Searching.ppt

UNIT III
Searching and Sorting

Searching
• Given a collection and an element (key) to
find…
• Output
– Print a message (“Found”, “Not Found)
– Return a value (position of key )

Linear Search: A Simple Search
• A search traverses the collection until
– The desired element is found
– Or the collection is exhausted
• If the collection is ordered, I might not
have to look at all elements
– I can stop looking when I know the
element cannot be in the collection.

Iterative Array Search
procedure Search(my_array Array,
target Num)
i Num
i <- 1
loop for (i=0;i <MAX ;i++)
if(a[i]==target)
print(“Target data found”)
break;
endloop
if(i > MAX) then
print(“Target data not found”)
endif
endprocedure // Search

my_array
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target = 13
target Num)
i Num
i <- 1
loop for (i=0;i <MAX ;i++)
if(a[i]==target)
break;
endloop
if(i > MAX) then
endif

procedure Search(my_array isoftype in NumArrayType,
target isoftype in Num)
i isoftype Num
i <- 1
loop
exitif((i > MAX) OR (my_array[i] = target))
i <- i + 1
endloop
if(i > MAX) then
else
endif
my_array
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target = 13

procedure Search(my_array isoftype in NumArrayType,
target isoftype in Num)
i isoftype Num
i <- 1
loop
i <- i + 1
endloop
if(i > MAX) then
else
endif
my_array
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1 2 3 4 5 6
target = 13
Target data found

Linear Search Analysis: Best Case
target Num)
i Num
i <- 1
loop
i <- i + 1
endloop
if(i > MAX) then
else
endif
Scan the array
Best Case: match with the first item
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target = 7
Best Case:
1 comparison

Linear Search Analysis: Worst Case
target Num)
i Num
i <- 1
loop
i <- i + 1
endloop
if(i > MAX) then
else
endif
Scan the array
Worst Case: match with the last item (or no match)
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target = 32
Worst Case:
N comparisons

The Scenario
• We have a sorted array
• We want to determine if a particular element is in the array
– Once found, print or return (index, boolean, etc.)
– If not found, indicate the element is not in the collection
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A Better Search Algorithm
• Of course we could use our simpler search
and traverse the array
• But we can use the fact that the array is
sorted to our advantage
• This will allow us to reduce the number of
comparisons

Binary Search
• Requires a sorted array or a binary search tree.
• Cuts the “search space” in half each time.
• Keeps cutting the search space in half until the
target is found or has exhausted the all possible
locations.

Binary Search Algorithm
look at “middle” element
if no match then
look left (if need smaller) or
right (if need larger)
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Look for 42

The Algorithm
look at “middle” element
if no match then
look left or right
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Look for 42

The Binary Search Algorithm
• Return found or not found (true or false), so it should
be a function.
• When move left or right, change the array boundaries
– We’ll need a first and last

The Binary Search Algorithm
calculate middle position
if (first and last have “crossed”) then
“Item not found”
elseif (element at middle = to_find) then
“Item Found”
elseif to_find < element at middle then
Look to the left
else
Look to the right

Looking Left
• Use indices “first” and “last” to keep track of
where we are looking
• Move left by setting last = middle – 1
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F L
M
L

Looking Right
• Use indices “first” and “last” to keep track of
where we are looking
• Move right by setting first = middle + 1
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F L
M F

Binary Search Example – Found
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Looking for 42
F L
M

Binary Search Example – Found
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42 found – in 3 comparisons
F
L
M

Binary Search Example – Not Found
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Looking for 89
F L
M

Binary Search Example – Not Found
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89 not found – 3 comparisons
F
L

Function Find return boolean (A Array, first, last, to_find)
middle <- (first + last) div 2
if (first > last) then
return false
elseif (A[middle] = to_find) then
return true
elseif (to_find < A[middle]) then
return Find(A, first, middle–1, to_find)
else
return Find(A, middle+1, last, to_find)
endfunction
Binary Search Function

Binary Search Analysis: Best Case
return false
return true
else
endfunction
Best Case: match from the firs comparison
Best Case:
1 comparison
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Target: 59

Binary Search Analysis: Worst Case
return false
return true
else
endfunction
Worst Case: divide until reach one item, or no match.
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How many
comparisons??

Binary Search Analysis: Worst Case
• With each comparison we throw away ½ of the list
N
N/2
N/4
N/8
1
………… 1 comparison
.
.
.
Number of steps is at
most Log2N

Summary
• Binary search reduces the work by half at each
comparison
• If array is not sorted  Linear Search
– Best Case O(1)
– Worst Case O(N)
• If array is sorted  Binary search
– Best Case O(1)
– Worst Case O(Log2N)

Searching.ppt

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