Saturday, December 15, 2007


I've heard people talking about the std::transform functions for ages, but I didn't really get around to exploring them until recently. What they bring to the table is a compact yet powerful way of performing a batch operations on stl containers, much like matlab's matrix math routines but with way more potential. You can perform common vector arithmetic such as addition, subtraction etc. but also select certain components of a container, such as the routine below that in one single line prints all the hash values of a std::map. Furthermore, by writing your code in std::transform notation you're very likely unleashing a higher degree of optimization potential in your compilers, both current and future ones. For example, the Intel compiler (I'm running 9.1 atm) immediately starts spewing out "LOOP WAS AUTO-PARALLELIZED" all over the place, which generally hasn't been too common a sight for me in the past.

Now for a couple examples, copied straight from :

Print all of a map's keys.
int main()
map M;
M[1] = 0.3;
M[47] = 0.8;
M[33] = 0.1;

transform(M.begin(), M.end(), ostream_iterator(cout, " "),
// The output is 1 33 47.
Each element in V3 will be the difference of the corresponding elements in V1 and V2
const int N = 1000;
vector V1(N);
vector V2(N);
vector V3(N);

iota(V1.begin(), V1.end(), 1);
fill(V2.begin(), V2.end(), 75);

assert(V2.size() >= V1.size() && V3.size() >= V1.size());
transform(V1.begin(), V1.end(), V2.begin(), V3.begin(),

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