Problem Formulation and Solution Overview

This article will show you how to find the most common element in a Python Dictionary. However, since all Dictionary Keys are unique, this article focuses on searching for the most common Dictionary Value.

To make it more fun, we have the following running scenario:

Marty Smart, a Math Teacher at Harwood High, has amassed his student’s grades for the semester and has come to you to write a script to determine the most common grade. Below is sample data.

students = {'Marc': 99, 'Amie': 76, 'Jonny': 98, 'Anne': 99, 'Andy': 77, 'Elli': 98, 'Acer': 67, 'Joan': 61, 'Mike': 54, 'Anna': 76, 'Bobi': 67, 'Kate': 99, 'Todd': 98, 'Emma': 49, 'Stan': 76, 'Harv': 99, 'Ward': 67, 'Hank': 54, 'Wendy': 98, 'Sven': 100}

Question: How would we write code to locate the most common value in a Dictionary?

We can accomplish this task by one of the following options:

• Method 1: Use statistics.mode()
• Method 2: Use collections.Counter
• Method 3: Use for loop and max()
• Method 4: Use max()

Method 1: Use statistics mode()

This example uses mode() from the statistics library. This function returns the single most common element found in the passed argument.

from statistics import mode

common_val = mode(students.values())

The above code calls in mode() from the statistics library.

The following line uses the mode() function and passes the values from the key:value pair of students as an argument. The results save to common_val.

If the contents of students.values() are output to the terminal, the following will display.

print(students.values())

Run the code below to find the most common value.

print(common_val)

This is correct!

Method 2: Use Collections.Counter

This example uses the collections library with the counter() function to keep track of each element count.

from collections import Counter common_val = Counter(students.values()).most_common

The above code imports Python’s built-in collections library and counter().

Next, the counter() function is called and is passed all values from the key:value pair of students as an argument. Then, most_common() is appended. The results save to common_val.

If this was output to the terminal, the following would display.

This isn’t the result we want. How can we get this result?

common_val = Counter(students.values()).most_common(1) 

If we append a (1) to the end of most_common, a List containing one Tuple returns.

To extract the data further, use slicing ([0]) to reference the Tuple and assign the output accordingly.

value, count = Counter(students.values()).most_common(1)[0]

print(value, count)

Much clearer! The grade of 99 appears 4 times in students.

Method 3: Use For Loop and max()

This example locates the most common value in a Dictionary using a for loop and max() without importing a library.

tally = {}

for k, v in students.items(): if v not in tally: tally[v] = 0 else: tally[v] += 1

print(max(tally, key=tally.get))

The above code declares an empty Dictionary tally.

Then a for loop is instantiated to loop through each key:value pair in the Dictionary students.

If v (the value) is not in the tally, then the count for is set to 0.

Otherwise, if tv (the value) is in tally, the count is increased by 1.

Once the iteration is complete, the max() function is called to get the most common value in tally and output to the terminal.

Method 4: Use max()

This example uses max() to retrieve the most common value in a Python dictionary. Simple, clean, efficient.

common_val = max(list(students.values()), key=list(students.values()).count)

The code above calls the max() function and passes two (2) arguments, the values of the key:value pairs of students and a List object.

If output to the terminal, these two (2) arguments contain the following.

print(list(students.values()))

print(list(students.values()).count)

To retrieve the most common element, run the following code.

print(common_val)

Summary

This article has provided four (4) ways to find the most common element in a Python Dictionary. These examples should give you enough information to select the best fitting for your coding requirements.

Good Luck & Happy Coding!

Programmer Humor – Blockchain

This article continues on the series we started the last time: Solidity smart contract examples, which implement a simplified real-world process.

Here, we’re walking through an example of a simple open auction.

Original Source Code: Solidity Docs

We’ll first lay out the entire smart contract example without the comments for readability and development purposes.

Then we’ll dissect it part by part, analyze it and explain it.

Following this path, we’ll get a hands-on experience with smart contracts, as well as good practices in coding, understanding, and debugging smart contracts.

