Looping through array and removing items without breaking for loop

Question

I have the following for loop  and when I use splice   to remove an item  I then get that  seconds  is undefined   I could check if it s undefined  but I feel there s probably a more elegant way to do this   The desire is to simply delete an item and keep on going     for  i   0  len   Auction auctions length  i  lt  len  i          auction   Auction auctions i       Auction auctions i   seconds   --      if  auction seconds  lt  0             Auction auctions splice i  1

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Auction auctions   Auction auctions filter function el      return --el  seconds    gt  0

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You can just look through and use shift

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This is a pretty common issue  The solution is to loop backwards   for  var i   Auction auctions length - 1  i  gt   0  i--        Auction auctions i  seconds--      if  Auction auctions i  seconds  lt  0             Auction auctions splice i  1             It doesn t matter if you re popping them off of the end because the indices will be preserved as you go backwards

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Recalculate the length each time through the loop instead of just at the outset  e g    for  i   0  i  lt  Auction auctions length  i            auction   Auction auctions i         Auction auctions i   seconds   --        if  auction seconds  lt  0               Auction auctions splice i  1             i--    decrement             That way you won t exceed the bounds   EDIT  added a decrement in the if statement

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Here is a simple linear time solution to this simple linear time problem  When I run this snippet  with n   1 million  each call to filterInPlace   takes  013 to  016 seconds   A quadratic solution  e g  the accepted answer  would take a million times that  or so   x000D   x000D     Remove from array every item such that  condition item   function filterInPlace array  condition       var iOut   0     for  var i   0  i  lt  array length  i         if  condition array i           array iOut      array i      array length   iOut        Try it out   A quadratic solution would take a very long time  var n   1 1000 1000  console log  constructing array       var Auction    auctions       for  var i   0  i  lt  n    i      Auction auctions push  seconds 1      Auction auctions push  seconds 2      Auction auctions push  seconds 0      console log  array length should be    3 n        Auction auctions length  filterInPlace Auction auctions  function auction   return --auction seconds  gt   0     console log  array length should be    2 n        Auction auctions length  filterInPlace Auction auctions  function auction   return --auction seconds  gt   0     console log  array length should be   n       Auction auctions length  filterInPlace Auction auctions  function auction   return --auction seconds  gt   0     console log  array length should be 0     Auction auctions length  x000D   x000D   x000D   Note that this modifies the original array in place rather than creating a new array  doing it in place like this can be advantageous  e g  in the case that the array is the program s single memory bottleneck  in that case  you don t want to create another array of the same size  even temporarily

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If you are e using ES6  - why not just use Array filter method  Auction auctions   Auction auctions filter  auction    gt      auction  seconds   --    return  auction seconds  gt  0        Note that modifying the array element during filter iteration only works for objects and will not work for array of primitive values

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Deleting Parameters         oldJson   firstName  s1  lastName  v1                      firstName  s2  lastName  v2                      firstName  s3  lastName  v3                     newJson   oldJson map      ele     gt              delete ele firstName            return ele                it deletes and and create new array and as we are using spread operator on each objects so the original array objects are also remains unharmed

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Two examples that work    Example ONE     Remove from Listing the Items Checked in Checkbox for Delete let temp products images   store state c products products images if  temp products images    null        for  var l   temp products images length  l--                 mark  is the checkbox field         if  temp products images l  mark    true                store state c products products images splice l 1              THIS WORKS                this  delete store state c products products images l       THIS ALSO WORKS                     Example TWO     Remove from Listing the Items Checked in Checkbox for Delete let temp products images   store state c products products images if  temp products images    null        let l   temp products images length     while  l--                    mark  is the checkbox field         if  temp products images l  mark    true                store state c products products images splice l 1              THIS WORKS                this  delete store state c products products images l       THIS ALSO WORKS

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Try to relay an array into newArray when looping   var auctions   Auction auctions  var auctionIndex  var auction  var newAuctions        for     auctionIndex   0     auctionIndex  lt  Auction auctions length    auctionIndex         auction   auctions auctionIndex      if  auction seconds  gt   0         newAuctions push        auction              Auction auctions   newAuctions

