Question

请有人可以帮助我了解为什么该程序的输出是：

def group_by_owners(files): dt = {} for i,j in files.items(): if j in dt.keys(): dt[j] = dt[j].append(i) # Re-assigning the element. This is where the issue is present. else: dt[j]=[i] return dt files = { 'Input.txt': 'Randy', 'Code.py': 'Stan', 'Output.txt': 'Randy' } print(group_by_owners(files))

而不是预期的输出：

Stopped at: NaN

谢谢

这段代码是用JS ES6编写的

Stopped at: 3

Answer 1

存在参考问题，已更正代码。

function StopWatch(){
    var me = this;
    this.watchtime = 0;
    this.started = 0;
    this.start = function(){
        if(this.started){
            d = new Date()
            console.log("Cannot start watch twice. Current time is", (d.getTime() - this.watchtime)/1000)
        }else if(this.watchtime == 0){
            this.started = 1
            d = new Date()
            this.watchtime = d.getTime()
        }
        else{
            d = new Date()
            console.log("Stopwatch re-started from", (d.getTime() - this.watchtime)/1000)
            this.watchtime = d.getTime() - this.watchtime
            this.started = 1
        }
    }
    this.stop = function(){
        if (this.started == 0){
            console.log("Stopwatch has not yet been started")
        }else{
            d = new Date()
            this.total_time = (d.getTime() - me.watchtime)/1000
            console.log("Stopped at:", this.total_time)
            this.started = 0
        }
    }
}
sw = new StopWatch;
sw.start();
setTimeout(sw.stop, 3000);

创建变量me，并用this对其进行初始化，然后在stop函数中，将this.watchtime替换为me.watchtime

Answer 2

使用ES6时，使用Arrow函数将解决参考问题。

function StopWatch() {
  this.watchtime = 0;
  this.started = 0;
  this.start = () => {
    if (this.started) {
      d = new Date()
      console.log("Cannot start watch twice. Current time is", (d.getTime() - this.watchtime) / 1000)
    } else if (this.watchtime == 0) {
      this.started = 1
      d = new Date()
      this.watchtime = d.getTime()
    } else {
      d = new Date()
      console.log("Stopwatch re-started from", (d.getTime() - this.watchtime) / 1000)
      this.watchtime = d.getTime() - this.watchtime
      this.started = 1
    }
  }
  this.stop = () => {
    if (this.started == 0) {
      console.log("Stopwatch has not yet been started")
    } else {
      d = new Date()
      this.total_time = (d.getTime() - this.watchtime) / 1000
      console.log("Stopped at:", this.total_time)
      this.started = 0
    }
  }
}
sw = new StopWatch
sw.start()
setTimeout(sw.stop, 3000)

如果您不了解Arrow函数，请阅读here。

JavaScript中的秒表-错误的输出

2 个答案: