Loops

Checking a condition more than once

Gregory S. DeLozier, PhD

Breaking Changes

while, break, and continue become keywords.

A Basic Loop

i = 0;
while (i < 3) {
    print(i);
    i = i + 1;
}

Prints 0, 1, 2.

Tracing a Counter

One more check than iterations.

Condition, Body, Update

Condition: i < 3 — checked before every pass.

Body: print(i) — what runs while it holds.

Update: i = i + 1 — what changes before the next check.

Accumulating a Sum

n = 5;
sum = 0;
k = 1;
while (k <= n) {
    sum = sum + k;
    k = k + 1;
}

sum ends at 15.

Accumulating a String

letters = "";
j = 0;
while (j < 3) {
    letters = letters + "x";
    j = j + 1;
}

letters ends at "xxx".

Parsing a While Statement

def parse_while_statement(tokens):
    tokens = require(tokens, "while", "Expected 'while'")
    tokens = require(tokens, "(", "Expected '(' after 'while'")
    condition, tokens = parse_expression(tokens)
    tokens = require(tokens, ")", "Expected ')' after while condition")
    body, tokens = parse_block(tokens)
    return {"tag": "while", "condition": condition,
            "body": body}, tokens

Break and Continue Are Just Keywords

break_statement ::= "break"
continue_statement ::= "continue"

No condition, no argument, no block.

Evaluating break and continue

if ast["tag"] == "break":
    return None, "break"

if ast["tag"] == "continue":
    return None, "continue"

Evaluating a Loop

if ast["tag"] == "while":
    while True:
        condition, status = evaluate(ast["condition"], environment)
        if status is not None:
            return condition, status
        if not condition:
            return None, None
        value, status = evaluate(ast["body"], environment)
        if status == "break":
            return None, None
        if status is not None and status != "continue":
            return value, status

Checking the Condition

After the Body Runs

Three outcomes, one box each.

Break Finishes the Nearest Loop

found = 0;
search = 0;
while (true) {
    search = search + 1;
    if (search == 4) {
        break;
    };
    found = search;
}

search ends at 4. found ends at 3.

Continue Returns to the Condition

total = 0;
m = 0;
while (m < 5) {
    m = m + 1;
    if (m == 3) {
        continue;
    };
    total = total + m;
}

total ends at 12: skips 3.

Nested Loops

outer = 0;
combined = 0;
while (outer < 2) {
    outer = outer + 1;
    inner = 0;
    while (true) {
        inner = inner + 1;
        if (inner == 2) { break; };
        combined = combined + 1;
    };
    combined = combined + 10;
}

combined ends at 22.

Misuse Outside a Loop

if status == "exit":
    sys.exit(value)
if status is not None:
    raise ValueError(f"Unhandled evaluation status: {status}")

Diagnosed at evaluation, not parsing.

Testing the Contract

Zero, one, and repeated iterations; the boolean-condition requirement.

break and continue skipping later body statements.

continue reevaluating the condition exactly once, checked by mocked input() call count.

Nested loops; exit and a failed assertion propagating out of a loop body.

Worth Checking Out: newton.v

guess = (guess + S / guess) / 2;

Newton’s method for square roots, in one line.

Questions for Thought

Why must continue return to the condition rather than directly to the body?

Which changes to loop state establish progress toward termination?

Why does an inner loop consume break but propagate exit?