# Inserting some zeros in a list by a rule of positions

I have a list:

mainlist={0.23, 0.34, 0.8, 0.0, -0.2, 0.4, -0.1};


I have to extend the above list to another one finalresult with Length=17. But in finalresult the elements of the mainlist must be in the positions ruled by the positionlist:

positionlist={3,4,8,9,10,13,14};

finalresult={0,0,0.23,0.34,0,0,0,0.8,0.0,-0.2,0,0,0.4,-0.1,0,0,0}


How can I reach this goal? I would be so glad to see an answer.

• I expect somebody to reference a duplicate, but you can use finalresult = ConstantArray[0, 17]; finalresult[[positionlist]]=mainlist in the meantime. – Carl Woll Aug 26 '17 at 14:23
• Related: (30339), (30647), (31648), (41877), (147876) – Mr.Wizard Aug 26 '17 at 21:01
• @CarlWoll I looked but I was unable to find a duplicate. Perhaps you would care to convert your comment to an answer, in the mean time. :^) – Mr.Wizard Aug 26 '17 at 21:33

Normal@SparseArray[Thread[positionlist -> mainlist], 17, 0]


Update: should emphasize C.E.'s nice comment:

Normal@SparseArray[positionlist -> mainlist, 17]


equivalent in result, more pleasantly laconic, and lack of explicit thread actually less expensive for longer lists.

• +1. Thread and the zero at the end can be omitted. – C. E. Aug 26 '17 at 14:27
• A usage note regarding the performance of this method: (148112) – Mr.Wizard Aug 26 '17 at 21:15
mainlist={0.23, 0.34, 0.8, 0.0, -0.2, 0.4, -0.1};
positionlist={3,4,8,9,10,13,14};


finalresult = ConstantArray[0, 17];