Smart contract – Simple Open Auction

// SPDX-License-Identifier: GPL-3.0

pragma solidity ^0.8.4; contract SimpleAuction { address payable public beneficiary; uint public auctionEndTime; address public highestBidder; uint public highestBid; mapping(address => uint) pendingReturns; bool ended; event HighestBidIncreased(address bidder, uint amount); event AuctionEnded(address winner, uint amount); error AuctionAlreadyEnded(); error BidNotHighEnough(uint highestBid); error AuctionNotYetEnded(uint timeToAuctionEnd); error AuctionEndAlreadyCalled(); constructor( uint biddingTime, address payable beneficiaryAddress ) { beneficiary = beneficiaryAddress; auctionEndTime = block.timestamp + biddingTime; } function bid() external payable { if (block.timestamp > auctionEndTime) revert AuctionAlreadyEnded(); if (msg.value <= highestBid) revert BidNotHighEnough(highestBid); if (highestBid != 0) { pendingReturns[highestBidder] += highestBid; } highestBidder = msg.sender; highestBid = msg.value; emit HighestBidIncreased(msg.sender, msg.value); } function withdraw() external returns (bool) { uint amount = pendingReturns[msg.sender]; if (amount > 0) { pendingReturns[msg.sender] = 0; if (!payable(msg.sender).send(amount)) { pendingReturns[msg.sender] = amount; return false; } } return true; } function auctionEnd() external { if (block.timestamp < auctionEndTime) revert AuctionNotYetEnded(auctionEndTime - block.timestamp); if (ended) revert AuctionEndAlreadyCalled(); ended = true; emit AuctionEnded(highestBidder, highestBid); beneficiary.transfer(highestBid); }

}

Code breakdown and analysis

// SPDX-License-Identifier: GPL-3.0

Compiles only with Solidity compiler version 0.8.4 and later, but before version 0.9.

Learn More: Layout of a Solidity File

pragma solidity ^0.8.4; contract SimpleAuction {

Parameters of the auction are variables beneficiary and auctionEndTime which we’ll initialize with contract creation arguments while the contract gets created, i.e. in the contract constructor.

Data type for time variables is unsigned integer uint, so that we can represent either absolute Unix timestamps (seconds since 1970-01-01) or time periods in seconds (seconds lapsed from the reference moment we chose).

 address payable public beneficiary; uint public auctionEndTime;

The current state of the auction is reflected in two variables, highestBidder and highestBid.

 address public highestBidder; uint public highestBid;

Previous bids can be withdrawn, that’s why we have mapping data structure to record pendingReturns.

 mapping(address => uint) pendingReturns;

Indicator flag variable for the auction end. By default, the flag is initialized to false; we’ll prevent changing it once it switches to true.

 bool ended;

When changes occur, we want our smart contract to emit the corresponding change events.

 event HighestBidIncreased(address bidder, uint amount); event AuctionEnded(address winner, uint amount);

We’re defining four errors to describe relevant failures. Along with these errors, we’ll also introduce “triple-slash” comments, commonly known as natspec comments. They enable users to see comments when an error is displayed or when users are asked to confirm the transaction.

Learn More: Natspec comments are formally defined in Ethereum Natural Language Specification Format.

 /// The auction has already ended. error AuctionAlreadyEnded(); /// There is already a higher or equal bid. error BidNotHighEnough(uint highestBid); /// The auction has not ended yet, the remaining seconds are displayed. error AuctionNotYetEnded(uint timeToAuctionEnd); /// The function auctionEnd has already been called. error AuctionEndAlreadyCalled();

Initialization of the contract with the contract creation arguments biddingTime and beneficiaryAddress.

 /// Create a simple auction with `biddingTime` /// seconds bidding time on behalf of the /// beneficiary address `beneficiaryAddress`. constructor( uint biddingTime, address payable beneficiaryAddress ) { beneficiary = beneficiaryAddress; auctionEndTime = block.timestamp + biddingTime; }

A bidder bids by sending the currency (paying) to the smart contract representing the beneficiary, hence the bid() function is defined as payable.

Learn More: What is payable in Solidity?

 /// Bid on the auction with the value sent /// together with this transaction. /// The value will only be refunded if the /// auction is not won. function bid() external payable {

The function call reverts if the bidding period ended.

 if (block.timestamp > auctionEndTime) revert AuctionAlreadyEnded();

The function rolls back the transaction to the bidder if the bid does not exceed the highest one.

 if (msg.value <= highestBid) revert BidNotHighEnough(highestBid);

The previous highest bidder was outbid and his bid is added to his previous bids reserved for a refund.

A direct refund is considered a security risk due to the possibility of executing an untrusted contract.

Instead, the bidders (recipients) will withdraw their bids themselves by using withdraw() function below.

 if (highestBid != 0) { pendingReturns[highestBidder] += highestBid; }

The new highest bidder and his bid are recorded; the event HighestBidIncreased is emitted carrying this information pair.

 highestBidder = msg.sender; highestBid = msg.value; emit HighestBidIncreased(msg.sender, msg.value); }

Bidders call the withdraw() function to retrieve the amount they bid.

