How to sum all values in std::map<std::string, size_t> collection without using for loop? The map resides as private member in a class. Accumulation is performed in public function call.
I do not want to use boost or other 3rd parties.
- 1I'd like to swim the atlantic without getting in the water? Or do you just want to avoid writing the for loop yourself?bmargulies– bmargulies2012-12-28 18:19:42 +00:00Commented Dec 28, 2012 at 18:19
- I'm looking LINQ like functions as in C#Chesnokov Yuriy– Chesnokov Yuriy2012-12-28 18:20:51 +00:00Commented Dec 28, 2012 at 18:20
- 1They use loops, it's just not so evident.Puppy– Puppy2012-12-28 18:26:05 +00:00Commented Dec 28, 2012 at 18:26
- I know behind the scenes is the loop. I want to use shortcut with just one line of codeChesnokov Yuriy– Chesnokov Yuriy2012-12-28 18:26:53 +00:00Commented Dec 28, 2012 at 18:26
- 9Why is this question closed as not constructive? It's a good question.SmallChess– SmallChess2015-12-02 01:02:05 +00:00Commented Dec 2, 2015 at 1:02
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2 Answers
You can do this with a lambda and std::accumulate. Note you need an up to date compiler (at least MSVC 2010, Clang 3.1 or GCC 4.6):
#include <numeric> #include <iostream> #include <map> #include <string> int main() { const std::map<std::string, std::size_t> bla = {{"a", 1}, {"b", 3}}; const std::size_t result = std::accumulate( std::begin(bla), std::end(bla), 0, // initial value of the sum [](const std::size_t previous, const std::pair<const std::string, std::size_t>& p) { return previous + p.second; } ); std::cout << result << "\n"; } If you use C++14, you can improve the readability of the lambda by using a generic lambda instead:
[](const std::size_t previous, const auto& element) { return previous + element.second; } 
4 Comments
Chesnokov Yuriy
#include <numeric> is missing
rubenvb
@ChesnokovYuriy fixed. GCC didn't catch that, thanks.
kebs
Great example for introducing lambdas !
Tigran Saluev
For those trying to shorten the function call, try
std::accumulate(bla.begin(), bla.end(), 0, [](const size_t previous, decltype(*bla.begin()) p) { return previous+p.second; });.Use std::accumulate. But it very likely will use loop behind the scenes.
8 Comments
Chesnokov Yuriy
how to use it? can you provide code sample?
Tomek
No. I have to feed my children now. Look for 4 parameter version.
Chesnokov Yuriy
Do you want your post to be marked as answer? I do not want to declare additional function to the 4th parameter and use some inline binding
rubenvb
@ChesnokovYuriy then use a lambda.
Chesnokov Yuriy
sure :) @BillyONeal did not know how to use lambda
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