如何过滤数组并获取过滤后的长度？

Question

以典型的json为例：

{ "store": {
    "book": [
      { "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century",
        "price": 8.95
      },
      { "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 12.99
      },
      { "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
      },
      { "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
      }
    ],
    "bicycle": {
      "color": "red",
      "price": 19.95
    }
  }
}

我要过滤图书，获取过滤结果数组大小

我将 json 路径写为：$..book[?(@.category in ['fiction'])].size()

预期值为 3。但实际上我得到了14.

试过 $..book[?(@.category in ['fiction'])].length()

仍然得到相同的结果：14

Answer 1

你为什么不用这样的东西：

int count = 0;
for (Book book : store) {
  count = book.category.equals('fiction')? count+1 : count;
}

Answer 2

您需要使用：

$..book[?(@.category=="fiction")]

很多例子在： https://www.npmjs.com/package/jsonpath

完整的js代码：

'use strict';
const $ = require( "jquery" );
const fs = require('fs');
var jp = require('jsonpath');

var store = { "store": {
    "book": [
      { "category": "reference",
        "author": "Nigel Rees",
        "title": "Sayings of the Century",
        "price": 8.95
      },
      { "category": "fiction",
        "author": "Evelyn Waugh",
        "title": "Sword of Honour",
        "price": 12.99
      },
      { "category": "fiction",
        "author": "Herman Melville",
        "title": "Moby Dick",
        "isbn": "0-553-21311-3",
        "price": 8.99
      },
      { "category": "fiction",
        "author": "J. R. R. Tolkien",
        "title": "The Lord of the Rings",
        "isbn": "0-395-19395-8",
        "price": 22.99
      }
    ],
    "bicycle": {
      "color": "red",
      "price": 19.95
    }
  }
};
var book = store["store"]
var len = jp.query(book, '$..book[?(@.category=="fiction")]').length
console.debug(len)

Answer 3

$.store.book[?(@.category=="fiction")].length()

//Output
[
   4,
   5,
   5
]

将给出每本书中的属性数。这就是 $..book[?(@.category == "fiction")].length()

得到 14 (4+5+5) 的原因

现在，如果您对生成的数组使用长度函数，您将得到 3，这是所需的结果。

$.length($.store.book[?(@.category == "fiction")].length())

在这里试试：Jayway JsonPath Evaluator

这是满足您要求的 work-around，github length() not working as expected

中有许多类似问题的未解决问题

如何过滤数组并获取过滤后的长度？

How to filter an array and get the filtered length?

java

jsonpath