Use class method as `op` argument to `accumulate`

Question

I'm trying to create a class that calculates its variance from a vector<float>. It should do this by using its previously calculated this->mean in diffSquaredSum. I'm trying to call the method diffSquaredSum inside of accumulate but have no idea what the magical syntax is.

What is the correct syntax to use the diffSquaredSum class method as the op argument to accumulate in setVariance?

float diffSquaredSum(float sum, float f) {
    // requires call to setMean
    float diff = f - this->mean;
    float diff_square = pow(diff,2);
    return sum + diff_square;
}

void setVariance(vector<float>& values) {
    size_t n = values.size();
    double sum = accumulate(
        values.begin(), values.end(), 0, 
        bind(this::diffSquaredSum));
    this->variance = sum / n;
}

When I call it with just `diffSquaredSum` I get `no matching function for call to ‘accumulate(std::vector::iterator, std::vector::iterator, int, )` — Chris Redford, May 13 '14 at 00:41
possible duplicate of [C++ vector accumulates](http://stackoverflow.com/questions/8227610/c-vector-accumulates) — Bryan Chen, May 13 '14 at 00:49
This question is not a duplicate because it uses the op argument of accumulate. It is not a duplicate of others that do use op because it is using a class method as op. The proposed duplicate doesn't even use op. — Chris Redford, May 13 '14 at 10:55

score 2 · Accepted Answer · answered May 13 '14 at 00:57

2

double sum = std::accumulate(
  values.begin(),
  values.end(),
  0.f,
  [&](float sum, float x){ return diffSquaredSum(sum,x);}
);

bind is only rarely useful. Prefer lambdas, they are easier to write and read.

You could instead get fancy with binding, but why?

answered May 13 '14 at 00:57

Yakk - Adam Nevraumont

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@bryanchen those are methods in the OP -- they use `this`. You end up having to bind `this`, and who has the time for that syntax? – Yakk - Adam Nevraumont May 13 '14 at 01:01
@Yakk Thank you for bringing some much needed competence to this question thread. – Chris Redford May 13 '14 at 01:03

Use class method as `op` argument to `accumulate`

1 Answers1