使用for循环遇到Array Splice的问题

Question

Above code splices only first element it doesn't work second time in for loop! Please help!

var answeredQuestions = [18,19];
var questions = [{"questionid":"18"},{"questionid":"19"},{...}];

for (var j = 0; j < questions.length; j++) {
    var pos = $.inArray(parseInt(questions[j].questionid), answeredQuestions);
    if(parseInt(pos) != -1) {
        questions.splice(j,1);
    }
}

Answer 1

When you modify the array (remove items from it) in the middle of the for loop, it causes your for loop to miss items in the array.

One way to work-around that is to process the array backwards (from end to front) so that when you remove the current item from the array, you aren't messing up any of the indexes in the next iterations of the for loop.

var answeredQuestions = [18,19];
var questions = [{"questionid":"18"},{"questionid":"19"},{...}];

for (var j = questions.length - 1; j >= 0; j--) {
    var pos = $.inArray(parseInt(questions[j].questionid, 10), answeredQuestions);
    if(pos !== -1) {
        questions.splice(j,1);
    }
}

Also, there is no need to use parseInt() on the result of $.inArray as it is already an integer.

---

Edit as of 2015/2016 , it's probably easier to use .filter() and let that method handle the modifying of the array for you:

var answeredQuestions = [18,19];
var questions = [{"questionid":"18"},{"questionid":"19"},{...}];
questions = questions.filter(function(item) {
    // keep only questions that haven't been answered already
    return answeredQuestions.indexOf(+item.questionid) === -1;
});

Answer 2

What you want to do is just filter the answered questions. You could use .filter :

var answeredQuestions = [18,19];
var questions = [{"questionid":"18"},{"questionid":"19"},{...}];

var result = questions.filter(function(e) {
  return answeredQuestions.indexOf(+e.questionid) == -1;
});