#fmt handles %f incorrectly when the number needs rounding (truncates instead) #1876

bzbarsky · 2012-02-21T16:30:18Z

Looks like %f always truncates instead of rounding to the desired precision. For example, this:

use std;
fn main() {
let f = 5.1;
std::io::println(#fmt("hello world %f!", f));
}

prints out "hello world 5.099999!" instead of the expected "hello world 5.100000!".

brson · 2012-02-21T22:29:01Z

See also #1375 for additional problems with float formatting.

This seems to fix issue rust-lang#1876, and some of the superficial parts of issue rust-lang#1375. The #fmt macro and the to_str functions will round, rather than truncate, floats as strings. Other issues remain, and I wrote more code here than intended, but the following should pass now. ``` fn x() { assert "3.1416" == #fmt["%.4f", 3.14159]; assert "3" == #fmt["%.0f", 3.14159]; assert "99" == #fmt["%.0f", 98.5]; assert "7.0000" == #fmt["%.4f", 6.999999999]; assert "3.141590000" == #fmt["%.9f", 3.14159]; } ```

This seems to fix issue #1876, and some of the superficial parts of issue #1375. The #fmt macro and the to_str functions will round, rather than truncate, floats as strings. Other issues remain, and I wrote more code here than intended, but the following should pass now. ``` fn x() { assert "3.1416" == #fmt["%.4f", 3.14159]; assert "3" == #fmt["%.0f", 3.14159]; assert "99" == #fmt["%.0f", 98.5]; assert "7.0000" == #fmt["%.4f", 6.999999999]; assert "3.141590000" == #fmt["%.9f", 3.14159]; } ```

brson · 2012-06-02T23:47:41Z

float::to_str rounding is fixed by @killerswan in a7359f5. The default precision is still an open issue #1375

Most files names are relatively explicit and probably don't need to be explicited. However `rt.rs` is really not clear, and I believe it clarifies the text to indicate this is runtime service implementation.

killerswan mentioned this issue Jun 2, 2012

(float) fix some rounding errors when showing as str #2488

Closed

brson closed this as completed Jun 2, 2012

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#fmt handles %f incorrectly when the number needs rounding (truncates instead) #1876

#fmt handles %f incorrectly when the number needs rounding (truncates instead) #1876

bzbarsky commented Feb 21, 2012

brson commented Feb 21, 2012

brson commented Jun 2, 2012

#fmt handles %f incorrectly when the number needs rounding (truncates instead) #1876

#fmt handles %f incorrectly when the number needs rounding (truncates instead) #1876

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bzbarsky commented Feb 21, 2012

brson commented Feb 21, 2012

brson commented Jun 2, 2012