How to adjust the slider's decimals?

[AurevoirXavier]

I use:

ui.add(
	Slider::new(&mut ctx.components.setting.ai.temperature, 0_f32..=2.)
		.fixed_decimals(1)
		.step_by(0.1),
);

The display shows 0.7, but when I save this setting to the file, it appears as 0.699999988079071.

[Creative0708]

You're using an f32, which is less precise than f64. I presume the file format you're using uses 64-bit floats, so you'll run into precision errors.

Try changing the type of ctx.components.setting.ai.temperature and the range to f64.

[AurevoirXavier]

I tried f64. The result changes to temperature = 0.7000000000000001.

I'm using serde and toml.

[emilk]

This sounds like a problem with your serializer, not with egui. Does the problem still persist if you assing temperature = 0.7; instead of using egui?

[AurevoirXavier]

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=f46c921d9a574688d43a4902b4493c28

It looks good.

[AurevoirXavier]

I disable the toml now. I just print the value every time I change the slider. I got:

[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.1
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.30000000000000004
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.5
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.7000000000000001
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.9
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.1
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.2000000000000002
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.3
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.4000000000000001
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.5
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.6
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.7000000000000002
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.8
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.9000000000000001
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 2.0
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.9000000000000001
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.8
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.7000000000000002
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.6
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.5
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.3
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.2000000000000002
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.1
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 1.0
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.9
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.8
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.7000000000000001
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.6000000000000001
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.5
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.4
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.30000000000000004
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.2
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.1
[src/ui/panel/setting.rs:117:5] ctx.components.setting.ai.temperature = 0.0

[AurevoirXavier]

Things quite clear now.

If I use f32 with the slider, the slider can get the value correctly. But toml has this issue toml-rs/toml#494.

If I use f64 with the slider, the toml can serialize the value correctly. But egui can not get the value correctly.

[AurevoirXavier]

@emilk this might be an issue of egui slider with f64.

[YgorSouza]

When a step is set, the slider does some arithmetic on the value after rounding it, which causes these rounding errors:

egui/crates/egui/src/widgets/slider.rs

Lines 538 to 544 in db8db50

	if let Some(max_decimals) = self.max_decimals {
	value = emath::round_to_decimals(value, max_decimals);
	}
	if let Some(step) = self.step {
	let start = *self.range.start();
	value = start + ((value - start) / step).round() * step;
	}

It is pretty easy to reproduce:

            ui.add(
                egui::Slider::new(&mut self.value, 0.0..=2.0)
                    .max_decimals(2)
                    .step_by(0.1),
            );
            ui.label(self.value.to_string());

It is the same effect whether we change the value with the slider, the drag functionality or just type it manually.

If we invert the order of the two if statements, this no longer happens.

So if there are no undesirable side effects, this could be an easy fix. I'll try to look into it and make a PR this week if no one does it first.

[AurevoirXavier]

I think I can help with this today later.