Your First Loop

Why Loops?

In competitive programming you're rarely processing just one thing. You might need to sum 1000 numbers, count how many are even, or keep reading until input ends. Without loops, you'd have to copy-paste the same line a thousand times.

The while Loop

A while loop keeps running as long as a condition is true. It's the simplest kind of loop — you check a condition, run the body, check again.

int i = 0;
while (i < 5) {
    cout << i << "\n";
    i++;
}

let mut i = 0;
while i < 5 {
    println!("{}", i);
    i += 1;
}

The for Loop

A for loop bundles three things into one line: a starting value, a condition to keep going, and what to do after each iteration. It's the most common loop in competitive programming.

// i goes 0, 1, 2, 3, 4
for (int i = 0; i < 5; i++) {
    cout << i << "\n";
}

// i goes 1 through 10
for (int i = 1; i <= 10; i++) {
    cout << i << "\n";
}

// 0..5 is exclusive (0, 1, 2, 3, 4)
for i in 0..5 {
    println!("{}", i);
}

// 1..=10 is inclusive (1 through 10)
for i in 1..=10 {
    println!("{}", i);
}

Pattern: Summing Numbers

This is the first algorithmic pattern you should memorize. You have an accumulator (a variable that starts at 0), and you add each number to it.

Sample input:
5
4 7 1 9 3

Expected output:
24

#include <bits/stdc++.h>
using namespace std;
int main() {
    int n; cin >> n;
    int total = 0;
    for (int i = 0; i < n; i++) {
        int x; cin >> x;
        total += x;
    }
    cout << total << "\n";
    return 0;
}

use std::io::{self, Read};
fn main() {
    let mut input = String::new();
    io::stdin().lock().read_to_string(&mut input).unwrap();
    let nums: Vec<i64> = input
        .split_whitespace()
        .map(|x| x.parse().unwrap())
        .collect();
    let n = nums[0] as usize;
    let total: i64 = nums[1..=n].iter().sum();
    println!("{}", total);
}

Pattern: Counting

Instead of summing values, you count how many items have a certain property. The pattern is the same — an accumulator, a loop, and a condition.

Sample input:
6
2 5 8 11 14 17

Expected output:
3

#include <bits/stdc++.h>
using namespace std;
int main() {
    int n; cin >> n;
    int count = 0;
    for (int i = 0; i < n; i++) {
        int x; cin >> x;
        if (x % 2 == 0) count++;
    }
    cout << count << "\n";
    return 0;
}

use std::io::{self, Read};
fn main() {
    let mut input = String::new();
    io::stdin().lock().read_to_string(&mut input).unwrap();
    let nums: Vec<i64> = input
        .split_whitespace()
        .map(|x| x.parse().unwrap())
        .collect();
    let n = nums[0] as usize;
    let count = nums[1..=n].iter().filter(|&x| x % 2 == 0).count();
    println!("{}", count);
}

Key Takeaways

• → while loops repeat while a condition is true. for loops package initialization, condition, and increment.
• → Use an accumulator variable to sum or count inside a loop.
• → for (int i = 0; i < n; i++) in C++, 0..n in Rust.
• → In Rust, 0..n is exclusive (0 to n-1), 0..=n is inclusive.
• → Never forget to update the loop variable — infinite loops crash the judge.

Practice

• Kattis: Sum Squared Digits
• Codeforces 231A: Team
• AtCoder ABC 087 B: Coins