Transform and Accumulate: Map and Fold

Two algorithms produce new data from old: std::transform (map) and std::accumulate (fold).

#include <algorithm>
#include <numeric>
#include <string>
#include <vector>

std::vector<int> v{1, 2, 3};
std::vector<int> doubled;
std::transform(v.begin(), v.end(), std::back_inserter(doubled),
               [](int x) { return x * 2; });

long total = std::accumulate(v.begin(), v.end(), 0L);
std::string joined = std::accumulate(
    words.begin(), words.end(), std::string{},
    [](const std::string& a, const std::string& b) {
        return a.empty() ? b : a + " " + b;
    });

std::back_inserter is required when the destination is empty: transform does not grow the container for you; writing through doubled.begin() on an empty vector is undefined behavior.

accumulate takes an initial value and a binary operation. Its type rule is subtle: the initial value's type is the accumulator's type — starting with 0 instead of 0.0 silently truncates a double sum. This is the classic beginner bug this lesson exists to prevent.