Vec::push() in a loop vs. pre-allocating with Vec::with_capacity()?

Key Performance Differences

Vec::push() in a Loop	Vec::with_capacity() + push()
Reallocates memory multiple times (grows exponentially).	Allocates once upfront.
O(n log n) time complexity (amortized).	O(n) time complexity.
May fragment memory due to repeated allocations.	Single contiguous block of memory.

Why Reallocations Are Costly

Growth Strategy

• A Vec starts with capacity 0 and doubles its capacity when full (e.g., 0 → 4 → 8 → 16...).
• Each reallocation involves:
  • Allocating new memory.
  • Copying all existing elements.
  • Freeing the old memory.

Example for 10 Elements

• push() with Vec::new(): 4 reallocations (capacity 0 → 4 → 8 → 16).
• push() with with_capacity(10): 0 reallocations.

Benchmark Comparison

use std::time::Instant;

fn main() {
    // Test with 1 million elements
    let n = 1_000_000;
    
    // Method 1: No pre-allocation
    let start = Instant::now();
    let mut v1 = Vec::new();
    for i in 0..n {
        v1.push(i);
    }
    println!("Vec::new(): {:?}", start.elapsed());
    
    // Method 2: Pre-allocate
    let start = Instant::now();
    let mut v2 = Vec::with_capacity(n);
    for i in 0..n {
        v2.push(i);
    }
    println!("Vec::with_capacity(): {:?}", start.elapsed());
}

Typical Results

Vec::new(): 1.8ms  
Vec::with_capacity(): 0.4ms  // 4.5x faster

When to Pre-Allocate

• Known Size: Use with_capacity(n) if you know the exact/maximum number of elements.
• Performance-Critical Code: Avoid reallocations in hot loops.
• Large Data: Prevent stack overflow for huge collections.

When Vec::new() is Acceptable

• Small/Unknown Sizes: For ad-hoc usage or short-lived vectors.
• Code Simplicity: When performance isn't critical.

Advanced Optimization: extend()

If you have an iterator, extend() is often faster than a loop with push():

let mut v = Vec::with_capacity(n);
v.extend(0..n);  // Optimized for iterators (avoids bounds checks)

Key Takeaways

✅ Use with_capacity() for:

• Predictable element counts.
• High-performance scenarios.

✅ Use Vec::new() for:

• Small/unknown sizes or prototyping.

🚀 Avoid unnecessary reallocations—they dominate runtime for large Vecs.

Real-World Impact

In the regex crate, pre-allocation is used for capture groups to avoid reallocations during pattern matching.

Try This: What happens if you pre-allocate too much (e.g., with_capacity(1000) but only use 10 elements)?

Answer: Wasted memory. Use shrink_to_fit() to release unused capacity.