/*
Architecture notes:
-------------------
This program demonstrates a modular design:
- count_digits: counts digits using integer math.
- has_even_digits: checks if a number has an even digit count.
- count_even_digit_numbers: processes a vector and returns the count.
- Multiple test cases in main.
Performance notes:
------------------
- All operations are O(n) for n vector elements.
- Digit counting uses integer division, avoiding string conversions.
- No heap allocations beyond the vector itself.
- Negative numbers handled safely.
Security notes:
---------------
- No unsafe blocks.
- No unchecked indexing.
- No external dependencies.
*/
/// Count the number of digits in an integer.
/// Complexity: O(log n) due to repeated division by 10.
/// Pitfall: negative numbers must be handled correctly.
fn count_digits(value: i32) -> u32 {
if value == 0 {
return 1; // zero has one digit
}
let mut n = value.abs();
let mut count = 0;
while n > 0 {
n /= 10;
count += 1;
}
count
}
/// Returns true if the number has an even number of digits.
fn has_even_digits(value: i32) -> bool {
count_digits(value) % 2 == 0
}
/// Count how many numbers in a vector have an even number of digits.
/// Complexity: O(n)
fn count_even_digit_numbers(v: &[i32]) -> usize {
v.iter().filter(|&&v| has_even_digits(v)).count()
}
/// Utility: print a vector in a readable format.
fn print_vec(v: &[i32]) {
print!("[ ");
for v in v {
print!("{v} ");
}
print!("]");
}
fn main() {
println!("=== Count Numbers with Even Number of Digits (Rust) ===\n");
// Test cases
let a1 = vec![12, 345, 2, 6, 7896];
let a2 = vec![0, -22, 1000, -7];
let a3 = vec![1, 3, 5]; // No even-digit numbers
let a4 = vec![10, 99, 1001]; // All even-digit numbers
let a5: Vec<i32> = vec![]; // Edge case: empty vector
let a6 = vec![-100000, 500000]; // Large numbers
// Test 1
print!("Test 1: ");
print_vec(&a1);
println!(" -> Count = {}", count_even_digit_numbers(&a1));
// Test 2
print!("Test 2: ");
print_vec(&a2);
println!(" -> Count = {}", count_even_digit_numbers(&a2));
// Test 3
print!("Test 3: ");
print_vec(&a3);
println!(" -> Count = {}", count_even_digit_numbers(&a3));
// Test 4
print!("Test 4: ");
print_vec(&a4);
println!(" -> Count = {}", count_even_digit_numbers(&a4));
// Test 5: empty vector
print!("Test 5: ");
print_vec(&a5);
println!(" -> Count = {}", count_even_digit_numbers(&a5));
// Test 6: large numbers
print!("Test 6: ");
print_vec(&a6);
println!(" -> Count = {}", count_even_digit_numbers(&a6));
}
/*
run:
=== Count Numbers with Even Number of Digits (Rust) ===
Test 1: [ 12 345 2 6 7896 ] -> Count = 2
Test 2: [ 0 -22 1000 -7 ] -> Count = 2
Test 3: [ 1 3 5 ] -> Count = 0
Test 4: [ 10 99 1001 ] -> Count = 3
Test 5: [ ] -> Count = 0
Test 6: [ -100000 500000 ] -> Count = 2
*/
