How to use nested arrays with a for loop in javascript

问题

My goal for this is to be able to access properties of a nested array in a for loop, for example given the array.

[[1 , "one"],
[0.2 , "two"],
[0.3 , "three"]]

I'd like to be able to access index 0 over a loop.

My current way of doing this is:

function printArray() {
  for (var i = arguments.length; i >= 0; i--) {
    print(arguments[i][0]);
  }
}

How ever this produces this error

Uncaught TypeError: Cannot read property '0' of undefined

When I expected

0.3
0.2
1

What is the correct way of looping over a nested array?

回答1:

There are a couple of ways to loop through an array.

Which one is the best?

It depends on what it's used for, and subjectivity.

Here's a few of them.

let arr = [[1 , "one"], [0.2 , "two"], [0.3 , "three"]];

// forEach loop
console.log('forEach loop');
arr.forEach((x, idx) => {
    console.log(x[0]);
});

//for of loop
console.log('for-of loop');
for (elem of arr){
   console.log(elem[0]);
}

//for in loop
console.log('for-in loop');
for (idx in arr){
    console.log(arr[idx][0]);
}

// old-school for loop
console.log('for loop');
for (let i = 0; i < arr.length; i++) { 
    console.log(arr[i][0]); 
}

// while loop
console.log('while loop');
let i = 0;
while (i < arr.length) {
    console.log(arr[i][0]);
    i++;
}

// do while loop
console.log('do while loop');
let j = 0;
do {
    console.log(arr[j][0]);
    j++;
} while (j < arr.length);

But about your function.
1) it's better to pass the array as a variable.
2) the last index of an array is it's length - 1

So if you want something that even works in older browsers.

To loop in ascending order of the index:

function printArray(arr) {
  for (var i = 0; i < arr.length; i++) {
    print(arr[i][0]);
  }
}

To loop in descending order of the index:

function printArray(arr) {
  for (var i = arr.length - 1; i >= 0; i--) {
    print(arr[i][0]);
  }
}

回答2:

Use for/of for iterables so you don't have to even worry about index dereferencing.

function printArray () {
  for (const arg of [...arguments]) {
    for (const nestedArg of arg) {
      console.log(nestedArg);
    }
  }
};

printArray([
  [1, "one"],
  [0.2, "two"],
  [0.3, "three"]
]);

回答3:

In the first iteration is i=3, that's past the array boundaries if used as index

let args = [
  [1, "one"],
  [0.2, "two"],
  [0.3, "three"]
];

for (let i = args.length - 1; i >= 0; i--) {
  console.log(args[i][0]);
}

回答4:

arguments is array of arguments pass to the function, in your case you should point to the first element of the argument array like this:

let data = [[1 , "one"],
    [0.2 , "two"],
    [0.3 , "three"]];


function printArray() {
  for (var i = arguments[0].length -1 ; i >= 0; i--) {
    console.log(arguments[0][i][0]);
  }
}

printArray(data);

The error :

Uncaught TypeError: Cannot read property '0' of undefined

come form arguments.length, that will cause index out of bound array error

来源：https://stackoverflow.com/questions/60045889/how-to-use-nested-arrays-with-a-for-loop-in-javascript

标签

javascript

arrays

multidimensional-array