Re: Is there a faster way to fetch data from stl map?

Steve555 <>
Fri, 26 Dec 2008 22:30:56 -0800 (PST)
On 27 Dec, 04:13, ""
<> wrote:

There are a few ways you can optimize this; all depend on what exactly
you're doing with the data:

On Dec 26, 1:09 pm, Steve555 <> wrote:

On 26 Dec, 16:38, LR <> wrote:

Steve555 wrote:

On 26 Dec, 14:50, wrote:

On Dec 26, 8:21 am, Steve555 <> wrote:

I'm retrieving data from a map of size approx 500,000, containing
structs and with an integer key.
I've been using myMap[myKey] most of the time as it's convenie=

nt and

is clear to read what's happening. But in a tight loop (2million+=


it's taking considerably longer to fetch values from a map like t=


than it is to do a large amount of number-crunching on the values
I've tried
data = myMap.find(myKey)->second;
and that helps a tiny bit, reducing the loop time from about 2.0 =


1.5 seconds, but without accessing the map at all the loop time i=


<0.1, so the bulk of the time is spent retrieving the data.
I'm quite prepared to re-write my code from the ground up if this=


best I can get from a map, but hoped there may be another way.

No, find() is the best way if you are trying to search for an
arbitrary key from an stl map.
In reality, it may only be doing something like 18 comparisons to =


the data. 1.5 seconds for the loop, where you are doing 2,000,0=

00 (or

more) look-ups means each look-up is averaging less than 750
nanoseconds. Just paging through the memory might be taking so=

me of

the time (cache issues). Many systems are memory bound instead=


processor bound. (i.e. once the data is in cache, thing fly along)

I would immediately ask two questions:
1) What comparison operator are you using? Is it comparing m=


member variables, or just one POD type?
2) How large are your objects? Each time though the loop you=


copying the object ("data = "). Could this be replaced with a=


reference on the lhs? If so, this "may" help some speedup

Joe Cook

Thanks Joe. The map is sorted by the integer key as in

typedef map<long, myObject*, less<long> > myMap;

so I guess this means the comparison operator is the default
operator< for longs. Is that what you meant?

Sorry my initial description was not quite right, the contents are
pointers to a custom class, which are large and contain a few array=


so they're approx 800 bytes.
In another post I was advised that storing values is more efficient
than storing pointers (which I generally do now), but in this case =


overhead of passing and copying such big objects will be too high.

Do they get copied much? Do they get passed by value and not by re=


I'm curious to know what your loop looks like. If you're not using=


iterator as the loop index and if that would work for your applicatio=


is it possible that would be better?

data = myMap.find(myKey)->second;

How do you initialize myKey?

You say the size() of your map is around 500k, but you imply that you=


going through the loop around 2m times.

Is it the case that every call to myMap.find(myKey) will not return

Would it be possible for you to post a small, compilable example of y=


code showing your problem? Because I feel a little fuzzy on the detai=



Thanks LR, it's a huge project so I can't extract a compilable version
but this is the essence of it:


struct SongRating
        long userID;
        double rating;};

//For each song,a list of users and their rating
typedef vector<SongRating> SongRatingVector;
typedef map<long, SongRatingVector*, less<short> >SongMap;//key=so=



class MyUserClass
{ ... contains about 800bytes of data describing each user }
// map of users, Key = UserID (this is the map I need to optimise)
typedef map<long, MyUserClass*, less<long> > UsersByID;

(At this point I am accessing: UsersByID usersByIDMap, SongMap
*mySongMap )

MyUserClass *userPtr;
long songID, count=0;
SongRatingVector *songRatings;
SongRating *thisRatingPtr;
SongRatingVector::iterator rateIter;
SongMap::const_iterator songIter;

//loop through all 5000+/- songs
for(songIter = mySongMap->begin();songIter!=mySongMap->end();+
  songID = songMapIter->first;
  songRatings = probeIter->second;
  //for each song loop through 400+/- rating
  for(rateIter = songRatings->begin();rateIter!=songRatings->end(=

); +

      thisRatingPtr = &*ratingsIter;
// Find the userID associated with this rating
      userID = thisRatingPtr->userID;
//Find one of 500,000 users with this userID
      userPtr = usersByIDMap.find(userID)->second;
// Do some numerical checks on the user's member data
       if(CheckUserSuitable(thisUser) == true)


=====================>>How do you=
 initialize myKey?

You say you load all data from a text file. What is the format of your
text file? You may consider changing the format a bit to optimize
things. For example, your inner loop above tries to map userID ->
MyUserClass* for 500,000 iterations for every user. This is a
bottleneck. You could save a lookup in a 500,000 entry map by simply
changing SongRating to this:

struct SongRating {
  MyUserClass *user;
  double rating;


Now you never have to look up the user ID in the map. However, this
means that when loading the text file, you have to load the user data
first, and then fill these structures as you load the song rating
data, doing the user map lookup there. Depending on how many times you
run through your huge loop below, doing the lookup (for every song and
rating) *once* when you load the file, rather than more than once,
will save you a lot of time.

