I'm not stringing you along, honest

"urn:schemas-microsoft-com:office:office" />JScript
and VBScript are often used to build large strings full of formatted text, particularly
in ASP. Unfortunately, naïve string concatenations are a major source of performance
problems.

Before
I go on, I want to note that it may seem like I am contradicting my earlier post,
by advocating some "tips and tricks" to make string concatenation faster. Do
not blindly apply these techniques to your programs in the belief that they will magically
make your programs faster! You always
need to first determine what is fast enough, next determine what is not fast enough,
and THEN try to fix it!

JScript
and VBScript use the aptly-named naïve concatenation
algorithm when building strings. Consider this silly JScript example:

var str
= "";

for (var
count = 0 ; count < 100 ; ++count)

  str
= "1234567890" + str;

The
result string has one thousand characters so you would expect that this would copy
one thousand characters into str.
Unfortunately, JScript has no way of knowing ahead of time how big that string is
going to get and naïvely assumes that every concatenation is the last one.

On
the first loop str is
zero characters long. The concatenation produces a new ten-character string and copies
the ten-character string into it. So far ten characters
have been copied. On the second time through
the loop we produce a new twenty-character
string and copy two ten-character strings into it. So far 10 + 20 = 30 characters
have been copied.

You
see where this is going. On the third time thirty more
characters are copied for a total of sixty,
on the fourth forty more
for a total of one hundred. Already the
string is only forty characters long and we have copied more than twice that many
characters. By the time we get up to the hundredth iteration over
fifty thousand characters have been copied to make that one thousand character string.
Also there have been ninety-nine temporary strings allocated and immediately thrown
away.

Moving
strings around in memory is actually pretty darn fast, though 50000 characters is
rather a lot. Worse, allocating and releasing
memory is not cheap. OLE Automation has
a string caching mechanism and the NT heap is pretty good about these sorts of allocations,
but still, large numbers of allocations and frees are going to tax the performance
of the memory manager.

If
you're clever, there are a few ways to make it better. (However,
like I said before, always make sure you're spending time applying cleverness in the
right place.)

One
technique is to ensure that you are always concatenating small strings to small strings,
large strings to large strings. Pop quiz:
what's the difference between these two programs?

for (var
count = 0 ; count < 10000 ; ++count)

  str
+= "1234567890" + "hello";

and

For count
= 1 To 10000

  str
= str & "1234567890" & "hello"

Next

? I
once had to debunk a widely distributed web article which claimed that VBScript was
orders of magnitude slower than JScript because the comparison that the author used
was to compare the above two programs. Though
they produce the same output, the JScript program is MUCH faster. Why's
that? Because this is not an apples-to-apples
comparison. The VBScript program is equivalent
to this JScript program:

for (var
count = 0 ; count < 10000 ; ++count)

  str
= (str + "1234567890") + "hello";

whereas
the JScript program is equivalent to this JScript program

for (var
count = 0 ; count < 10000 ; ++count)

  str
= str + ("1234567890" + "hello");

See,
the first program does two concatenations of a small string to a large string in one
line, so the entire text of the large string gets moved twice every time through the
loop. The second program concatenates
two small strings together first, so the small strings move twice but the large string
only moves once per loop. Hence, the
first program runs about twice as slow. The
number of allocations remains unchanged, but the number of bytes copied is much lower
in the second.

In
hindsight, it might have been smart to add a multi-argument string concatenation opcode
to our internal script interpreter, but the logic actually gets rather complicated
both at parse time and run time. I still
wonder occasionally how much of a perf improvement we could have teased out by adding
one. Fortunately, as you'll see below,
we came up with something better for the ASP case.

The
other way to make this faster is to make the number of allocations smaller, which
also has the side effect of not moving the bytes around so much.

var str
= "1234567890"; 10

str =
str + str; 20

var str4
= str + str; 40

str =
str4 + str4; 80

str =
str + str; 160

var str32
= str + str; 320

str =
str32 + str32; 640

str =
str + str32 960

str =
str + str4; 1000

This
program produces the same result, but with 8 allocations instead of 100, and only
moves 3230 characters instead of 50000+. However,
this is a rather contrived example -- in the real world strings are not usually composed
like this!

Now,
those of you who have written programs in languages like C where strings are not first-class
objects know how to solve this problem efficiently. You
build a buffer that is bigger than the string you want to put in, and fill it up. That
way the buffer is only allocated once and the only copies are the copies into the
buffer. If you don't know ahead of time
how big the buffer is, then a double-when-full strategy is quite optimal -- pour stuff
into the buffer until it's full, and when it fills up, create a new buffer twice as
big. Copy the old buffer into the new
buffer and continue. (Incidentally, this
is another example of one of the "no worse than 200% of optimal" strategies that I
was discussing earlier -- the amount of used memory is never more than twice the size
of the memory needed, and the number of unnecessarily copied bytes is never more than
twice the size of the final buffer.)

Another
strategy that you C programmers probably have used for concatenating many small strings
is to allocate each string a little big, and use the extra space to stash a pointer
to the next string. That way concatenating
two strings together is as simple as sticking a pointer in a buffer. When
you're done all the concatenations, you can figure out the size of the big buffer
you need, and do all the allocations and copies at once. This
is very efficient, wasting very little space (for the pointers) in common scenarios.

Can
you do these sorts of things in script? Actually,
yes. Since JScript has automatically
expanding arrays you can implement a quick and dirty string builder by pushing
strings onto an array, and when you're done, joining
the array into one big string. In VBScript
it's not so easy because arrays are fixed-size, but you can still be pretty clever
with fixed size arrays that are redimensioned with a "double when full" strategy. But
surely there is a better way than these cheap tricks.

Well,
in ASP there is. You know, I used to
see code like this all the time:

str =
"<blah>"

str =
str + blah

str =
str + blahblah

str =
str + whatever

' etc,
etc, etc -- the string gets longer and longer, we have some loops, etc.

str =
str + "</blah>"

Response.Write str

Oh,
the pain. The Response object
is an efficient string buffer written in C++ . Don't
build up big strings, just dump 'em out into the HTML stream directly. Let
the ASP implementers worry about making it efficient.

"Hold
on just a minute. Mister Smartypants Lippert," I hear you say, "Didn't
you just tell us last week that eliminating calls to COM objects is usually a better
win than micro-optimizing small stuff like string allocations? "

Yes,
I did. But in this case, that advice
doesn't apply because I know something you don't know.

We
realized that all the work that the ASP implementers did to ensure that the string
buffer was efficient was being overshadowed by the inefficient late-bound call to Response.Write. So
we special-cased VBScript so that it detects when it is compiling code that contains
a call to Response.Write and
there is a named item in the global namespace called Response that
implements IResponse::Write