 /// Withdraw a bid that was overbid. function withdraw() external returns (bool) { uint amount = pendingReturns[msg.sender]; if (amount > 0) {

It is possible to call the withdraw() function again before the send() function returns. That’s the reason why we need to disable multiple sequential withdrawals from the same sender by setting the pending returns for a sender to 0.

 pendingReturns[msg.sender] = 0;

Variable type of msg.sender is not address payable, therefore we need to convert it explicitly by using function payable() as a wrapping function.

If the send() function ends with an error, we’ll just reset the pending amount and return false.

 if (!payable(msg.sender).send(amount)) { // No need to call throw here, just reset the amount owing pendingReturns[msg.sender] = amount; return false; } } return true; }

The auctionEnd() function ends the auction and sends the highest bid to the beneficiary.

The official Solidity documentation recommends dividing the interacting functions into three functional parts:

• checking the conditions,
• performing the actions, and
• interacting with other contracts.

Otherwise, by combining these parts rather than keeping them separated, more than one calling contract could try and modify the state of the called contract and change the called contract’s state.

 /// End the auction and send the highest bid /// to the beneficiary. function auctionEnd() external {

Checking the conditions…

 if (block.timestamp < auctionEndTime) revert AuctionNotYetEnded(auctionEndTime - block.timestamp); if (ended) revert AuctionEndAlreadyCalled();

…performing the actions…

 ended = true; emit AuctionEnded(highestBidder, highestBid);

…and interacting with other contracts.

 beneficiary.transfer(highestBid); }

}

Our smart contract example is a simple, but a powerful one, enabling us to bid an amount of currency to the beneficiary.

When the contract instantiates via its constructor, it sets the auction end time and its beneficiary, i.e. beneficiary address.

The contract has three simple features, implemented via dedicated functions: bidding, withdrawing the bids and ending the auction.

A new bid is accepted only if its amount is strictly larger than the current highest bid. A new bid acceptance means that the current highest bid is added to the bidder’s balance for later withdrawal. The new highest bidder becomes the current highest bidder and the new highest bid becomes the current highest bid.

Bid withdrawing returns all summed previous bids to each bidder (mapping pendingReturns).

Contract Test Scenario

Open auction duration (in seconds): 240

Beneficiary: 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4

Testing/demonstration steps:

1. Account 0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2 bids 10 Wei;
2. Account 0x4B20993Bc481177ec7E8f571ceCaE8A9e22C02db bids 25 Wei;
3. Account 0x78731D3Ca6b7E34aC0F824c42a7cC18A495cabaB bids 25 Wei (rejected);
4. Account 0x617F2E2fD72FD9D5503197092aC168c91465E7f2 bids 35 Wei;
5. Account 0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2 bids 40 Wei + initiates premature auction end;
6. Account 0xAb8483F64d9C6d1EcF9b849Ae677dD3315835cb2 withdraws his bids;
7. Account 0x4B20993Bc481177ec7E8f571ceCaE8A9e22C02db withdraws his bids;
8. Account 0x78731D3Ca6b7E34aC0F824c42a7cC18A495cabaB withdraws his bids;
9. Account 0x78731D3Ca6b7E34aC0F824c42a7cC18A495cabaB initiates timely auction end;
10. Account 0x617F2E2fD72FD9D5503197092aC168c91465E7f2 withdraws his bids;

Appendix – The Contract Arguments

In this section is additional information for running the contract. We should expect that our example accounts may change with each refresh/reload of Remix.

Our contract creation arguments are the open auction duration (in seconds) and the beneficiary address (copy this line when deploying the example):

300, 0x5B38Da6a701c568545dCfcB03FcB875f56beddC4

Info: we could’ve used any amount of time, but I went with 300 seconds to timely simulate both a rejected attempt of ending the auction and the successful ending of the auction.

Conclusion

We continued our smart contract example series with this article that implements a simple open auction.

First, we laid out clean source code (without any comments) for readability purposes. Omitting the comments is not recommended, but we love living on the edge – and trying to be funny!

Second, we dissected the code, analyzed it, and explained each possibly non-trivial segment. Just because we’re terrific, safe players who never risk it and do everything by the book

Programmer Humor – Blockchain

PHP has a core function session_destroy() to clear all the session values. It is a simple no-argument function that returns a boolean true or false.

The PHP session ID is stored in a cookie by default. Generally that session cookie file is name PHPSESSID. The session_destroy function will not unset the session id in the cookie.

To destroy the session ‘completely’, the session ID must also be unset.

