Summary: The struct module in Python, taken from the C programming language, lets you convert bytes to and from floating point numbers.

Problem: How to convert bytes to floating point numbers?

A quick look at the solution:

A Brief Introduction to Struct

The struct module has three main methods for data conversion:

• unpack(),
• pack()
• calcsize().

➦ You can use the unpack() method to convert bytes to floating point numbers. The method accepts data format and the byte to be converted.

struct.unpack("f", <byte>)

➦ On the other hand, the pack() method helps in converting data types to bytes.

struct.pack("f", <floating point number>)

Where f stands for floats. As you will see in other parts of this tutorial, the struct module handles different data types such as characters, integers and floats. But before that, you should understand bytes, floating point numbers, and the concept of structs.

Definition Of Bytes And Floating Point Numbers

This section focuses on the roots of bytes and data formats.

Unlike humans that represent numbers in base 10 (0 to 9 digits), computers understand the language of 1s and 0s. A pair of 1 and 0 is a binary digit, shortened as a bit. So, the computer converts data into a series of 1s and 0s (bits) before storing them in the memory.

Likewise, non-numerical data get stored as bytes. For instance, a character occupies 1 byte of memory. An array of characters forms a string.

Now that you understand how a computer interprets various data types, it would be best to learn how the struct module uses them to convert bytes to floating point numbers. Since struct has been taken from the C programming language, hence, a deeper understanding of how it works in C is crucial.

Struct Padding vs Packing

In the C programming language, a struct is a user-defined data type. Unlike other variables, a struct combines different data types in a structure. Here is an example.

struct Fruit { char firstLetter; int total;

};

We are creating a blueprint of a Fruit with a firstLetter character, and total integer. We can create a banana from the Fruit model, assigning it b as firstLetter and 23 as total.

struct Fruit fruit1 = { 'b', 23 };

Printing the size of each attribute,

printf("firstLetter is %d bytes and total is %d bytes \n", sizeof(fruit1.firstLetter),sizeof(fruit1.number));

we get the result as

firstLetter is 1 bytes and total is 4 bytes

The size of fruit1 should be (1 + 4 =) 5 bytes, right? Let’s check.

Size of fruit1 is 8 bytes

It turns out our computer uses more memory to store smaller data quantities. That happens due to a concept called padding.

CPU reads 4 bytes of data per cycle. For the first cycle, it adds three bytes of space to the available (1) byte and returns the result. Next, it finds 4 bytes and returns them. In total, it records (4 + 4 =) 8 bytes.

Padding wastes memory while reducing CPU cycles. Struct packing comes in to store more bytes in less memory.

#include <stdio.h>

#pragma pack(1) struct Fruit { char firstLetter; int number;

}; int main () { struct Fruit fruit1 = { 'b', 23 }; // printf("firstLetter is %d bytes and total is %d bytes \n", sizeof(fruit1.firstLetter),sizeof(fruit1.number)); printf("Size of fruit1 is %d bytes \n", sizeof(fruit1)); return 0;

}

We include #pragma pack(1) header with pack function of value 1 to reduce the memory wastage when compiling and assembling data. This time around, the struct size is what we expect: 1 byte + 4 bytes = 5 bytes.

Size of fruit1 is 5 bytes 

The key takeaway is that struct is a C programming language data structure for storing user-defined data types. Unlike other data types, it combines a continuous stream of different data types. The various data types consume more memory due to padding, something we can control using struct packing.

We can apply the concept of struct to convert data types to bytes in Python, as illustrated below.

How To Use The Struct Module In Python

Unlike C, which speaks to the memory through data types and compiling, Python is a high-level, dynamically-typed programming language. Most operations occur through modules (classes) that get translated and compiled to produce bytes. One such module is the struct module.
The struct module has three methods: pack(), unpack(), and calcsize(). The pack method accepts two parameters: format and data to be converted to bytes. Like the struct blueprint in C, the format part of the pack function in Python accepts various data types. For example,

struct.pack('iif', 2, 4, 7.68)

This means converting integer 2, integer 4 and float 7.68 to a stream of bytes. i stands for an integer while f represents floats.

You can represent repeating integers by a numeral. For example, iii can map to 3i. Also, you can separate the data types with a space. For example, 3i f is another representation for 3if.

We can check the format size by importing the module,

import struct 

and using its calcsize() method.

struct.calcsize('3if')

In the same way, we can convert a floating point number into bytes. Assume we want to convert 3.76 to bytes. We can do that using the following code.

byteData = struct.pack('f', 3.76) print(byteData)

Output:

b'\xd7\xa3p@'

Here, b stands for bytes. The other parts may differ as per computer depending on the memory address system and endianness. Let’s now find floating point numbers from bytes.

