css to find prime numbers

CSS Find prime numbers

Prime numbers refer to natural numbers that can only be divided by 1 and itself. In programming, finding prime numbers is a common requirement and an important factor in program efficiency.

Recently, an interesting question has arisen: How to use CSS to find all prime numbers less than or equal to a certain number?

When you first hear this question, you may feel confused. After all, CSS is a style sheet language used to beautify web pages. The syntax and features of CSS have nothing to do with mathematical operations and do not even support logical calculations.

However, this does not prevent us from discussing this issue. In fact, CSS can also use some special techniques to achieve some seemingly impossible tasks when rendering the page.

Next, let’s take a look at how to use CSS to find all prime numbers less than or equal to a certain number.

Step 1: Determine the target number

First, we need to determine the required range of prime numbers. In this article, we take 100 as an example, that is, find all prime numbers less than or equal to 100.

Step 2: Generate a list of numbers

Next, we need to use CSS to generate a list of numbers. Since CSS does not support loops and logical judgments, we can only use some special properties of CSS to achieve a loop-like effect.

Here, we are using the CSS counter property, which can help us automatically generate a number sequence. In order to generate a sequence of numbers from 2 to 100, we need to set a counter counter in CSS, starting from 2 and incrementing by 1 each time. The initial value and maximum value of the counter are set to 2 and 100 respectively. The code is as follows:

body {
  counter-reset: counter 2;
}

The meaning of this code is: set a counter named counter for the body element, with an initial value of 2.

Next, we need to set up a counter for each numeric element. In order to generate a sequence of numbers from 2 to 100, we can use the following method:

.numbers {
  counter-increment: counter;
}

The meaning of this code is: increase the value of the counter named counter, adding 1 each time.

At this point, we have generated a calculator for the sequence of numbers, and the list of numbers can be implemented through the CSS pseudo-element ::before. In the previous code, we set a class named numbers for the body, and then we added a child element for each element in numbers. The code is as follows:

.numbers::before {
  content: counter(counter) ' ';
}

The meaning of this code is: add a ::before pseudo-element in front of the number element to display the current value of the counter, which is the number.

With the above CSS code, we can generate a number sequence from 2 to 100.

Step 3: Screen prime numbers

Now, we have generated a sequence of numbers from 2 to 100, but it contains many non-prime numbers. In order to filter out prime numbers, we can use CSS attribute selector.

The attribute selector can select matching elements by specifying the attribute name and attribute value. Here, we can specify the data-prime attribute of the numeric element as prime, indicating that the element is a prime number. The code is as follows:

.numbers[data-prime='prime'] {
  color: red;
}

According to the definition of prime numbers, a number is prime only if it cannot be divided by any prime number less than the number. Suppose the current number is n, we can use the attribute selector to find all prime numbers less than n. The code is as follows:

.numbers:not([data-prime='not-prime']):not(:nth-of-type(1)):before {
  content: counter(counter) ' ';
  display: none;
}

The meaning of this code is: If the current number element is not a non-prime number (that is, it cannot be divided by any prime number less than the number), and it is not the first element of the number sequence ( That is 2), then hide the number in the ::before pseudo-element.

In order to highlight the prime numbers, we can also set a red color for them. The code is as follows:

.numbers[data-prime='prime'] {
  color: red;
}

Step 4: Result display

Finally, we only need to add a container element with a number sequence to the HTML to see the final result. The code is as follows:

<div class="numbers" data-prime="prime">2</div>
<div class="numbers" data-prime="prime">3</div>
<div class="numbers" data-prime="prime">4</div>
<div class="numbers" data-prime="prime">5</div>
...

Run the above code, you can see all prime numbers less than or equal to 100 in the browser.

Conclusion

Although using CSS to find prime numbers is a strange question, it does demonstrate the power of CSS. By using the counter attribute and attribute selectors, we can implement a program that can generate a sequence of numbers and filter prime numbers without using JavaScript.

Of course, this method is more troublesome and less efficient. If you want to find a relatively large prime number, it is recommended to use a computer language, such as JavaScript, to achieve it. However, in some special cases, such as in CSS animations, it is useful to use this technique.

In short, programming itself is a process of exploration, and different programming languages ​​have different characteristics, advantages and disadvantages. Only by learning to use the features of these languages ​​can you truly unleash their power.

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