Multiple item search in an unsorted list in Python

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- By Vasudev Ram - Online Python training / SQL training / Linux training

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Hi, readers,

I was reviewing simple algorithms with a view to using some as examples or exercises in my Python programming course. While doing so, I thought of enhancing simple linear search for one item in a list, to make it search for multiple items.

Here are a couple of program versions I wrote for that task. They use straightforward logic. There are just a few additional points:

- In both programs, I use a generator to yield the values found (the index and the item).
- In the first program, I print out the index and item for each item found.
- In the second program, I mark where the items are found with text "arrows".

This is the first program, mult_item_search_unsorted_list.py:

# mult_item_search_unsorted_list.py 
# Purpose: To search for multiple items in an unsorted list.
# Prints each item found and its index.
# Author: Vasudev Ram
# Copyright 2019 Vasudev Ram
# Training: https://jugad2.blogspot.com/p/training.html
# Blog: https://jugad2.blogspot.com
# Web site: https://vasudevram.github.io
# Product store: https://gumroad.com/vasudevram

from __future__ import print_function
import sys
from random import sample, shuffle

def mult_item_search_unsorted_list(dlist, slist):
    for didx, ditem in enumerate(dlist):
        for sitem in slist:
            if sitem == ditem:
                yield (didx, ditem)

def main():
    # Create the search list (slist) with some items that will be found 
    # and some that will not be found in the data list (dlist) below.
    slist = sample(range(0, 10), 3) + sample(range(10, 20), 3)
    # Create the data list.
    dlist = range(10)
    for i in range(3):
        # Mix it up, DJ.
        shuffle(slist)
        # MIX it up, DEK.
        shuffle(dlist)
        print("\nSearching for:", slist)
        print("    in:", dlist)
        for didx, ditem in mult_item_search_unsorted_list(dlist, slist):
            print("        found {} at index {}".format(ditem, didx))
    
main()

Output of a run:

$ python mult_item_search_unsorted_list.py

Searching for: [1, 18, 3, 15, 19, 4]
    in: [8, 9, 1, 2, 0, 7, 5, 3, 6, 4]
        found 1 at index 2
        found 3 at index 7
        found 4 at index 9

Searching for: [4, 19, 18, 15, 1, 3]
    in: [7, 5, 8, 2, 9, 4, 0, 3, 6, 1]
        found 4 at index 5
        found 3 at index 7
        found 1 at index 9

Searching for: [1, 3, 4, 18, 19, 15]
    in: [9, 6, 1, 8, 7, 4, 3, 0, 2, 5]
        found 1 at index 2
        found 4 at index 5
        found 3 at index 6

And this is the second program, mult_item_search_unsorted_list_w_arrows.py:

# mult_item_search_unsorted_list_w_arrows.py 
# Purpose: To search for multiple items in an unsorted list.
# Marks the position of the items found with arrows.
# Author: Vasudev Ram
# Copyright 2019 Vasudev Ram
# Training: https://jugad2.blogspot.com/p/training.html
# Blog: https://jugad2.blogspot.com
# Web site: https://vasudevram.github.io
# Product store: https://gumroad.com/vasudevram

from __future__ import print_function
import sys
from random import sample, shuffle

def mult_item_search_unsorted_list(dlist, slist):
    for didx, ditem in enumerate(dlist):
        for sitem in slist:
            if sitem == ditem:
                yield (didx, ditem)

def main():
    # Create the search list (slist) with some items that will be found 
    # and some that will not be found in the data list (dlist) below.
    slist = sample(range(10), 4) + sample(range(10, 20), 4)
    # Create the data list.
    dlist = range(10)
    for i in range(3):
        # Mix it up, DJ.
        shuffle(slist)
        # MIX it up, DEK.
        shuffle(dlist)
        print("\nSearching for: {}".format(slist))
        print("    in: {}".format(dlist))
        for didx, ditem in mult_item_search_unsorted_list(dlist, slist):
            print("---------{}^".format('---' * didx))
    
main()

Output of a run:

$ python mult_item_search_unsorted_list_w_arrows.py

Searching for: [16, 0, 15, 4, 6, 1, 10, 12]
    in: [8, 9, 0, 1, 5, 4, 7, 2, 6, 3]
---------------^
------------------^
------------------------^
---------------------------------^

Searching for: [6, 16, 10, 0, 1, 4, 12, 15]
    in: [2, 7, 0, 8, 1, 4, 6, 3, 9, 5]
---------------^
---------------------^
------------------------^
---------------------------^

Searching for: [0, 12, 4, 10, 6, 16, 1, 15]
    in: [8, 1, 0, 7, 9, 6, 2, 5, 4, 3]
------------^
---------------^
------------------------^
---------------------------------^

In a recent post, Dynamic function creation at run time with Python's eval built-in, I had said:

"Did you notice any pattern to the values of g(i)? The values are 1, 4, 9, 16, 25 - which are the squares of the integers 1 to 5. But the formula I entered for g was not x * x, rather, it was x * x + 2 * x + 1. Then why are squares shown in the output? Reply in the comments if you get it, otherwise I will answer next time."

No reader commented with a solution. So here is a hint to figure it out:

What is the expansion of (a + b) ** 2 (a plus b the whole squared) in algebra?

Heh.

The drawing of the magnifying glass at the top of the post is by:

Yours truly.

( The same one that I used in this post:
Command line D utility - find files matching a pattern under a directory )

I'll leave you with another question: What, if any, could be the advantage of using Python generators in programs like these?
Notice that I said "programs like these", not "these programs".

Enjoy.

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- Vasudev Ram - Online Python training and consulting

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I conduct online courses on Python programming, Unix / Linux commands and shell scripting and SQL programming and database design, with course material and personal coaching sessions.

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