Strange array element assignment behavior

Question

Today I came across some strange behavior of an Array element:

arr = ["a","b"] arr2 = [1,2] arr.unshift(arr2) #= [[1, 2], "a", "b"] arr.push(arr2) #=> ["a", "b", [1, 2]] 

This makes sense, however:

 arr[0,0] = arr2 #=> [1, 2, "a", "b"] 

I know that in [0,0] first zero is index , and the second is the number of elements that will be executed in this array, starting from index .

In my opinion, it should be the same as unshift , but it is not.

Can someone explain the behavior?

Answer 1

If we dive into the ruby ​​source code, we will find a function called rb_ary_splice when an array with three arguments is assigned (for example, index, length and new value):

 static VALUE rb_ary_aset(int argc, VALUE *argv, VALUE ary) { long offset, beg, len; if (argc == 3) { rb_ary_modify_check(ary); beg = NUM2LONG(argv[0]); len = NUM2LONG(argv[1]); rb_ary_splice(ary, beg, len, argv[2]); return argv[2]; } [...] 

And if we follow in rb_ary_splice , we will have a place where the magic happens:

 static void rb_ary_splice(VALUE ary, long beg, long len, VALUE rpl) { long rlen; long olen; if (len < 0) rb_raise(rb_eIndexError, "negative length (%ld)", len); olen = RARRAY_LEN(ary); [...] if (len != rlen) { RARRAY_PTR_USE(ary, ptr, MEMMOVE(ptr + beg + rlen, ptr + beg + len, VALUE, olen - (beg + len))); ARY_SET_LEN(ary, alen); } if (rlen > 0) { MEMMOVE(RARRAY_PTR(ary) + beg, RARRAY_CONST_PTR(rpl), VALUE, rlen); } } RB_GC_GUARD(rpl); } 

First, it creates a space in the array for the new elements and updates the length:

 RARRAY_PTR_USE(ary, ptr, MEMMOVE(ptr + beg + rlen, ptr + beg + len, VALUE, olen - (beg + len))); ARY_SET_LEN(ary, alen); 

Then, through the magic of C pointers, he inserts the new element (s):

 MEMMOVE(RARRAY_PTR(ary) + beg, RARRAY_CONST_PTR(rpl), VALUE, rlen);