STL layer

Write data filtering with STL containers and algorithms.

STL layer: vector

In this lesson we collect Task objects in a std::vector. A std::vector is a dynamic array; you add elements with push_back.

Example: std::vector<Task> tasks; tasks.push_back(Task("Homework"));. This keeps tasks in a list. You access the first element with tasks[0]. STL algorithms work on this vector: std::count_if counts elements that satisfy a condition. We take the Task class and printItem template from previous lessons and add the vector layer on top.

count_if and lambda

std::count_if counts elements in a vector that satisfy a condition. The condition is given as a lambda.

Example: std::count_if(tasks.begin(), tasks.end(), [](const Task& t) { return t.done; });. Here the lambda checks t.done for each task. If true the task counts as done. A lambda is a short way to write small anonymous functions. [](const Task& t) { return t.done; } takes a Task and returns its done flag. This pattern is very common with STL algorithms.

Exercise Add tasks to a vector and count done

After the Task class and printItem template in the starting code, create a std::vector<Task> in main. Add three tasks (Homework, Sport, Book). Mark the first done with tasks[0].done = true;. Count completed tasks with std::count_if and print the count with std::cout.

Show example solution

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

class Task {
public:
    std::string title;
    bool done;
    Task(std::string t) : title(t), done(false) {}
};

template <typename T>
void printItem(const T& item) {
    std::cout << item;
}

int main() {
    std::vector<Task> tasks;
    tasks.push_back(Task("Homework"));
    tasks.push_back(Task("Sport"));
    tasks.push_back(Task("Book"));
    tasks[0].done = true;
    int n = std::count_if(tasks.begin(), tasks.end(),
        [](const Task& t) { return t.done; });
    std::cout << n;
    return 0;
}

Test yourself

Match each piece to its role

std::vector<Task>

Task list
push_back

Adding an element
tasks[0]

First element
std::count_if

Counting by condition

Complete the vector definition

std::____<Task> tasks;

Show answer

std::vector<Task> tasks;

What does std::count_if do?
```
std::count_if(tasks.begin(), tasks.end(),
    [](const Task& t) { return t.done; });
```
- Counts those satisfying the condition
- Deletes the vector
- Returns the first element
- Sorts the vector
Show answer

Counts those satisfying the condition — count_if counts elements satisfying the condition.

Next lesson: File I/O

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