Welcome to the introduction to Standard Template Library (STL) algorithms in C++! In today's fast-paced development environment, efficiency and maintainability are crucial. Using tools like STL algorithms can help you achieve these goals by providing pre-built, robust functions.

By the end of this lesson, you will:

1. Understand what STL algorithms are and why they are beneficial.
2. Learn the usage and syntax of the std::for_each algorithm.
3. See how std::for_each can be used in a practical example.

STL algorithms are a collection of functions provided by the Standard Template Library (STL) in C++. They perform common operations on sequences of data. Using STL algorithms saves time and effort by leveraging well-tested and optimized operations for tasks like searching, sorting, and transforming data. Why Use STL Algorithms?

• Efficiency: STL algorithms are highly optimized and can make your code faster.
• Readability: They offer a clear and expressive way to handle data structures.
• Reusability: These algorithms have undergone extensive testing, ensuring reliability.

Some commonly used STL algorithms include:

• std::for_each: Applies a function to a range of elements.
• std::sort: Sorts a range of elements.
• std::find: Searches for a value in a range of elements.

In this lesson, we will focus on std::for_each.

std::for_each lets you execute a specified function on every element within a range. Here is its basic signature:

C++
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1std::for_each(InputIterator first, InputIterator last, Function fn);

• InputIterator first: An iterator pointing to the start of the range.
• InputIterator last: An iterator pointing to one past the end of the range.
• Function fn: A function or function object (often a lambda) to apply to the elements in the range.

This function is especially useful when you need to perform operations on all elements of a container, such as displaying, modifying, or accumulating values.

Here’s a basic example demonstrating the usage of std::for_each to print elements of a vector:

C++
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1#include <iostream>
2#include <vector>
3#include <algorithm>
4
5int main() {
6    std::vector<int> data = {1, 2, 3, 4, 5};
7
8    // Applying std::for_each
9    std::for_each(data.begin(), data.end(), [](int n) {
10        std::cout << n << ' ';
11    });
12    std::cout << '\n';  // Output: 1 2 3 4 5
13
14    return 0;
15}

Explanation of the Code:

1. Initialization: A std::vector is initialized with values {1, 2, 3, 4, 5}.
2. Calling std::for_each: We call std::for_each with three arguments:
   • data.begin(): An iterator pointing to the beginning of the vector.
   • data.end(): An iterator pointing to one past the end of the vector.
   • Lambda function: This lambda function takes an integer n and prints it, followed by a space.

When std::for_each is executed, the lambda function is called for each element, resulting in the elements being printed to the console.

Imagine you have a list of grades, and you want to normalize them by subtracting the mean. Here's how you could use std::for_each to accomplish this:

C++
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1#include <iostream>
2#include <vector>
3#include <algorithm>
4
5int main() {
6    std::vector<int> grades = {75, 85, 95, 80, 70};
7
8    // Calculate the mean
9    double sum = 0;
10    for (int grade : grades) {
11        sum += grade;
12    }
13    double mean = sum / grades.size(); // mean = 81.0
14
15    // Normalize the grades by subtracting the mean
16    std::for_each(grades.begin(), grades.end(), [mean](int &grade) {
17        grade -= mean;
18    });
19
20    // Print the normalized grades
21    std::for_each(grades.begin(), grades.end(), [](int grade) {
22        std::cout << grade << ' ';
23    });
24    std::cout << '\n';  // Output: -6 4 14 -1 -11
25
26    return 0;
27}

Explanation of the Real-World Example

1. Initialization: A vector of grades {75, 85, 95, 80, 70} is initialized.
2. Calculating the Mean: The sum of the grades is computed, and dividing by the vector size gives the mean.
3. Normalizing the Grades: We use std::for_each to subtract the mean from each grade. Note that the lambda captures the mean by value.
4. Printing the Grades: Another call to std::for_each prints the normalized grades.

By using std::for_each, we applied a custom operation to each element in the vector seamlessly.

We have covered the essentials of STL algorithms in C++, focusing on std::for_each. We learned:

• The purpose and benefits of using STL algorithms.
• The syntax and functionality of std::for_each.
• How to apply std::for_each in both simple and real-world scenarios.

By leveraging STL algorithms, you can write more concise, readable, and efficient code. In the next section, you will get hands-on practice with std::for_each, applying what you've learned and exploring more advanced use cases.