Convert Bytes To Floating Point Numbers

The unpack function accepts format, and the byte stream then converts it to a floating point number. For example, we can decode b'\xd7\xa3p@' as follows.

byteData = struct.unpack('f', b'\xd7\xa3p@') print(byteData)

The result is a tuple containing a floating point number with a massive number of decimal points.

(3.759999990463257,)

We can extract the result by surrounding the input with square brackets.

[byteData] = struct.unpack('f', b'\xd7\xa3p@')

The result of printing the output is 3.759999990463257.

The extended decimal output from a reduced input size shows the essence of scientific notation in computing. It also proves the reason for the preference of floating point numbers over integers.

Apart from efficiency, handling floating point numbers comes with speed since much work has gone into building floating point numbers over the years.

Conclusion

The struct module with its unpack() method helps convert bytes to floating point numbers. It would help to understand other methods such as pack() and calcsize because, from them, you can generate bytes from various data types.

Another way to ease handling the conversion is understanding the ins and outs of the struct module, as explained in this tutorial.

• Recommended Reads:
  
  • Converting Integer to String in Python
  • Python String to Float – A Simple Illustrated Guide
  • Converting String to Integer In Python

File upload is an important component in building websites. This article will help you to implement a file upload to the server feature with PHP and a MySQL database.

Example use cases of where the file upload may be needed in a website,

Let us see how to code PHP file upload for a website. You can split this article into the following sections.

1. PHP file upload – A quick example.
2. File upload – server-side validation.
3. Upload file to the server with database.
4. Upload file as a blob to the database.

<?php if (isset($_FILES['upload_file'])) { move_uploaded_file($_FILES["upload_file"]["tmp_name"], $_FILES["upload_file"]["name"]);

}

?>

<form name="from_file_upload" action="" method="post" enctype="multipart/form-data"> <div class="input-row"> <input type="file" name="upload_file" accept=".jpg,.jpeg,.png"> </div> <input type="submit" name="upload" value="Upload File">

</form>

This quick example shows a simple code to achieve PHP file upload. It has an HTML form to choose a file to upload. Let the form with the following attributes for supporting file upload.

1. method=post
2. enctype=multipart/form-data

By choosing a file, it will be in a temporary directory. The $_FILES[“upload_file”][“tmp_name”] has that path. The PHP move_uploaded_file() uploads the file from this path to the specified target.

The form allows multi-file upload by having an array of file input.

PHP file upload configuration

Ensure that the server environment has enough settings to upload files.

• Check with the php.ini file if the file_uploads = on. Mostly, it will be on by default.

More optional directives to change the default settings

• upload_tmp_dir – to change the system default.
• upload_max_filesize – to exceed the default limit.
• max_file_uploads – to break the per-request file upload limit.
• post_max_size – to breaks the POST data size.
• max_input_time – to set the limit in seconds to parse request data.
• max_execution_time – time in seconds to run the file upload script.
• memory_limit – to set the memory limit in bytes to be allocated.

File upload – server-side validation

When file uploading comes into the picture, then there should be proper validation. It will prevent unexpected responses from the server during the PHP file upload.

This code checks the following 4 conditions before moving the file to the target path. It validates,

• If the file is not empty.
• If the file does not already exist in the target.
• If the file type is one of the allowed extensions.
• If the file is within the limit.

It shows only the PHP script for validating the uploaded file. The form HTML will be the same as that of the quick example.

file-upload-validation.php

<?php

if (isset($_POST["upload"])) { // Validate if not empty if (!empty($_FILES['upload_file']["name"])) { $fileName = $_FILES["upload_file"]["name"]; $isValidFile = true; // Validate if file already exists if (file_exists($fileName)) { echo "<span>File already exists.</span>"; $isValidFile = false; } // Validate file extension $allowedFileType = array( 'jpg', 'jpeg', 'png' ); $fileExtension = strtolower(pathinfo($fileName, PATHINFO_EXTENSION)); if (! in_array($fileExtension, $allowedFileType)) { echo "<span>File is not supported. Upload only <b>" . implode(", ", $allowedFileType) . "</b> files.</span>"; $isValidFile = false; } // Validate file size if ($_FILES["upload_file"]["size"] > 200000) { echo "<span>File is too large to upload.</span>"; $isValidFile = 0; } if ($isValidFile) { move_uploaded_file($_FILES["upload_file"]["tmp_name"], $fileName); } } else { echo "No files have been chosen."; }

}

?>

Upload file to the server with database

This section gives a full-fledged PHP file upload example. It is with add, edit, preview, list images features. The add/edit allows users to choose an image file to upload to the server.

The home page displays a list of uploaded images with edit, delete action controls. The edit screen will show the preview of the existing file.

PHP file upload and add a new row to the database

This code is for showing a HTML form with a file upload option. This allows users to choose files to upload to the server.

The PHP code receives the uploaded file data in $_FILES. In this code, it checks for basic file validation to make sure of its uniqueness.