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Here is another example for the proper use of splice  This example is about to remove  attribute  from  array    for  var i   array length  i--         if  array i       attribute             array splice i  1

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Another simple solution to digest an array elements once   while Auction auctions length          From first to last        var auction   Auction auctions shift           From last to first        var auction   Auction auctions pop            Do stuff with auction

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Although your question is about deleting elements from the array being iterated upon and not about removing elements  in addition to some other processing  efficiently  I think one should reconsider it if in similar situation   The algorithmic complexity of this approach is O n 2  as splice function and the for loop both iterate over the array  splice function shifts all elements of array in the worst case   Instead you can just push the required elements to the new array and then just assign that array to the desired variable  which was just iterated upon    var newArray       for  var i   0  len   Auction auctions length  i  lt  len  i          auction   Auction auctions i       auction seconds--      if   auction seconds  lt  0             newArray push auction           Auction auctions   newArray      Since ES2015 we can use Array prototype filter to fit it all in one line   Auction auctions   Auction auctions filter auction   gt  --auction seconds  gt   0

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Give this a try  RemoveItems forEach  i  j    gt        OriginalItems splice  i - j   1

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The array is being re-indexed when you do a  splice    which means you ll skip over an index when one is removed  and your cached  length is obsolete   To fix it  you d either need to decrement i after a  splice    or simply iterate in reverse     var i   Auction auctions length while  i--                if                  Auction auctions splice i  1              This way the re-indexing doesn t affect the next item in the iteration  since the indexing affects only the items from the current point to the end of the Array  and the next item in the iteration is lower than the current point

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There are lot of wonderful answers on this thread already  However I wanted to share my experience when I tried to solve  remove nth element from array  in ES5 context    JavaScript arrays have different methods to add remove elements from start or end  These are   arr push ele  - To add element s  at the end of the array  arr unshift ele  - To add element s  at the beginning of the array arr pop   - To remove last element from the array  arr shift   - To remove first element from the array    Essentially none of the above methods can be used directly to remove nth element from the array       A fact worth noting is that this is in contrast with java iterator s   using which it is possible to remove nth element for a collection   while iterating    This basically leaves us with only one array method Array splice to perform removal of nth element  there are other things you could do with these methods as well  but in the context of this question I am focusing on removal of elements    Array splice index 1  - removes the element at the index    Here is the code copied from original answer  with comments     x000D   x000D  var arr     one    two    three    four    x000D  var i   arr length    initialize counter to array length  x000D   x000D  while  i--    decrement counter else it would run into IndexOutBounds exception x000D    x000D    if  arr i       four     arr i       two     x000D        splice modifies the original array x000D      arr splice i  1     never runs into IndexOutBounds exception  x000D      console log  Element removed  arr      x000D   x000D      else   x000D      console log  Element not removed  arr      x000D      x000D    console log arr   x000D    x000D   x000D   x000D    Another noteworthy method is Array slice  However the return type of this method is the removed elements  Also this doesn t modify original array  Modified code snippet as follows    x000D   x000D  var arr     one    two    three    four    x000D  var i   arr length    initialize counter to array length  x000D   x000D  while  i--    decrement counter  x000D    x000D    if  arr i       four     arr i       two     x000D      console log  Element removed  arr      x000D      console log arr slice i  i   1    x000D      console log  Original array      x000D      console log arr   x000D      x000D    x000D   x000D   x000D    Having said that  we can still use Array slice to remove nth element as shown below  However it is lot more code  hence inefficient     x000D   x000D  var arr     one    two    three    four    x000D  var i   arr length    initialize counter to array length  x000D   x000D  while  i--    decrement counter  x000D    x000D    if  arr i       four     arr i       two     x000D      console log  Array after removal of ith element      x000D      arr   arr slice 0  i  concat arr slice i   1    x000D      console log arr   x000D      x000D   x000D    x000D   x000D   x000D       The Array slice method is extremely important to achieve   immutability in functional programming    la redux

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for  i   0  len   Auction auctions length  i  lt  len  i          auction   Auction auctions i       Auction auctions i   seconds   --      if  auction seconds  lt  0            Auction auctions splice i  1           i--          len--

[javascript] Looping through array and removing items, without breaking for loop

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