This quick example uses session_destroy() to destroy the session. It uses the set_cookie() method to kill the entirety by expiring the PHP session ID.

<?php

// Always remember to initialize the session,

// even before attempting to destroy it. // Destroy all the session variables.

$_SESSION = array(); // delete the session cookie also to destroy the session

if (ini_get("session.use_cookies")) { $cookieParam = session_get_cookie_params(); setcookie(session_name(), '', time() - 42000, $cookieParam["path"], $cookieParam["domain"], $cookieParam["secure"], $cookieParam["httponly"]);

} // as a last step, destroy the session.

session_destroy();

Note:

1. Use session_start() to reinitiate the session after the PHP session destroy.
2. Use PHP $_SESSION to unset a particular session variable. For an older PHP version, use session_unset().

About this login session_destroy() example

Let’s create a login example code to use PHP session, session_destroy and all. It allows users to login and logout from the current session. Use this code if you are looking for a complete user registration and login in PHP script.

This example provides an automatic login session expiry feature.

Landing page with a login form

This form posts the username and the password entered by the user. It verifies the login credentials in PHP.

On successful login, it stores the logged-in state into a PHP session. It sets the expiry time to 30 minutes from the last login time.

It stores the last login time and the expiry time into the PHP session. These two session variables are used to expire the session automatically.

login.php

<?php

session_start();

$expirtyMinutes = 1;

?>

<html>

<head>

<title>PHP Session Destroy after 30 Minutes</title>

<link rel='stylesheet' href='style.css' type='text/css' />

<link rel='stylesheet' href='form.css' type='text/css' />

</head>

<body> <div class="phppot-container"> <h1>Login</h1> <form name="login-form" method="post"> <table> <tr> <td>Username</td> <td><input type="text" name="username"></td> </tr> <tr> <td>Password</td> <td><input type="password" name="password"></td> </tr> <tr> <td><input type="submit" value="Sign in" name="submit"></td> </tr> </table> </form>

<?php

if (isset($_POST['submit'])) { $usernameRef = "admin"; $passwordRef = "test"; $username = $_POST['username']; $password = $_POST['password']; // here in this example code focus is session destroy / expiry only // refer for registration and login code https://phppot.com/php/user-registration...-download/ if ($usernameRef == $username && $passwordRef == $password) { $_SESSION['login-user'] = $username; // login time is stored as reference $_SESSION['ref-time'] = time(); // Storing the logged in time. // Expiring session in 30 minutes from the login time. // See this is 30 minutes from login time. It is not 'last active time'. // If you want to expire after last active time, then this time needs // to be updated after every use of the system. // you can adjust $expirtyMinutes as per your need // for testing this code, change it to 1, so that the session // will expire in one minute // set the expiry time and $_SESSION['expiry-time'] = time() + ($expirtyMinutes * 60); // redirect to home // do not include home page, it should be a redirect header('Location: home.php'); } else { echo "Wrong username or password. Try again!"; }

}

?>

</div>

</body>

</html>

Dashboard validates PHP login session and displays login, and logout links

This is the target page redirected after login. It shows the logout link if the logged-in session exists.

Once timeout, it calls the destroy-session.php code to destroy all the sessions.

If the 30 minutes expiry time is reached or the session is empty, it asks the user to log in.

home.php

<?php

session_start();

?>

<html>

<head>

<title>PHP Session Destroy after 30 Minutes</title>

<link rel='stylesheet' href='style.css' type='text/css' />

<link rel='stylesheet' href='form.css' type='text/css' />

</head>

<body> <div class="phppot-container">

<?php

if (! isset($_SESSION['login-user'])) { echo "Login again!<br><br>"; echo "<a href='login.php'>Login</a>";

} else { $currentTime = time(); if ($currentTime > $_SESSION['expiry-time']) { require_once __DIR__ . '/destroy-session.php'; echo "Session expired!<br><br><a href='login.php'>Login</a>"; } else { ?> <h1>Welcome <?php echo $_SESSION['login-user'];?>!</h1> <a href='logout.php'>Log out</a>

<?php }

}

?>

</div>

</body>

</html>

This PHP code is used for users who want to log out before the session expiry time.

It destroys the session by requiring the destroy-session.php code. Then, it redirects the user to the login page.

logout.php

<?php

session_start();

require_once __DIR__ . '/destroy-session.php';

header('Location: login.php');

?>

I hope this example helps to understand how to destroy PHP sessions. And also, this is a perfect scenario that is suitable for explaining the need of destroying the session.
Download

↑ Back to Top