CODEWITHSUNDEEP
javascriptarraysslice
Björn
Björn

Reputation: 13207

Extract x items from array (slice) but continue from beginning if end is reached

Lets say I have:

var array = [0,1,2,3,4,5,6,7,8,9]

I define:

var itemsToExtract  = 5
var startindex = 7
var direction = "forward"

I want to be able to do:

array.someMethod(startindex, itemsToExtract, direction)

and get

[7,8,9,0,1]

I also want it to work backwards, if I set direction to "backward" (slicing from right to left).

I wasnt too lazy and tried something already, see here: http://jsfiddle.net/MkNrr/

I am looking for something "tidier" if there is, also is there a name for this method, is it a known problem?

Background: I am trying to build a sequential pre image loader (load one image(src) after the other) for use in an image gallery. Maybe even such a libary already exists?

Upvotes: 6

Views: 1346

Answers (6)

omma2289
omma2289

Reputation: 54619

My approach:

function overslice(array, startindex, count, direction) {
    var retarray = [];
    var increment = (direction === 'backward') ? -1 : 1;

    for(var c=0, i = startindex; c<count; i+=increment, c++){
        retarray.push(array[(i + array.length)%array.length]);
    }
    return retarray;
}

Working fiddle

UPDATE

Another version using the count variable to dictate the direction using positive/negative values and with a fix to allow use of count larger than the array length:

function overslice(array, startindex, count) {
    var retarray = [];
    var increment = (count >= 0) ? 1 : -1;
    count = Math.abs(count);
    for(var i = startindex, c = 0;c<count;i+=increment, c++){
        if(i<0) i= array.length-1;  
        retarray.push(array[i%array.length]);
    }
    return retarray;
}

Demo fiddle

Upvotes: 1

rab
rab

Reputation: 4144

Hi try this code .

function myslice( ar , start , count , dir ) {

    dir = dir || 'F';

    var out = [] ,
        times = Math.ceil( ( start + count )/ ar.length  ) + 1;

    while( times-- ) {
        [].push.apply( out, ar );
    }

    if ( dir == 'B' ) {
        start = ar.length - start - 1;
        out = out.reverse() ;
    }

    return out.slice( start  , start+count );
}

myslice ( [0,1,2,3,4,5,6,7,8,9] , 7 , 5 , 'B' );

Upvotes: 0

yaoxing
yaoxing

Reputation: 4183

To filter invalid positions, I add some validations, without which the code can be even shorter.

var arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
var countToExtract = 10
var whereToBegin = -1
var direction = "backword"

function overslice(array, startIndex, count, direction) {
    var retArray = [];
    var step = direction == "backword" ? -1 : 1;
    var startPos = (startIndex + array.length) % array.length;
    var endPos = (startPos + (count % array.length) * step + array.length) % array.length;

    for (var i = 0; i < count; i++) {
        var pos = (startPos + (i * step) % array.length + array.length) % array.length;
        retArray.push(array[pos]);
    }
    return retArray;
}

var lampa = overslice(arr, whereToBegin, countToExtract, direction)

alert(lampa)

with code above, you can:
begin from a minus position, which will count back from the other end.
count can be longer than array length, which will return you numbers repeatedly.

Upvotes: 0

elclanrs
elclanrs

Reputation: 94101

How about a function that returns the forward and back arrays? That way you don't need a switch. Something like this:

function overslice(arr, idx, items) {

  var fwd = arr.filter(function(v,i){ return i >= idx }).slice(0, items),
      back = arr.filter(function(v,i){ return i <= idx }).slice(-items);

  while (fwd.length < items) {
    fwd = fwd.concat(arr).slice(0, items);
  }

  while (back.length < items) {
    back = arr.concat(back).slice(-items);
  }

  return { fwd: fwd, back: back };
}

Then you can use it like:

var array = [0,1,2,3,4,5,6,7,8,9]
var result = overslice(array, 7, 5);

console.log(result.fwd, result.back); //=> [7, 8, 9, 0, 1] [3, 4, 5, 6, 7]

Upvotes: 1

Roland Jansen
Roland Jansen

Reputation: 2783

Just create copy of the array and append the original array until it is long enough:

var arr = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
var start = 7
var count = 5
var tmparr = arr

while (tmparr.length < start + count) {
   tmparr = tmparr.concat(arr);
}

var result = tmparr.slice(start, start + count);

alert(result);

Upvotes: 0

Akhil Sekharan
Akhil Sekharan

Reputation: 12683

Its not much, but looks fine JS Bin:

function overSlice(arr, start, count, dir){
  if(dir==='backward'){
      arr = arr.reverse();
      start = arr.length-start-1;
  }
   var lastIndex = start+count;
  return arr.slice(start,  lastIndex>arr.length?arr.length: lastIndex)
  .concat(arr.slice(0,  lastIndex>arr.length?lastIndex-arr.length: 0));
}

var arr = [0,1,2,3,4,5,6,7,8,9];
alert(overSlice(arr,7,5,'backward'));

Upvotes: 0

Related Questions