Then, you can optimize your text file format to eliminate user map
lookups even on load. For example, say your text file is (for

<songid> <song info...>
<songid> <song info...>
<songid> <song info...>
begin user <userid> <user info...>
  <songid> <rating>
  <songid> <rating>
  <songid> <rating>
  <songid> <rating>
end user

You are now explicitly associating SongRatings with users in your text
file. This eliminates any user map lookups on load at all. Loading is
simply this:

  1. Load all song information into songID -> song map.
  2. Load all user information, for each user:
     - Create MyUserClass and load user info into it.
     - Create entry in userID -> user map (if you even need to).
     - Create the SongRatings, and you already know exactly which
MyUserClass they're associated with!

The key there is grouping song ratings by user in the data file means
you always know precisely which user is associated with a song rating,
as you load it, without having to look that user up by ID.

That's the biggest optimization I can think of given how you say you
are using the data. It eliminates all userID -> MyUserClass lookups

That is only one suggestion. Read on!

There is another possible optimization you can make. It appears that
you are examining every single rating for every single song, and
counting the total number of ratings made by a given user where
CheckUserSuitable(user) is true.

Well, CheckUserSuitable depends only on the user. It does not depend
on the song or the rating. So why iterate through all the songs and
ratings in an outer loop? You say the data never changes, so simply
count the number of SongRatings each user is associated with on load,
store that count in MyUserClass, and *all* you need to do to compute
'count' is iterate through your users and sum up all the counts for
users where CheckUserSuitable is true:

class MyUserClass {
  int getNumberOfSongRatings () const;


You can store the users in a map, or even a vector, if it turns out
you don't need to actually do the userID -> MyUserClass lookups any
more; here I'll use a map:

map<long,MyUserClass *> users;
map<long,MyUserClass *>::const_iterator user;
int count = 0;

for (user = users.begin(); user != users.end(); ++ user)
  if (CheckUserSuitable((*user).second))
    count += (*user).second->getNumberOfSongRatings();

However, note that if you're counting the number of song ratings each
user has on load, you still may have to do some map lookups on load.
Well, if you change your text file format to the example I gave above,
counting song ratings per user becomes trivial -- since song ratings
are grouped per user, you do not need to do any map lookups at all to
count song ratings as you load them and store that count in

In general, it sounds like you have a huge amount of data, but you're
not storing it in a way that is optimal for what you're actually doing
with it. You need to look at what operations are the most time
consuming, and the figure out how to get rid of them: Looking up users
in the user map is time consuming, to get rid of it you can store the
MyUserClass* instead of the user ID in SongRating. To optimize that,
you can modify your text data format accordingly. Iterating over every
song and rating when CheckUserSuitable depends on neither is also time
consuming, you can do some "preprocessing" on load by counting the
number of song ratings per user to eliminate those loops from the

There are plenty of other easy optimizations you can make depending on
how you are accessing the data. For example, if you believe you will
frequently be looking up users with the same ID, then cache, say, the
last 10 userID -> MyUserClass* entries, and search your cache, hitting
the map only if...

read more =BB

Thanks Jason

struct SongRating {
  MyUserClass *user;
  double rating;


Fantastic, dunno why I didn't think of that! Just this change has made
enough difference on it's own. The loop is about 0.3 seconds now.

Thanks for the suggestions about loading the database. I should add
the disclaimer that being fairly new to this I can never quite judge
how much information to add when trying to describe the core of my
problem in a newsgroup post, so I left out a few details which now
seem relevant.
=95 The database is only loaded once on program startup (takes about 10
seconds), everything from then on is done from ram.
=95 The program is experimenting with lots of statistical and A.I.
techniques to ascertain if it's possible to measure how good a
particular user is at rating songs, and, whether some users can act as
reliable predictors for other users. CheckUserSuitable() is doing the
bulk of this analysis, which I didn't show as I thought it was a bit
off-topic. However, from yours and LR's posts it's clear that on load
I could create various copies of stripped-down vectors reflecting
different subsets of the data in the big UsersByID Map.
Again, thanks for your post, there are so many ideas there that apply
to other parts of the project where I'm throwing this data around, I'd
got tunnel-vision and didn't think of creating other subset maps and
vectors as the need arises.
Using MySQL is tempting, as are the speeds you quoted, but interfacing
it with my AI code is complicated. This is as much a learning exercise
as anything else, so with this group's help I feel better that I know
what's happening in the code with no black boxes.


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