecessary variables• Using variables for information which can be accurately found elsewhere leaves room for careless errors• In this example there is no need for udepth and vdepth when the tree keeps track of the node’s depth
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = null
if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root
while (lca == null) doif (u == v) thenlca = uelseif T.depth(u) > T.depth(v) then
u = T.parent(u)
else if T.depth(v) > T.depth(u)v = T.parent(v)
elseu = T.parent(u)v = T.parent(v)
return lca
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = null
if (T.isroot(u) == true) or (T.isroot(v) == true) then lca = T.root
while (lca == null) doif (u == v) thenlca = uelseif T.depth(u) > T.depth(v) then
u = T.parent(u)
else if T.depth(v) > T.depth(u)v = T.parent(v)
elseu = T.parent(u)v = T.parent(v)
return lca
Redundant equality checks
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = null
if T.isroot(u) or T.isroot(v) then lca = T.root
while (lca == null) doif (u == v) thenlca = uelseif T.depth(u) > T.depth(v) then
u = T.parent(u)
else if T.depth(v) > T.depth(u)v = T.parent(v)
elseu = T.parent(u)v = T.parent(v)
return lca
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = null
if T.isroot(u) or T.isroot(v) then lca = T.root
while (lca == null) doif (u == v) thenlca = uelseif T.depth(u) > T.depth(v) then
u = T.parent(u)
else if T.depth(v) > T.depth(u)v = T.parent(v)
elseu = T.parent(u)v = T.parent(v)
return lca
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nullif T.isroot(u) or T.isroot(v) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif 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 removing some whitespace to fit on slides
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Ways to improve psuedocode
• If a method returns a boolean, there is no need to check if return value == true
• Logical operators can be used on boolean return values • !T.isroot(u)equals T.isroot(u)==false
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nullif T.isroot(u) or T.isroot(v) then
lca = T.rootwhile (lca == null) do
if (u == v) thenlca = uelseif 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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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nullif T.isroot(u) or T.isroot(v) then
lca = T.rootreturn lca
while (lca == null) doif (u == v) thenlca = uelseif 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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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nullif T.isroot(u) or T.isroot(v) then
lca = T.rootreturn lca
while (lca == null) doif (u == v) thenlca = ureturn lcaelseif 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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Ways to improve psuedocode
• As soon as you know you’ve found the answer, return it!• This avoids going through unnecessary code
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nullif T.isroot(u) or T.isroot(v) then
lca = T.rootreturn lca
while (lca == null) doif (u == v) thenlca = ureturn lcaelseif 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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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vlca = nullif T.isroot(u) or T.isroot(v) then
lca = T.rootreturn lca
repeatif (u == v) thenlca = ureturn lcaelseif 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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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.rootrepeat
if (u == v) then
return uelseif 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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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vif T.isroot(u) or T.isroot(v) then
return T.rootrepeat
if (u == v) thenreturn uelseif 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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Ways to improve psuedocode
• If a variable is only used for the purpose of returning something, simply return the value
• This avoids having to keep track of an unnecessary variable and makes your code shorter and clearer
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vif T.isroot(u) or T.isroot(v) then
return T.rootrepeat
if (u == v) thenreturn uelseif 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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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vwhile 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.rootrepeat
if (u == v) thenreturn uelseu = T.parent(u)v = T.parent(v)
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Ways to improve psuedocode
• If you know you will not enter a conditional after one iteration through a while loop, attempt to use two separate while loops to simplify• In this example, if u is at a lower depth than v, the algorithm will not allow u to get to a higher depth than v. Therefore, it is unnecessary to continue checking both of these within one while loop
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vwhile 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.rootrepeat
if (u == v) thenreturn uelseu = T.parent(u)v = T.parent(v)
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vwhile 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 urepeat
if (u == v) thenreturn uelseu = T.parent(u)v = T.parent(v)
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vwhile 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 urepeat
if (u == v) thenreturn uelseu = T.parent(u)v = T.parent(v)
Can be simplified!
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Ways to improve psuedocode
• Avoid unnecessary conditional checks• In this example, after the two while loops, u and v have the same depth. If either is a root, they both are the same root
• Try to be as concise as possible!
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vwhile 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 urepeat
if (u == v) thenreturn uelseu = T.parent(u)v = T.parent(v)
Condense into a single loop!
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function LCA(u, v, T):// Input: two nodes u, v in a tree T// Output: the lowest common ancestor of u and vwhile 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 a clunky 19 lines to an elegant 8 line beauty!
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Improve psuedocode – yes you can!
• Now that you have seen how easily psuedocode can be simplified, you are expected to be making similar improvements to your own psuedocode
• Good psuedocode is BOTH accurate and concise
Thursday, February 26, 2015
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