Welcome back! In our previous lesson, we explored the fundamentals of STL algorithms in C++ and how they can enhance your code's efficiency and readability. Today, we will continue this journey by diving into more advanced STL algorithms, with a focus on std::accumulate.

By the end of this lesson, you should understand:

1. How to use std::accumulate to find the sum of numbers in a vector.
2. Advanced uses of std::accumulate with custom functionalities using lambda expressions.

std::accumulate in the <numeric> header performs a fold or reduction operation over a range of elements. It combines the elements using a binary operation, starting with an initial value.

The basic syntax of std::accumulate:

C++
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1std::accumulate(first, last, init);

• First: Iterator to the beginning of the range.
• Last: Iterator to the end of the range.
• Init: Initial value to start the accumulation.

Here is an example using std::accumulate to sum the elements of a vector:

C++
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1#include <iostream>
2#include <vector>
3#include <numeric>  // For std::accumulate
4
5int main() {
6    std::vector<int> data = {1, 2, 3, 4, 5};
7
8    // Using std::accumulate
9    int sum = std::accumulate(data.begin(), data.end(), 0);
10    std::cout << "Sum: " << sum << '\n';  // Sum: 15
11
12    return 0;
13}

Explanation:

1. Initialize a vector: We initialize a vector data with {1, 2, 3, 4, 5}.
2. Use std::accumulate: We pass the range defined by data.begin() and data.end(), and the initial value 0.
3. Result: The function sums the elements starting from 0. The final sum, 15, is outputted.

Consider computing the product of elements:

C++
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1#include <iostream>
2#include <vector>
3#include <numeric>  // For std::accumulate
4#include <functional>  // For std::multiplies
5
6int main() {
7    std::vector<int> data = {1, 2, 3, 4, 5};
8
9    // Using std::accumulate with std::multiplies
10    int product = std::accumulate(data.begin(), data.end(), 1, std::multiplies<int>());
11    std::cout << "Product: " << product << '\n';  // Product: 120
12
13    return 0;
14}

Explanation:

1. Initial Value: Use 1 as the initial value.
2. Custom Operation: Use std::multiplies<int>() for multiplication.

Let's look at custom logic with lambda expressions.

Finding the Sum of Squares:

C++
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1#include <iostream>
2#include <vector>
3#include <numeric>  // For std::accumulate
4
5int main() {
6    std::vector<int> data = {1, 2, 3, 4, 5};
7
8    // Using std::accumulate with a lambda expression
9    int sum_of_squares = std::accumulate(data.begin(), data.end(), 0, [](int sum, int val) {
10        return sum + val * val;
11    });
12    std::cout << "Sum of Squares: " << sum_of_squares << '\n';  // Sum of Squares: 55
13
14    return 0;
15}

Custom Lambda: We pass a lambda that computes the square of each element and adds it to the sum. In the lambda expression:

• sum: Accumulator that holds the ongoing sum of squares.
• val: Current element from the vector being processed.

Combining two vectors into a single accumulated sum:

C++
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1#include <iostream>
2#include <vector>
3#include <numeric>  // For std::accumulate
4
5int main() {
6    std::vector<int> data1 = {1, 2, 3, 4, 5};
7    std::vector<int> data2 = {6, 7, 8, 9, 10};
8
9    // Combining two vectors element-wise and accumulating the result
10    int combined_sum = std::accumulate(data1.begin(), data1.end(), 0, [&data2, i = 0](int sum, int val) mutable {
11        return sum + val + data2[i++];
12    });
13    std::cout << "Combined Sum: " << combined_sum << '\n';  // Combined Sum: 55
14
15    return 0;
16}

Lambda with Capture: We capture data2 by reference and use a mutable lambda to add elements of both vectors. In the lambda expression:

• sum: Accumulator that holds the ongoing sum of combined elements from both vectors.
• val: Current element from data1 being processed.
• data2[i++]: Corresponding element from data2 being added, with mutable allowing us to modify the captured i to iterate through data2.

In this lesson, we explored:

• The importance and utility of STL algorithms in C++.
• How to use std::accumulate for simple and complex operations.
• Examples with built-in and custom operations via lambda expressions.

Now that you've learned to use std::accumulate, it's time to practice. Move on to the hands-on exercises to reinforce your understanding and enhance your skills. Good luck, and happy coding!