遍历数组并打印特定范围的值？

Question

var months = ["January", "February", "March", "April", "May", "June", "July","August", "September", "October","November", "December"];
// For Loop
for (var i = 0; i = months.slice(4, 8); i++) {
alert(a);

I'm trying to loop through this array and alert the values in a specific range, May - August. I can't seem to figure this out. Thanks!

Answer 1

For a for loop, specify your start index (May = 4) and end index (August = 7), and use this pattern:

var months = ["January", "February", "March", "April", "May", "June", "July","August", "September", "October","November", "December"];
// For Loop
for (var i = 4; i <= 7; i++) {
  alert(months[i]);
}

Answer 2

If you just want those items, and you know the contents of the array, you have a few options:

var months = ["January", "February", "March", "April",
              "May", "June", "July","August", "September",
              "October","November", "December"];

// For Loop
for (var i = 4; i <= 7; i++) {
   alert(months[i]);
}

There's no need to slice while using the for loop. You already know which indices you're interested in, so you can just iterate across that range. It's also the most efficient of these choices, generally.

// Slice and for-loop
var selected = months.slice(4,7);
for (var i = 0; i < selected.length; i++) {
   alert(selected[i]);
}

You could slice first and then iterate across the whole (new) array in a for loop. Generally, you wouldn't do this. However, if your conditions for narrowing the array were more complex, there might be reasons to build the array out first and then loop over it, so here's a clean example of how to do that.

// Slice and forEach
months.slice(4,7).forEach(function(x) {alert(x);});

This would be my personal choice, because it's concise. It's effectively the same as the slice-and-loop example (slightly less performant, but not in a way you're ever likely to need to care about), just expressed using Array.prototype.forEach() instead of a separate for loop.

Answer 3

You need to store the sliced array in a temporary variable and loop over that.

Example: https://jsfiddle.net/epkkstqz/1/

var months = ["January", "February", "March", "April", "May", "June", "July","August", "September", "October","November", "December"];

for (var i = 0, slice = months.slice(4, 8); i < slice.length; i++) {
  alert(slice[i]);
}