Ruby - Array.join versus String Concatenation (Efficiency)

Question

I recall getting a scolding for concatenating Strings in Python once upon a time. I was told that it is more efficient to create an List of Strings in Python and join them later

Answer 1

Yes, it's the same principle. I remember a ProjectEuler puzzle where I tried it both ways, calling join is much faster.

If you check out the Ruby source, join is implemented all in C, it's going to be a lot faster than concatenating strings (no intermediate object creation, no garbage collection):

/*
 *  call-seq:
 *     array.join(sep=$,)    -> str
 *  
 *  Returns a string created by converting each element of the array to
 *  a string, separated by sep.
 *     
 *     [ "a", "b", "c" ].join        #=> "abc"
 *     [ "a", "b", "c" ].join("-")   #=> "a-b-c"
 */

static VALUE
rb_ary_join_m(argc, argv, ary)
    int argc;
    VALUE *argv;
    VALUE ary;
{
    VALUE sep;

    rb_scan_args(argc, argv, "01", &sep);
    if (NIL_P(sep)) sep = rb_output_fs;

    return rb_ary_join(ary, sep);
}

where rb_ary_join is:

 VALUE rb_ary_join(ary, sep)
     VALUE ary, sep;
 {
     long len = 1, i;
     int taint = Qfalse;
     VALUE result, tmp;

     if (RARRAY(ary)->len == 0) return rb_str_new(0, 0);
     if (OBJ_TAINTED(ary) || OBJ_TAINTED(sep)) taint = Qtrue;

     for (i=0; ilen; i++) {
     tmp = rb_check_string_type(RARRAY(ary)->ptr[i]);
     len += NIL_P(tmp) ? 10 : RSTRING(tmp)->len;
     }
     if (!NIL_P(sep)) {
     StringValue(sep);
     len += RSTRING(sep)->len * (RARRAY(ary)->len - 1);
     }
     result = rb_str_buf_new(len);
     for (i=0; ilen; i++) {
     tmp = RARRAY(ary)->ptr[i];
     switch (TYPE(tmp)) {
       case T_STRING:
         break;
       case T_ARRAY:
         if (tmp == ary || rb_inspecting_p(tmp)) {
         tmp = rb_str_new2("[...]");
         }
         else {
         VALUE args[2];

         args[0] = tmp;
         args[1] = sep;
         tmp = rb_protect_inspect(inspect_join, ary, (VALUE)args);
         }
         break;
       default:
         tmp = rb_obj_as_string(tmp);
     }
     if (i > 0 && !NIL_P(sep))
         rb_str_buf_append(result, sep);
     rb_str_buf_append(result, tmp);
     if (OBJ_TAINTED(tmp)) taint = Qtrue;
     }

     if (taint) OBJ_TAINT(result);
     return result;
}