Then, it calls functions to upload and insert the file path to the database.

The uploadImage runs PHP move_upload_files() to put the uploaded file in a directory.

The insertImage calls database handlers to insert the uploaded path to the database.

image-upload-list-preview-edit/insert.php

<?php

namespace Phppot; use Phppot\DataSource;

require_once __DIR__ . '/lib/ImageModel.php';

$imageModel = new ImageModel();

if (isset($_POST['send'])) { if (file_exists('../uploads/' . $_FILES['image']['name'])) { $fileName = $_FILES['image']['name']; $_SESSION['message'] = $fileName . " file already exists."; } else { $result = $imageModel->uploadImage(); $id = $imageModel->insertImage($result); if (! empty($id)) { $_SESSION['message'] = "Image added to the server and database."; } else { $_SESSION['message'] = "Image upload incomplete."; } } header('Location: index.php');

} ?>

<html>

<head>

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

</head>

<body> <div class="form-container"> <h1>Add new image</h1> <form action="" method="post" name="frm-add" enctype="multipart/form-data" on‌submit="return imageValidation()"> <div Class="input-row"> <input type="file" name="image" id="input-file" class="input-file" accept=".jpg,.jpeg,.png"> </div> <input type="submit" name="send" value="Submit" class="btn-link"> <span id="message"></span> </div> </form> <script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script> <script src="assets/validate.js"></script>

</body>

</html> 

List of uploaded images with edit, delete actions

This is an extension of a usual PHP file upload example. It helps to build a file library interface in an application.

This page reads the uploaded images from the database using PHP.  The getAllImages function returns the image path data in an array format.

The list page iterates this array and lists out the result. And it links the records with the appropriate edit, delete controls.

image-upload-list-preview-edit/index.php

<?php

namespace Phppot; use Phppot\DataSource;

require_once __DIR__ . '/lib/ImageModel.php';

$imageModel = new ImageModel();

?>

<html>

<head>

<title>Display all records from Database</title>

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

</head>

<body> <div class="image-datatable-container"> <a href="insert.php" class="btn-link">Add Image</a> <table class="image-datatable" width="100%"> <tr> <th width="80%">Image</th> <th>Action</th> </tr> <?php $result = $imageModel->getAllImages(); ?> <tr> <?php if (! empty($result)) { foreach ($result as $row) { ?> <td><img src="<?php echo $row["image"]?>" class="profile-photo" alt="photo"><?php echo $row["name"]?> </td> <td><a href="update.php?id=<?php echo $row['id']; ?>" class="btn-action">Edit</a> <a on‌click="confirmDelete(<?php echo $row['id']; ?>)" class="btn-action">Delete</a></td> </tr> <?php } } ?> </table> </div> <script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script> <script type="text/javascript" src="assets/validate.js"></script>

</body> </html> 

Edit form with file preview

This shows an edit form with an uploaded file preview. It allows replacing the file by uploading a new one.

The form action passes the id of the record to update the file path in the database.

The PHP code calls the updateImage with the upload result and the record id.

image-upload-list-preview-edit/update.php

<?php

namespace Phppot; use Phppot\DataSource;

require_once __DIR__ . '/lib/ImageModel.php';

$imageModel = new ImageModel();

if (isset($_POST["submit"])) { $result = $imageModel->uploadImage(); $id = $imageModel->updateImage($result, $_GET["id"]);

}

$result = $imageModel->selectImageById($_GET["id"]); ?>

<html>

<head>

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

</head>

<body> <div class="form-container"> <h1>View/Edit image</h1> <form action="?id=<?php echo $result[0]['id']; ?>" method="post" name="frm-edit" enctype="multipart/form-data" on‌submit="return imageValidation()"> <div class="preview-container"> <img src="<?php echo $result[0]["image"]?>" class="img-preview" alt="photo"> <div>Name: <?php echo $result[0]["name"]?></div> </div> <div Class="input-row"> <input type="file" name="image" id="input-file" class="input-file" accept=".jpg,.jpeg,.png" value=""> </div> <button type="submit" name="submit" class="btn-link">Save</button> <span id="message"></span> </div> </form> <script src="https://code.jquery.com/jquery-3.6.0.min.js" integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script> <script src="assets/validate.js"></script>

</body>

</html> 

The delete action is triggered after user confirmation. It removes the file path from the database.

delete.php

<?php

namespace Phppot; use Phppot\DataSource;

require_once __DIR__ . '/lib/ImageModel.php';

$imageModel = new ImageModel();

$id=$_REQUEST['id'];

$result = $imageModel->deleteImageById($id);

header("Location: index.php");

?> 

PHP model class to upload file

It contains functions to upload files to a directory and to the database. The PHP file upload function sets a target to put the uploaded file.

