C++ course for Peru, taught live in English

Source: Comité Organizador Olimpiada Peruana de Informática, Reglamento OPI 2025 (published on opi.utec.edu.pe), read on 9 November 2026. The same rules say both phases run on the omegaUp judge, problems come from topics in the official IOI syllabus, contestants must be at most 20 years old on 1 July 2025 and enrolled in a Peruvian school in 2025 or have finished in 2024, and generative AI is forbidden. The OII 2025 was set for 14 and 15 June 2025 and the IOI 2025 for 27 July to 3 August 2025 in Sucre, Bolivia.

Teenagers interested in other computing olympiads, including UTEC's artificial intelligence contest, can read our Spanish page on the curso de IA para adolescentes, and pupils in their final school year our page on programación para 5to de secundaria.

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<long long> a = {4, 7, 1, 9, 3, 8, 2, 6};
    int n = a.size();

    // Version 1: add up every range again
    long long adds = 0;
    auto slow = [&](int l, int r) {
        long long s = 0;
        for (int i = l; i <= r; i++) { s += a[i]; adds++; }
        return s;
    };

    // Version 2: prefix sums, p[i] = a[0] + ... + a[i-1]
    vector<long long> p(n + 1, 0);
    for (int i = 0; i < n; i++) p[i + 1] = p[i] + a[i];
    auto fast = [&](int l, int r) { return p[r + 1] - p[l]; };

    int queries[3][2] = {{0, 7}, {2, 5}, {3, 3}};
    for (auto& q : queries)
        cout << "sum " << q[0] << ".." << q[1] << ": "
             << slow(q[0], q[1]) << " " << fast(q[0], q[1]) << "\n";
    cout << "additions, version 1: " << adds << "\n";
    cout << "additions to build p: " << n << "\n";
    cout << "wrong formula p[r] - p[l] for 3..3: " << p[3] - p[3] << "\n";
}

#include <iostream>
using namespace std;

int main() {
    long long n = 200000, q = 200000;
    cout << "worst case, version 1: " << n * q << " additions\n";
    cout << "version 2: " << n + q << " additions and subtractions\n";
}

Forty billion operations against four hundred thousand: that is the difference between a program that runs out of time and one that finishes almost instantly. The sizes of 200,000 numbers and 200,000 questions are our own example, not limits from any OPI problem. Notice too that we store the totals in long long; with large values, a running total can outgrow an int, and C++ will not warn you.

The habit this lesson builds is the one that matters in every contest: work out the cost before you write the loop, and test the smallest case, a range of one element, where off-by-one bugs live. The numbers and exercises are ours, written for our classes; no problem from the OPI or any other olympiad is reproduced here.

1. By hand. Write out p for the eight numbers on paper. Check that p[8] equals the sum of the whole array.
2. Break it on purpose. Change the formula to p[r] - p[l] and find every query in the program that now gives a wrong answer. Explain why in English.
3. Two dimensions. Extend the idea to a grid, so the sum of any rectangle takes a few additions and subtractions. Draw the picture first.
4. Count, then code. For a problem with a million numbers and a million questions, estimate both versions' operation counts before writing anything.

Source: the IANA time zone database, America/Lima and Asia/Kolkata, neither of which uses daylight saving. Conversions are ours.

Mentors explain slowly, repeat in simpler words when needed and type key terms into the chat. Learners in the capital who want to know more about studying in English can read about English-medium coding classes in Lima.