Checking a Wind Range With &&

Checking a Wind Range With &&

Ruth’s chain had two postponed branches, one for too little wind and one for too much. She merges them, so the racing test reads like the club’s rule: the fleet races when the wind is from 6 knots up to, but not including, 25, in one chained comparison. The else picks the reason with a conditional expression:

wind_knots = 4
gust_knots = 7

if gust_knots >= 30:
    decision = "cancelled"
    reason = "gusts of 30 knots or more"
elif 6 <= wind_knots < 25:
    decision = "racing"
    reason = "good racing wind"
else:
    decision = "postponed"
    reason = "too little wind" if wind_knots < 6 else "too much wind"

print(f"{decision} ({reason})")

On the calm morning, 4 knots, Ruth’s script prints postponed (too little wind). In this lesson, you ported that range test to JavaScript, where a line that looks right can make the wrong call.

• Your JavaScript port already had the old chain, with decision and reason declared with let above it, and each branch in braces. You updated it the same way Ruth did, and the reason became a ternary, with the condition first.
• The comparison signs are the same in both languages, so the range test looks like it carries across as it’s written, 6 <= windKnots < 25. It runs with no error and no warning, but on the calm morning, the port posts racing (good racing wind), where Python posts postponed (too little wind).
• JavaScript has no chained comparisons, so it reads the test left to right, one pair at a time: 6 <= 4 is false, and that false is compared with 25.
• Less than compares numbers, so JavaScript converts the boolean: false becomes 0, and true becomes 1. 0 < 25 is true, so the test is true.
• Python agrees with that step, (6 <= wind_knots) < 25 is True, since its booleans count as 0 and 1 too. But its chain compares the wind with each number, never a boolean with 25.
• The first step only ever gives false or true, 0 or 1, and both are below 25, so the test is true for any wind. On the strong morning, 26 knots, Python postpones for too much wind, and the port races.
• On the main morning, 14 knots, both race, so testing only that morning would miss the slip.
• The same reading applies to plain numbers: JavaScript prints true for 1 < 5 < 3, because 1 < 5 is true, and true counts as 1, which is less than 3. Python prints False, because 5 isn’t less than 3. It can go the other way, too: 3 > 2 > 1 is false in JavaScript and True in Python.
• Python’s chain is shorthand for two comparisons joined with and: 6 <= wind_knots and wind_knots < 25. JavaScript has no shorthand, so you write both comparisons in full and join them with &&, JavaScript’s and. The wind stays in the middle, between 6 and 25, so the test still reads as a range.