Why does the variable in the two-dimensional array value of php class report an error?

In PHP, we can use classes to define some complex data types to achieve more flexible code modularization and reuse, and two-dimensional arrays are one of the commonly used data types in PHP. Can be used to store complex structured data. But sometimes when using classes and two-dimensional arrays, you may encounter errors caused by variables contained in the values. The following will discuss how to solve this problem through case analysis and solutions.

Case Background

In a certain PHP project, some student information needs to be stored in a two-dimensional array and grouped according to grade and class. First, a class named Student is defined, the code is as follows:

class Student {
    public $name;
    public $age;
    public $grade;
    public $class;
}

Then a two-dimensional array named $students is defined to store multiple Student instances, the code is as follows:

$students = [
    ['name' => '张三', 'age' => 18, 'grade' => 1, 'class' => 1],
    ['name' => '李四', 'age' => 19, 'grade' => 1, 'class' => 2],
    ['name' => '王五', 'age' => 18, 'grade' => 2, 'class' => 1],
    ['name' => '赵六', 'age' => 19, 'grade' => 2, 'class' => 2]
];

Next , these student information need to be grouped by grade and class, and then count the number of people in each group. The code is as follows:

$count = [];
foreach ($students as $student) {
    $count[$student['grade']][$student['class']]++;
}

But when running the above code, you will find an error message:

PHP Warning:  Undefined offset: 1

Problem Analysis

In the above code, the $count variable is a two-dimensional array. $student['grade'] and $student['class'] are used to obtain the grade and class information of each student. These two information are used as the secondary subscripts of the $count array to implement group statistics of student information. However, in the $count array, the corresponding second-level subscripts start counting from 0, while the grade and class information in $student['grade'] and $student['class'] both start counting from 1. , therefore, a deviation occurs when accessing a two-dimensional array.

In addition, the value in the $count array is initially null and the value type is integer. Therefore, when adding 1 to it, value type conversion will occur and an error will occur. This is also one of the causes of errors.

Solution

In order to solve the above problem, the code needs to be adjusted appropriately. Specifically, the values ​​of $student['grade'] and $student['class'] can be reduced by 1 respectively to adapt to the subscript requirements of the $count array. At the same time, the $count array needs to be initialized to avoid type conversion errors.

The refactored code is as follows:

$count = [];
foreach ($students as $student) {
    $grade = $student['grade'] - 1;
    $class = $student['class'] - 1;
    if (!isset($count[$grade][$class])) {
        $count[$grade][$class] = 0;
    }
    $count[$grade][$class]++;
}

In this new code, we first decrease the values ​​​​of $student['grade'] and $student['class'] by 1 respectively. , and then determine whether the corresponding secondary subscript exists in the $count array. If it does not exist, it is initialized to 0. Afterwards, when adding 1 to the $count array, there will be no type conversion errors and out-of-bounds subscript problems.

Conclusion

In PHP, classes and arrays are commonly used data types, but their complexity and flexibility sometimes lead to some problems. Starting from an example, this article analyzes in detail the reasons why variables in values ​​​​in classes and arrays report errors, as well as how to adjust and improve solutions. I hope this content can be inspiring and helpful to PHP developers.

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