PHP Algorithm Exercise 7: Judgment Method for Implementing Geometric Sequences

In "PHP Algorithm Exercise 6: Determine whether a value appears three times in a row in an array", we introduce to you how to determine whether a value appears three times in a row in an array. This idea is also It’s worth learning. Interested friends can find out~

The topic of today’s article is how to use PHP to determine whether a set of data is a geometric sequence? It may be a little difficult for PHP newbies, but I believe that after reading this article, you will have a clear understanding of such issues.

First of all, everyone needs to know what a geometric sequence is:

A geometric sequence means that starting from the second term, the ratio of each term to its previous term is equal to the same constant. A kind of sequence, commonly represented by G and P. This constant is called the common ratio of the geometric sequence. The common ratio is usually represented by the letter q (q≠0), and the geometric sequence a1≠0. Each item in {an} is not 0. Note: When q=1, an is a constant column.

For example, the sequence: 2, 4, 8, 16,······

The ratio of each item to the previous one: 4÷2=8÷4=16÷8 =2, so this sequence is a geometric sequence, and its common ratio is 2.

The general term formula and summation formula of a geometric sequence are as follows:

After briefly understanding what a geometric sequence is, we will get to the topic.

The following is the judgment method I gave to realize the geometric sequence:

<?php
function is_geometric($arr)
{
    if (sizeof($arr) <= 1)
        return True;
    # 计算比率
    $ratio = $arr[1]/$arr[0];

    # 检查剩余的比例
    for($i=1; $i<sizeof($arr); $i++)
    {
        if (($arr[$i]/($arr[$i-1])) != $ratio)
        {
            return "不是等比数列";
        }
    }
    return "是等比数列";
}
$my_arr1 = array(2, 6, 18, 54);
$my_arr2 = array(10, 5, 2.5, 1.20);

print_r(is_geometric($my_arr1)."<br>");
print_r(is_geometric($my_arr2));

The judgment result is:

是等比数列
不是等比数列

In the above code, I gave two Judgment arrays are $my_arr1 = array(2, 6, 18, 54); and $my_arr2 = array(10, 5, 2.5, 1.20);.

Obviously $my_arr1 is a geometric sequence, and its common ratio is 3. And $my_arr2 is not a geometric sequence.

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