Why does the problem of result accumulation occur when using callback functions in Foreach loops? How to solve it?

The cumulative problem and solution of callback functions in foreach loop

This article discusses the cumulative result problems that may arise when using callback functions in foreach loops and provides corresponding solutions.

Problem description

When using foreach loop to iterate over the data and call the callback function getchildareaid to get the sub-region ID, it is found that the results are accumulated. The code is as follows:

 foreach ($towns as $key => $val) {
    $areaidarr = getchildareaid($val['id']);
    $result[$val['name']] = $worker::where('area_id', 'in', $areaidarr)->count();
}

The callback function getchildareaid is defined as follows:

 function getchildaiid($id) {
    static $area;
    $area = $area ?? new \app\common\model\area;
    $result = collection($area->where(['pid' => $id])->order('id desc')->select())->toArray();
    static $res = [];
    if ($result) {
        foreach ($result as $key => $val) {
            $res[] = $val['id'];
            getchildareaiid($val['id']);
        }
    }
    return $res;
}

The problem is that the getchildareaid function uses the static variable $res , which causes the previous result to accumulate every time it is called, rather than to calculate independently each time.

Problem analysis

static $res = []; declares a static variable in getchildareaid function. Static variables will not be destroyed after the function call is finished, and their values ​​will be retained in subsequent calls. Therefore, every time foreach loop calls getchildareaid , $res accumulates new results, which eventually leads to an error in the result.

Solution

To solve this problem and avoid using static variables, the following two methods can be used:

1. Rewrite the function, use local variables: Create a new function, instead of using the static variable $res , but use local variables to store the results of each call.

2. Modify existing functions and eliminate static variables: Modify getchildareaid function, remove static variable $res , and return a new array when called recursively to avoid the accumulation of results. The modified function is as follows:

 function getChildAreaId($id) {
    $area = new \app\common\model\Area;
    $result = collection($area->where(['pid' => $id])->order('id desc')->select())->toArray();
    $res = [];
    if ($result) {
        foreach ($result as $key => $val) {
            $res[] = $val['id'];
            $res = array_merge($res, getChildAreaId($val['id']));
        }
    }
    return $res;
}

This modified function creates a new $res array every time it is called recursively and merges the recursive results into the current $res , thus avoiding the problem of result accumulation. Which solution to choose depends on the overall structure and maintainability of the code. Usually, rewriting functions is clearer and easier to understand.

Through the above methods, the problem of callback function results accumulation in foreach loop can be effectively solved, ensuring that the correct result can be obtained every time the callback function is called.

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