从PI API取数时循环第二轮返回undefined问题排查

我通过PI Api获取圆周率数字，为提升效率，在每个105位的块（5×21位）中检查连续21位数字是否为回文数。测试时我的代码在for循环的第二轮返回undefined，请问这是什么原因？

注：105位块和2轮循环仅用于测试。

import fetch from "node-fetch";
import fs from "fs";

const contentPI = async (start, size) => {
  return await fetch(`https://api.pi.delivery/v1/pi?start=${start}&numberOfDigits=${size}`)
  .then(response => response.json())
  .then(data => data.content);
} 

let start = 1;
let size = 105;
let group = 0;

for (let y = 0; y < 2; y += 1) {
  const pi = await contentPI(start, size);

  let sequence = '';

  for (let i = 0; i < 5; i += 1) {
    let count = 0;
    for (let j = group; j < group + 21; j += 1) {
      sequence += pi[j];
    }
  
    console.log(sequence, ' - ', i, 'seq');
    for (let x = 2; x <= 10; x += 1) {
      if (sequence % x === 0) {
        count++;
      }
    }
    const result = (count > 0) ? 'not prime' : 'prime';
    if (result === 'prime') {
      const reverseSequence = sequence.split('').reverse().join('');
      if (sequence === reverseSequence) {
        console.log(`${sequence} - palindromic prime - position: ${i * 9}`);
      }
    }
    sequence = '';
    group += 21;
    count = 0;
    if (i === 4) {
      start += 105;
    }
  }
}

问题根源 #

group是全局变量，第一轮循环处理完5个21位片段后，group的值会累加为5*21=105。而第二轮循环获取的新PI块长度只有105位（索引范围是0到104），此时用j=105访问pi[j]会得到undefined，导致sequence拼接出包含undefined的字符串，后续逻辑全部出错。

修复方案 #

1. 重置group变量：将group移到外层for循环内部，每次处理新的PI块时重置为0，确保每个105位块都从索引0开始截取片段：

import fetch from "node-fetch";
import fs from "fs";

const contentPI = async (start, size) => {
  return await fetch(`https://api.pi.delivery/v1/pi?start=${start}&numberOfDigits=${size}`)
  .then(response => response.json())
  .then(data => data.content);
} 

let start = 1;
let size = 105;

for (let y = 0; y < 2; y += 1) {
  let group = 0; // 移到此处，每次循环重置为0
  const pi = await contentPI(start, size);

  let sequence = '';

  for (let i = 0; i < 5; i += 1) {
    let count = 0;
    for (let j = group; j < group + 21; j += 1) {
      sequence += pi[j];
    }
  
    console.log(sequence, ' - ', i, 'seq');
    for (let x = 2; x <= 10; x += 1) {
      if (sequence % x === 0) {
        count++;
      }
    }
    const result = (count > 0) ? 'not prime' : 'prime';
    if (result === 'prime') {
      const reverseSequence = sequence.split('').reverse().join('');
      if (sequence === reverseSequence) {
        console.log(`${sequence} - palindromic prime - position: ${i * 9}`);
      }
    }
    sequence = '';
    group += 21;
    count = 0;
    if (i === 4) {
      start += 105;
    }
  }
}

2. 额外优化建议：
   • 增加存在性检查：在拼接sequence时，先判断pi[j]是否存在，避免拼接undefined：if (pi[j]) sequence += pi[j];
   • 修正质数判断逻辑：当前用sequence % x === 0判断质数的方法完全错误——21位数字远超JavaScript安全整数范围，取余运算会失真，且判断质数需要检查到该数的平方根，而非仅到10。

内容的提问来源于stack exchange，提问作者Kelson Batista