Question

我编写了一个简单的javascript程序来执行递归。由于某些原因，它无法正常运行。在递归的一级，当我应该获得cd时，我得到了bcd。

任何帮助将不胜感激。

function recursion_test(i_s) {
    console.log('starting string:' + i_s);
    if (i_s.length <= 1) {
        console.log('base case: ' +i_s);
        return
    } else {
        midPoint = Math.floor(i_s.length /2);
        console.log('mid:' + midPoint, 'l: ' + i_s.substring(0,midPoint), ' r: '+ i_s.substring(midPoint, i_s.length));
        recursion_test(i_s.substring(0, midPoint));
        recursion_test(i_s.substring(midPoint, i_s.length));
    }

}

var start_str = "abcd";
console.log("Done: " + recursion_test(start_str));


The starting string:abcd
rec_test:32 mid:2 l: ab  r: cd
rec_test:26 starting string:ab
rec_test:32 mid:1 l: a  r: b
rec_test:26 starting string:a
rec_test:28 base case: a
rec_test:26 starting string:b
rec_test:28 base case: b
rec_test:26 starting string:bcd
rec_test:32 mid:1 l: b  r: cd
rec_test:26 starting string:b
rec_test:28 base case: b
rec_test:26 starting string:cd
rec_test:32 mid:1 l: c  r: d
rec_test:26 starting string:c
rec_test:28 base case: c
rec_test:26 starting string:d
rec_test:28 base case: d
rec_test:40 Done: undefinedoutput:

Answer 1

您应该在let之前使用midPoint，否则它将成为全局变量

function recursion_test(i_s) {
    console.log('starting string:' + i_s);
    if (i_s.length <= 1) {
        console.log('base case: ' +i_s);
        return
    } else {
        let midPoint = Math.floor(i_s.length /2);
        console.log('mid:' + midPoint, 'l: ' + i_s.substring(0,midPoint), ' r: '+ i_s.substring(midPoint, i_s.length));
        recursion_test(i_s.substring(0, midPoint));
        recursion_test(i_s.substring(midPoint, i_s.length));
    }

}
var start_str = "abcd";
console.log("Done: " + recursion_test(start_str));

Answer 2

这是因为您定义的变量midPoint是全局变量。

        recursion_test(i_s.substring(0, midPoint));
        recursion_test(i_s.substring(midPoint, i_s.length));

上面的第一个recursion_test会将midPoint的值更改为1，从而使第二个函数（其中i_s=='abcd'）以'abcd'.substring(1, 4) -> bcd作为输入。

要解决此问题，只需在变量var之前添加midPoint

Answer 3

function Write(str){
     console.log(str);
}

function getData(s,start,end=0){
   if (end == 0) {
       end = s.length;
   }
   return s.substring(start, end)
}

function getMidPoint(s) {
  var midPoint = Math.floor(s.length / 2);
 var left  = getData(s,0, midPoint);
 var right = getData(s,midPoint);
  Write('mid:' + midPoint + ' L: ' + left + ' R: ' + right );
  return midPoint;
}

function recursion_test(i_s) {
  Write(`Starting String: ` + i_s);
  if (i_s.length <= 1) {
    Write('base case: ' + i_s);
  } else {
    let midPoint = getMidPoint(i_s);
    let left = getData(i_s,0,midPoint);
    let right = getData(i_s,midPoint);
    let arr = [left,right];
    arr.forEach(function(e) {
        recursion_test(e);
    });
    return "All Done";
  }

}
var start_str = "abcd";
Write(recursion_test(start_str));

此解决方案只是对代码进行重构，因此一目了然。另外，请注意，用let声明的变量仅存在于块范围内。

不了解JS递归

3 个答案: