9. Control flow: if and for

`if`/`elif`/`else` behave as expected. Loops are more restricted than Solidity: Vyper only has `for`, and the loop bound must be known at compile time (a literal or a `range()` with a constant bound), which guarantees gas usage can't spiral out of control from a single call.

@external
@pure
def classify(n: int128) -> String[8]:
    if n > 0:
        return "positive"
    elif n < 0:
        return "negative"
    else:
        return "zero"

@external
@pure
def total(xs: uint256[5]) -> uint256:
    t: uint256 = 0
    for x in xs:
        t += x
    return t

There is no `while` loop and no `break`-free infinite loop — every loop must terminate by construction.

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