Lesson 11 · 11 of 20

Working with vectors · C++

Change a std::vector in C++ with insert, erase and pop_back, then sort it and find its smallest and largest values with <algorithm>.

C++ · Working with vectors in DMarlon Studio: the example's code and its output in the terminal

Besides push_back, `insert()` adds an element anywhere. It takes the position as an iterator: list.begin() points at the first element and list.begin() + 2 at the third. So coffee goes in at the start and butter before position 2.

`erase()` removes the element at a position (here milk, the second one) and `pop_back()` removes the last one. The others shift along and size() is updated: from 6 elements down to 4.

`std::sort`, from <algorithm>, sorts a range that goes from begin() to end(), the position just after the last element. Strings are sorted alphabetically.

<algorithm> has many more functions that work on ranges: `std::min_element` and `std::max_element` return an iterator to the smallest and largest, and the * in front gets the value. With a third argument, std::greater<int>() (from <functional>), sort goes from highest to lowest, like a high-score table.

`clear()` empties the vector and `empty()` tells you whether it has no elements left.

Code · main.cpp

#include <algorithm>#include <functional>#include <iostream>#include <string>#include <vector>int main() {    std::vector<std::string> list = {"milk", "bread", "eggs"};    list.push_back("apples");                 // at the end    list.insert(list.begin(), "coffee");      // at the start    list.insert(list.begin() + 2, "butter");  // before position 2    for (auto item : list) std::cout << item << ' ';    std::cout << "(" << list.size() << " items)\n";    // erase removes by position (here, the second item); pop_back, the last one.    list.erase(list.begin() + 1);    list.pop_back();    // sort from <algorithm> puts a range in order: begin() to end().    std::sort(list.begin(), list.end());    for (auto item : list) std::cout << item << ' ';    std::cout << "(" << list.size() << " items)\n";    // Game scores: lowest, highest and a top list from high to low.    std::vector<int> scores = {450, 120, 899, 75, 300};    std::cout << "Lowest: " << *std::min_element(scores.begin(), scores.end())              << ", highest: " << *std::max_element(scores.begin(), scores.end()) << '\n';    std::sort(scores.begin(), scores.end(), std::greater<int>());    for (int s : scores) std::cout << s << ' ';    list.clear();    std::cout << "\nList empty? " << std::boolalpha << list.empty() << '\n';    return 0;}

Output · c++ -std=c++17 main.cpp -o main && ./main

coffee milk butter bread eggs apples (6 items)
bread butter coffee eggs (4 items)
Lowest: 75, highest: 899
899 450 300 120 75 
List empty? true

Key points

v.begin() is the first position and v.end() the one after the last.
insert(v.begin() + i, x) adds before position i.
erase(v.begin() + i) removes position i; pop_back() the last one.
std::sort(v.begin(), v.end()) sorts from lowest to highest.
std::min_element and std::max_element find the extremes.

Exercise

Before removing a product, look for it with std::find(list.begin(), list.end(), "eggs"): if the result is list.end(), print that it is not on the list; otherwise remove it with erase.