Loops

Loops are used to repeat a block of code multiple times. Vyper only uses for keyword to define loops.

For loops can be used together with the range function. This allows looping over a fixed range.

Vyper also have special statements to control for loop flow:

break: to terminate for loop.
continue: to start the next cycle of for loop in the code block.

# pragma version 0.4.0

# Function below loops and sum the elements from 0 to 9
@external
@pure
def basic_sum_range() -> uint256:
    total: uint256 = 0

    for i: uint256 in range (10):
        total += i

    return total

# Function with control statements
@external
@pure
def controlled_sum_range() -> uint256:
    total: uint256 = 0

    for i: uint256 in range (10):

        # Applying `break` here would return `total` = 10
        # Applying `continue` here would return `total` = 40
        if i == 5:
            break
        else:
            total += i

    return total

# Function below evaluates sum of input in a fixed sized array of 5
@external
@pure
def fixed_array_loop(arr: uint256[5]) -> uint256:
    total: uint256 = 0

    for i: uint256 in arr:
        total += i

    return total

# Loop below is restricted by a bound size of 5 and it returns the final value of the loop before `higher_bound`
@external
@pure
def for_loop_bound(lower_bound: uint256, higher_bound: uint256) -> uint256:
    final_loop_value: uint256 = lower_bound

    for i: uint256 in range(lower_bound, higher_bound, bound = 5):
        final_loop_value = i

    return final_loop_value

Vyper Differentiators

The n for range(n) must be a compile-time constant (meaning it cannot be a variable).
Vyper does not have while and do-while keywords for loops.

Loops

Further Reading: