101. f64::total_cmp — Sort Floats Without the NaN Panic

Tried v.sort() on a Vec<f64> and hit the trait Ord is not implemented for f64? Then reached for .sort_by(|a, b| a.partial_cmp(b).unwrap()) and now a stray NaN is about to panic your service at 3am? f64::total_cmp is the one-liner that makes both problems disappear.

Why f64 doesn’t implement Ord

Floats form a partial order because NaN is not equal to anything — not even itself. So f64: PartialOrd but not Ord, which means sort() flat out refuses to compile:

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let mut temps: Vec<f64> = vec![21.5, 18.0, 23.0, 19.5];
// temps.sort(); // ❌ the trait `Ord` is not implemented for `f64`

The classic workaround is partial_cmp().unwrap():

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let mut temps: Vec<f64> = vec![21.5, 18.0, 23.0, 19.5];
temps.sort_by(|a, b| a.partial_cmp(b).unwrap());

assert_eq!(temps, [18.0, 19.5, 21.5, 23.0]);

Works — until a NaN sneaks in. Then partial_cmp returns None, the unwrap fires, and your sort becomes a panic.

Enter total_cmp

f64::total_cmp implements the IEEE 754 totalOrder predicate: a real total ordering on every f64 bit pattern, including all the NaNs. It returns Ordering directly — no Option, no panic:

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let mut temps: Vec<f64> = vec![21.5, 18.0, 23.0, 19.5];
temps.sort_by(f64::total_cmp);

assert_eq!(temps, [18.0, 19.5, 21.5, 23.0]);

Same result for well-behaved input, but now NaN won’t take the process down:

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let mut values: Vec<f64> = vec![3.0, f64::NAN, 1.0, f64::NEG_INFINITY, 2.0];
values.sort_by(f64::total_cmp);

// Finite values in order, -∞ at the front, NaN at the back.
assert_eq!(values[0], f64::NEG_INFINITY);
assert_eq!(values[1], 1.0);
assert_eq!(values[2], 2.0);
assert_eq!(values[3], 3.0);
assert!(values[4].is_nan());

min and max too

partial_cmp poisons more than just sort. Any time you reach for iter().max_by(|a, b| a.partial_cmp(b).unwrap()), you’ve written the same latent panic. total_cmp fits there too:

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let readings = [3.2_f64, 1.4, 4.8, 2.1];

let peak = readings.iter().copied().max_by(f64::total_cmp).unwrap();
let low  = readings.iter().copied().min_by(f64::total_cmp).unwrap();

assert_eq!(peak, 4.8);
assert_eq!(low, 1.4);

Sorting structs by a float field

Because total_cmp takes two &f64s and returns Ordering, it slots straight into sort_by:

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struct Trade { symbol: &'static str, price: f64 }

let mut book = vec![
    Trade { symbol: "AAPL", price: 172.40 },
    Trade { symbol: "NVDA", price: 915.10 },
    Trade { symbol: "MSFT", price: 419.80 },
];

book.sort_by(|a, b| a.price.total_cmp(&b.price));

assert_eq!(book[0].symbol, "AAPL");
assert_eq!(book[2].symbol, "NVDA");

When to reach for it

Any time you’re about to type partial_cmp(...).unwrap() for a float, stop and use total_cmp instead. f32::total_cmp works the same way. Available since Rust 1.62 — the fix has been hiding in plain sight for years.