app crash when there's more than 3000 entries in std::map ?

Hi everyone I have this strange behaviour happening with this code which I can't explain. On my computer when I set nt with a value greater than 3000 it crashes. Is there a max number of keys I can use with a std::map ? thanks a lot #include <vector> #include <iostream> #include <map> #include <string> int main() { float t = clock(); int nt = 3000; #if 1 hash_map<std::string, int, hash<std::string> > mymap; for ( int i = 0; i < nt; ++i ) { string tmp = "test" + i; mymap[ tmp ] = i; } for ( int i = 0; i < 10e5; ++i ) { int a = (int)(drand48() * nt); string tmp = "test" + a; hash_map<std::string, int, hash<std::string> >::iterator it = mymap.find( tmp ); if ( it != mymap.end() ) { } else { printf("not found\n"); } } unsigned max_size = mymap.max_size(); printf("%d\n", max_size ); #else std::map<std::string, int> mymap; for ( int i = 0; i < nt; ++i ) { string tmp = "test" + i; mymap[ tmp ] = i; } for ( int i = 0; i < 10e5; ++i ) { int a = (int)(drand48() * nt); string tmp = "test" + a; std::map<std::string, int>::iterator it = mymap.find( tmp ); if ( it != mymap.end() ) { } else { printf("not found\n"); } } unsigned max_size = mymap.max_size(); printf("%d\n", max_size ); #endif printf("time %f\n", (clock() - t ) / float( CLOCKS_PER_SEC ) ); return 0; } #endif

Sorry you need to add #include <ext/hash_map> using namespace __gnu_cxx; namespace __gnu_cxx { // hash specialisation to allow hashing of strings template<> struct hash<std::string> { size_t operator()(const std::string &__s) const { return __stl_hash_string(__s.c_str()); } }; } // namespace __gnu_cxx OR std at the top if you want this to compile. But hash_map and map behaves the same using c++ (GCC) 4.1.2 20080704 (Red Hat 4.1.2-44) On Aug 12, 6:33=A0pm, mast4as <mast...@yahoo.com> wrote: > Hi everyone > > I have this strange behaviour happening with this code which I can't > explain. On my computer when I set nt with a value greater than 3000 > it crashes. Is there a max number of keys I can use with a std::map ? > > thanks a lot > > #include <vector> > #include <iostream> > #include <map> > > #include <string> > > int main() > { > =A0 =A0 =A0 =A0 float t =3D clock(); > =A0 =A0 =A0 =A0 int nt =3D 3000; > #if 1 > =A0 =A0 =A0 =A0 hash_map<std::string, int, hash<std::string> > mymap; > =A0 =A0 =A0 =A0 for ( int i =3D 0; i < nt; ++i ) > =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 string tmp =3D "test" + i; > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 mymap[ tmp ] =3D i; > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 for ( int i =3D 0; i < 10e5; ++i ) > =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 int a =3D (int)(drand48() * nt); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 string tmp =3D "test" + a; > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 hash_map<std::string, int, hash<std::stri= ng> >::iterator it =3D > mymap.find( tmp ); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if ( it !=3D mymap.end() ) > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 else > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 printf("not found\n"); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 unsigned max_size =3D mymap.max_size(); > =A0 =A0 =A0 =A0 printf("%d\n", max_size ); > > #else > =A0 =A0 =A0 =A0 std::map<std::string, int> mymap; > =A0 =A0 =A0 =A0 for ( int i =3D 0; i < nt; ++i ) > =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 string tmp =3D "test" + i; > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 mymap[ tmp ] =3D i; > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 for ( int i =3D 0; i < 10e5; ++i ) > =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 int a =3D (int)(drand48() * nt); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 string tmp =3D "test" + a; > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 std::map<std::string, int>::iterator it = =3D mymap.find( tmp ); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 if ( it !=3D mymap.end() ) > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 else > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 printf("not found\n"); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 } > > =A0 =A0 =A0 =A0 unsigned max_size =3D mymap.max_size(); > =A0 =A0 =A0 =A0 printf("%d\n", max_size ); > > #endif > =A0 =A0 =A0 =A0 printf("time %f\n", (clock() - t ) / float( CLOCKS_PER_SE= C ) ); > > =A0 =A0 =A0 =A0 return 0; > > } > > #endif

