Control Flow

Mojo supports if/else, for, and while with Python-like syntax. The key difference: the compiler can analyze loop bounds and branch patterns to optimize execution. Predictable control flow enables vectorization and loop unrolling.

Code

fn main():
    # Conditionals
    var x: Int = 10
    if x > 5:
        print("greater")
    elif x == 5:
        print("equal")
    else:
        print("less")

    # For loop with range
    for i in range(5):
        print(i)

    # While loop
    var count: Int = 0
    while count < 3:
        print(count)
        count += 1

Loop Structure Impact

The compiler can optimize loops with known bounds. A for i in range(N) with constant N can be unrolled. A while loop with a dynamic condition cannot:

fn sum_fixed() -> Int:
    var total: Int = 0
    for i in range(1000):    # compiler knows: 1000 iterations
        total += i
    return total

Constraint

Write a loop that sums integers from 0 to N. First use for i in range(N), then rewrite it as a while loop. Both should produce the same result.

Why It Matters

Branch prediction failures cost 10-20 CPU cycles each. Loops with predictable bounds let the compiler unroll iterations and eliminate branch checks. This is the foundation for SIMD vectorization in Phase 3 — the compiler needs to know loop trip counts to generate vector instructions.

← Types Next: Functions →