It processes the PHP $_FILES array to get the file data. It prepares the database query by using the file parameter to perform read, write.

imageModel.php

<?php

namespace Phppot; use Phppot\DataSource; class ImageModel

{ private $conn; function __construct() { require_once 'DataSource.php'; $this->conn = new DataSource(); } function getAllImages() { $sqlSelect = "SELECT * FROM tbl_image"; $result = $this->conn->select($sqlSelect); return $result; } function uploadImage() { $imagePath = "uploads/" . $_FILES["image"]["name"]; $name = $_FILES["image"]["name"]; $result = move_uploaded_file($_FILES["image"]["tmp_name"], $imagePath); $output = array( $name, $imagePath ); return $output; } public function insertImage($imageData) { print_r($imageData); $query = "INSERT INTO tbl_image(name,image) VALUES(?,?)"; $paramType = 'ss'; $paramValue = array( $imageData[0], $imageData[1] ); $id = $this->conn->insert($query, $paramType, $paramValue); return $id; } public function selectImageById($id) { $sql = "select * from tbl_image where id=? "; $paramType = 'i'; $paramValue = array( $id ); $result = $this->conn->select($sql, $paramType, $paramValue); return $result; } public function updateImage($imageData, $id) { $query = "UPDATE tbl_image SET name=?, image=? WHERE id=?"; $paramType = 'ssi'; $paramValue = array( $imageData[0], $imageData[1], $_GET["id"] ); $id = $this->conn->execute($query, $paramType, $paramValue); return $id; } /* * public function execute($query, $paramType = "", $paramArray = array()) * { * $id = $this->conn->prepare($query); * * if (! empty($paramType) && ! empty($paramArray)) { * $this->bindQueryParams($id, $paramType, $paramArray); * } * $id->execute(); * } */ function deleteImageById($id) { $query = "DELETE FROM tbl_image WHERE id=$id"; $result = $this->conn->select($query); return $result; }

}

?> 

Upload image as a blob to the database

Though this example comes as the last, I guess it will be very useful for most of you readers.

Uploading the file as a blob to the database helps to move the file binary data to the target database. This example achieves this with very few lines of code.

Uploading image file blob using database insert

This file receives the uploaded file from PHP $_FILES. It extracts the file binary data by using PHP file_get_contents() function.

Then, it binds the file MIME type and the blob to the prepared query statement.

The code specifies the parameter type as ‘b’ for the file blob data. First, it binds a NULL value for the blob field.

Then, it sends the file content using send_long_data() function. This function specifies the query parameter index and file blob to bind it to the statement.

file-blob-upload/index.php

<?php

if (!empty($_POST["submit"])) { if (is_uploaded_file($_FILES['userImage']['tmp_name'])) { $conn = mysqli_connect('localhost', 'root', '', 'blog_eg'); $imgData = file_get_contents($_FILES['userImage']['tmp_name']); $imageProperties = getimageSize($_FILES['userImage']['tmp_name']); $null = NULL; $sql = "INSERT INTO tbl_image_data(image_type ,image_data) VALUES(?, ?)"; $stmt = $conn->prepare($sql); $stmt->bind_param("sb", $imageProperties['mime'], $null); $stmt->send_long_data(1, $imgData); $stmt->execute(); $currentId = $stmt->insert_id; }

}

?> <html>

<head>

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

</head>

<body> <div class="form-container"> <h1>Upload Image Blob</h1> <form action="" method="post" name="frm-edit" enctype="multipart/form-data" > <?php if(!empty($currentId)) { ?> <div class="preview-container"> <img src="image-view.php?image_id=<?php echo $currentId; ?>" class="img-preview" alt="photo"> </div> <?php } ?> <div Class="input-row"> <input type="file" name="userImage" id="input-file" class="input-file" accept=".jpg,.jpeg,.png" value="" required> </div> <input type="submit" name="submit" class="btn-link" value="Save"> <span id="message"></span> </div> </form>

</body>

</html>

This file is to read a blob from the database and show the preview. It will be specified in the <img> tag ‘src’ attribute with the appropriate image id.

file-blob-upload/image-view.php

<?php $conn = mysqli_connect('localhost', 'root', '', 'blog_eg'); if(isset($_GET['image_id'])) { $sql = "SELECT image_type,image_data FROM tbl_image_data WHERE id = ?"; $stmt = $conn->prepare($sql); $stmt->bind_param("i", $_GET['image_id']); $stmt->execute(); $result = $stmt->get_result(); $row = $result->fetch_array(); header("Content-type: " . $row["image_type"]); echo $row["image_data"]; } mysqli_close($conn);

?>

Conclusion

Thus, we have seen a detailed article to learn file upload. I swear we have covered most of the examples on PHP file upload.

We saw code in all levels from simple to elaborate file upload components. I hope, this will be helpful to know how to build this on your own.
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