Loops

A loop is a sequence of statements which is specified once but which may be carried out several times in succession. The code "inside" the loop (the body of the loop) is obeyed a specified number of times, or once for each of a collection of items, or until some condition is met, or indefinitely.

Basic loop

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for (int i = 0; i < 10; ++i) {
    std::cout << "Hello world!" << '\n';
}

Changing the number of repetitions

std::cout << "How many hello worlds? ";
int z;
std::cin >> z;
for (int i = 0; i < z; ++i) {
    std::cout << "Hello world" << '\n';
}

Similar tasks according to the counter

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for (int i = 0; i < 10; ++i) {
    std::cout << "Hello world " << i << "!\n";
}

Changing external variables

int total = 0;
for (int i = 0; i < 10; ++i) {
    total += i;
}
std::cout << "0+1+2+3+4+...+9: " << total << '\n';

Loop + conditional

total = 0;
for (int i = 0; i < 50; ++i) {
    const bool is_multiple_of_three = i % 3 == 0;
    if (is_multiple_of_three) {
        total += i;
    }
}
std::cout << "0+3+6+9+12+...+48: " << total << '\n';

Changing initialization

// 5/3 + 7/4 + 9/5 + 11/6 + ...
float series = 0.;
for (int i = 3; i < 50; ++i) {
    const auto i_float = static_cast<float>(i);
    series += (i_float * 2 - 1) / i_float;
}
std::cout << "5/3+7/4+9/5+...+97/49: " << series << '\n';

Changing update statement to +=

total = 0;
for (int i = 7; i < 500; i += 7) {
    total += i;
}
std::cout << "7+14+21+...+497: " << total << '\n';

Changing update statement to *=

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for (int i = 2; i <= 1024; i *= 2) {
    std::cout << i << '\n';
}

Index-based loop for containers with subscript operator[]

constexpr int n = 100;
std::array<double, n> prices{};
for (size_t i = 0; i < prices.size(); ++i) {
    prices[i] = 4.0 + 0.1 * i;
}

Range-based loop for containers

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for (double& price : prices) {
    std::cout << "Item price: " << price << '\n';
}

Range-based loop with constant references

double sum = 0.;
for (const auto &price : prices) {
    sum += price;
}
std::cout << "Average price: " << sum / n << '\n';

Range-based loop with maps

std::map<int, int> t;
t[7] = 2;
t[8] = 4;
t[4] = 6;
for (const auto &p : t) {
    std::cout << p.first << ": " << p.second << '\n';
}

Range-based loop with value pairs

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for (const auto &[key, value] : t) {
    std::cout << key << ": " << value << '\n';
}

2D arrays

constexpr int n1 = 100;
constexpr int n2 = 3;
std::array<std::array<double, n2>, n1> grades{};
for (int i = 0; i < n1; ++i) {
    for (int j = 0; j < n2; ++j) {
        grades[i][j] = 6. + i * (2. / n1) + j * (2. / n2);
    }
}

2D range based loop

for (const auto &row : grades) {
    double row_sum = 0.0;
    for (const auto &col : row) {
        row_sum += col;
    }
    std::cout << "Average student grade: " << row_sum / row.size() << '\n';
}

Columns in the external for

for (int j = 0; j < n2; ++j) {
    double col_sum = 0.0;
    for (int i = 0; i < n1; ++i) {
        col_sum += grades[i][j];
    }
    std::cout << "Average score on test " << j + 1 << ": " << col_sum / n1
         << '\n';
}

While loop

int num = 1;
while (num != 0) {
    std::cout << "Enter a number: ";
    std::cin >> num;
    std::cout << num << "^2 = " << num * num << '\n';
}

Do-while loop

do {
    std::cout << "Enter a number: ";
    std::cin >> num;
    std::cout << num << "^2 = " << num * num << '\n';
} while (num != 0);

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