On Aug 12, 12:33=A0pm, mast4as <mast...@yahoo.com> wrote: > Hi everyone > > I have this strange behaviour happening with this code which I can't > explain. On my computer when I set nt with a value greater than 3000 > it crashes. Is there a max number of keys I can use with a std::map ? > It's because this doesn't do what you think it does: > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 string tmp =3D "test" + i; It does *not* make a string named "test1", "test2", etc. Instead, it does pointer arithmetic on the const char* that points to "test". So ("test" + 0) is a pointer to the string "test" ("test" + 1) is a pointer to the string "est" ("test" + 2) is a pointer to the string "st" etc. Once i passes the null byte in the string literal, though, you're into some random chunk of memory. All bets are off. Eventually you're reading outside of memory that belongs to your application and you get a segfault. --Jonathan

On Aug 12, 6:49=A0pm, Jonathan Lee <jonathan.lee....@gmail.com> wrote: > On Aug 12, 12:33=A0pm, mast4as <mast...@yahoo.com> wrote: > > > Hi everyone > > > I have this strange behaviour happening with this code which I can't > > explain. On my computer when I set nt with a value greater than 3000 > > it crashes. Is there a max number of keys I can use with a std::map ? > > It's because this doesn't do what you think it does: > > > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 string tmp =3D "test" + i; > > It does *not* make a string named "test1", "test2", etc. > Instead, it does pointer arithmetic on the const char* > that points to "test". > > So > =A0 =A0("test" + 0) is a pointer to the string "test" > =A0 =A0("test" + 1) is a pointer to the string "est" > =A0 =A0("test" + 2) is a pointer to the string "st" > > etc. > > Once i passes the null byte in the string literal, though, > you're into some random chunk of memory. All bets are off. > Eventually you're reading outside of memory that belongs > to your application and you get a segfault. > > --Jonathan Thanks a lot yes ... I just figured (long day at work ;-( Thank you so much std::string createRandomString() { char alpha[ 6 ] =3D { 'a', 'b', 'c', 'd', 'e', 'f' }; char str[ 10 ]; for ( int i =3D 0 ; i < 10 ; i++ ) { int a =3D int(drand48()*5); str[ i ] =3D a; } return std::string( str ); } int main() { float t =3D clock(); int nt =3D 10000; #if 1 hash_map<std::string, int, hash<std::string> > mymap; for ( int i =3D 0; i < nt; ++i ) { string tmp =3D createRandomString(); mymap[ tmp ] =3D i; } for ( int i =3D 0; i < 10e5; ++i ) { int a =3D (int)(drand48() * nt); string tmp =3D createRandomString(); hash_map<std::string, int, hash<std::string> >::iterator it =3D mymap.find( tmp ); if ( it !=3D mymap.end() ) { } else { //printf("not found\n"); } } unsigned max_size =3D mymap.max_size(); printf("%d\n", max_size ); #else std::map<std::string, int> mymap; for ( int i =3D 0; i < nt; ++i ) { string tmp =3D createRandomString(); mymap[ tmp ] =3D i; } for ( int i =3D 0; i < 10e5; ++i ) { int a =3D (int)(drand48() * nt); string tmp =3D createRandomString(); std::map<std::string, int>::iterator it =3D mymap.find( tmp ); if ( it !=3D mymap.end() ) { } else { //printf("not found\n"); } } unsigned max_size =3D mymap.max_size(); printf("%d\n", max_size ); #endif printf("time %f\n", (clock() - t ) / float( CLOCKS_PER_SEC ) ); return 0; } #endif

On Aug 12, 12:56=A0pm, mast4as <mast...@yahoo.com> wrote: > Thanks a lot yes ... I just figured (long day at work ;-( > > Thank you so much > std::string createRandomString() > { > =A0 =A0 =A0 =A0 char alpha[ 6 ] =3D { 'a', 'b', 'c', 'd', 'e', 'f' }; > =A0 =A0 =A0 =A0 char str[ 10 ]; > =A0 =A0 =A0 =A0 for ( int i =3D 0 ; i < 10 ; i++ ) > =A0 =A0 =A0 =A0 { > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 int a =3D int(drand48()*5); > =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 str[ i ] =3D a; > =A0 =A0 =A0 =A0 } > =A0 =A0 =A0 =A0 return std::string( str ); > > } No problem. BTW, if you want to avoid these random strings you could probably use next_permutation from <algorithm> to get some string names. Make a char array of distinct letters, permute them, and use string::append to get your dummy string. (Not entirely related, just something I've found useful before) --Jonathan

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