在javascript中从数组中删除项目

Question

This code is supposed to remove numbers from an array. The expected output of the code below should be the strings "cat", "dog", and "bird" logged to the console. The actual output is actually leaves the console blank. Note that for the website I am programming this on, removeItem() is the correct notation for removing an array item.

var animals = ["cat", "dog", 2, 11, "bird", 13];
var i;

for (i = 0; ((isNaN(animals[i])) == false) && (i < animals.length - 1); i++) {
  removeItem(animals, i);
  console.log(animals);
}

Answer 1

The problem is the second argument to for needs to be the MAX value for the loop not what you have.

var animals = ["cat", "dog", 2, 11, "bird", 13];
var i;

for (i = 0; i<animals.length; i++) {
  if(isNaN(animals[i])){
    removeItem(animals, i);
  }
}

console.log(animals);

for a complete replacement this will replace your need for the removeItem function also

var animals = animals.filter(function(el) {
    return el.length && el==+el;
});

Answer 2

There are two conceptual issues here. First, you have a condition on a specific value in the loop's condition. In other words, the loop will terminate when the first number is encountered. Second, you're modifying the array while iterating over it, which will probably mess up your result.

Using a filter call would address both issues and also clean up your code:

animals = animals.filter(x => isNaN(x));

Answer 3

The for loop should look something like this:

for (i = 0; i < animals.length; i++) {
    // check if isNaN inside here then remove..
}

But I would suggest using filter() instead.

var animals = ['cat', 'dog', 2, 11, 'bird', 13];
var result = animals.filter(item => isNaN(item));
console.log(result);

Answer 4

The reason the loop never runs is because the first element is not a number

therefore the condition in the for loop fails on the first test

You need to move the test for NaN inside the loop

Assuming removeItem(animals, i); mutates the array, you'll also have another issue

If you remove an item in the array, then everything to the right of the removed item shifts one to the left ... but the for loop will increase i, therefore the next element will be skipped

so, whenever you have 2 numbers in a row, the second will not be removed - ie in this case, the 11

Ignoring the mis-placed isNaN test, you also loop until i < animals.length - 1 - which means you'd never test the last element - so in this case, the 13 will not be removed either

 function removeItem(arr, i) { arr.splice(i, 1); } var animals = ["cat", "dog", 2, 11, "bird", 13]; var i; for (i = 0; i < animals.length - 1; i++) { if (!isNaN(animals[i])) { removeItem(animals, i); } } console.log(animals);

In the above, when i == 2 , the value of animals[i] is 2 ... this gets removed ... now i == 2 still, and animals[i] is 11 ... but before the next iteration of the loop occurs, i becomes 3 and the 11 value slips through the code

You could simply put --i in the if condition after removeItem ... or even use removeItem(animals, i--) - that way removeItem is called with the current value of i, which then gets decreased, then increased by the i++ before the next iteration

That's could look misleading to some people, but works

 function removeItem(arr, i) { arr.splice(i, 1); } var animals = ["cat", "dog", 2, 11, "bird", 13]; var i; for (i = 0; i < animals.length; i++) { if (!isNaN(animals[i])) { removeItem(animals, i--); } } console.log(animals);

A cleaner and easier way to fix that is to iterate through the array from end to start

 function removeItem(arr, i) { arr.splice(i, 1); } var animals = ["cat", "dog", 2, 11, "bird", 13]; var i; for (i = animals.length - 1; i >= 0; --i) { if (!isNaN(animals[i])) { removeItem(animals, i); } } console.log(animals);

Answer 5

There is some issue that needs to be changed:

• Loop backwards throught the array because you are removing items.
• Create a condition to remove desidered items.
• Log the result outside the loop.

After it your code will be like this:

var animals = ["cat", "dog", 2, 11, "bird", 13];

for (var i = animals.length - 1; i >= 0; i--) {
    if (isNaN(animals[i])) { 
        removeItem(animals, i);
    }
}

console.log(animals);