PHP recursively finds the minimum value of an array

In PHP, recursion is a very useful technology that can solve many complex problems. When dealing with arrays, recursion can also help us find the minimum value in the array. In this article, we will discuss how to calculate the minimum value of an array in PHP using recursion.

What is recursion?

Recursion is a technique where a function calls itself. In a recursive function, the problem-solving method calls itself to solve smaller sub-problems. When the problem becomes too small to be decomposed any further, the recursive function stops calling itself and returns the result. Recursion is often used to solve complex problems such as traversal of tree structures, graph searches, and sorting and search algorithms.

Recursive implementation

Let us start with a simple example: calculating the sum of an array. We can implement this algorithm using recursion:

function sum($arr){
    if(count($arr) == 0){
        return 0;
    } else {
        $first = array_shift($arr);
        return $first + sum($arr);
    }
}

// 测试
$arr = array(1, 2, 3, 4, 5);
echo sum($arr); // 输出 15

In the above code, we first check if the array is empty. If yes, return 0. Otherwise, we pop the first element in the array and add it with the sum() function recursively calling and passing the rest of the array. This process continues until we have processed the entire array. Finally, we return the results.

Recursive function call stack

Note: Recursive techniques are very useful, but they can also cause problems. This is because each function call adds a new frame to the stack, and the stack size is limited. If the depth of the recursion is too great, the stack may be exhausted. In PHP, by default, the stack size is 1000 function calls. To avoid this, we can use iteration instead of recursion or increase PHP's maximum stack size.

Calculate the minimum value of an array

Next, let’s see how to use recursion to find the minimum value in a PHP array. The idea of ​​implementing this algorithm is similar to calculating the sum of an array:

function findMinimum($arr){
    // 如果数组为空，则返回NULL
    if(count($arr) == 0){
        return NULL;
    } else if(count($arr) == 1){
        // 如果数组只有一个元素，则返回它
        return $arr[0];
    } else {
        // 否则，递归地调用自身，并比较子数组的最小值
        $first = $arr[0];
        $rest = array_slice($arr,1);
        $min = findMinimum($rest);
        if($min < $first){
            return $min;
        } else {
            return $first;
        }
    }
}

// 测试
$arr = array(1, 3, 2, 5, 4);
echo findMinimum($arr); // 输出 1

First, we check the size of the array. If the array is empty, NULL is returned. If there is only one element, return it. Otherwise, we save the first element of the array into the variable $first, and the remaining elements into the variable $rest. Next, we call itself recursively, passing the $rest array as argument. This will return the minimum value of the subarray. Finally, we compare $min and $first and return the minimum of the two.

Summary

In this article, we discussed the method of calculating the minimum value in a PHP array using recursion. Although recursion is a very useful technique, it can also lead to problems such as stack overflows. Therefore, we need to be careful when using recursion. If the possibility of stack overflow is high, we can consider using an iterative algorithm or increasing PHP's maximum stack size.

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