How does a C++ Lambda expression return a result?

C++ Lambda expressions can use the return statement to return results: Syntax: [capture-list] (parameters) -> return-type { // Function body // return expression; } Practical combat: Lambda expressions can be used to filter odd numbers and calculate its sum: int sum_odd = std::accumulate(numbers.begin(), numbers.end(), 0, [](int sum, int num) { if (num % 2 == 1) { return sum + num; } return sum; });

C++ Lambda expression: mechanism for returning results

Lambda expression is an anonymous function in C++ , can be used to represent short and simple blocks of code. We can return results in a Lambda expression by using the return statement in its syntax.

Syntax:

[capture-list] (parameters) -> return-type {
  // 函数体
  return expression; // 返回结果
};

Practical case:

Assume we have a container numbers, we Need to return the sum of all odd numbers in the container. We can use Lambda expression as follows:

#include <vector>
#include <algorithm>

int main() {
  std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9};

  // 使用 Lambda 表达式过滤奇数
  int sum_odd = std::accumulate(numbers.begin(), numbers.end(), 0, [](int sum, int num) {
    if (num % 2 == 1) {
      return sum + num;
    }
    return sum;
  });

  std::cout << "奇数总和：" << sum_odd << std::endl;

  return 0;
}

Explanation:

• In this Lambda expression, [sum, num] is the capture list, which contains the external variables we need.
• (sum, num) is a parameter list, which receives two parameters: sum is the accumulated value, num is the current value in the container value.
• -> int specifies the return type, in this case int.
• The function body uses conditional statements to check whether num is an odd number. If so, add it to sum and return it, otherwise return sum.
• Finally, the std::accumulate function uses a Lambda expression as a binary operator to accumulate all odd numbers in the container into sum_odd.

With the above method, we can easily return results from functions using Lambda expressions.

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