Nodemcu-firmware: string.format("%.0f")

[vsky279]

Expected behavior

=("%.0f"):format(3.141)
3

Actual behavior

=("%.0f"):format(3.141)
3.141000

On desktop in Lua 5.1 and 5.3 formatting string %.0f works as expected:

Lua 5.3.1  Copyright (C) 1994-2015 Lua.org, PUC-Rio
> print(("%.0f"):format(3.141))
3

Lua 5.1.5  Copyright (C) 1994-2012 Lua.org, PUC-Rio
> print(("%.0f"):format(3.141))
3

Test code

as above

NodeMCU startup banner

NodeMCU 3.0.0.0 built with Docker provided by frightanic.com
    branch: dev
    commit: 86f3419835b6ed92265f054cf2271795b0757606
    release: 3.0-master_20200610 +7
    release DTS: 202006182133
    SSL: false
    build type: float
    LFS: 0x10000 bytes total capacity
    modules: enduser_setup,file,gpio,net,node,rtcmem,rtctime,sjson,sntp,tmr,uart,wifi
 build 2020-06-19 11:06 powered by Lua 5.1.4 on SDK 3.0.1-dev(fce080e)

Applies also to Lua 5.3.

Hardware

ESP8266 ESP-01

[KT819GM]

Had same problem some time ago, did not bothered to check on pc version (thought my limited knowledge was the reason), so wen't with math.floor or with 5.3 // when doing division. Just checked example on build 2020-06-19 06:37 powered by Lua 5.3.5 on SDK 3.0.1-dev(fce080e) and got the same problem.

[vsky279]

Yep, workaround is quite simple. I think %f uses rounding, so suggested solutions would need to be more sophisticated to replicate the functionality. It would be nice to implement it correctly.

[TerryE]

Having stepped through the code with the debugger, the problem is app/libc/stdio.c:L766 if (prec == 0) prec = 6; This version does not differentiate between precision not specified which should default to 6 and an explicit prec=0. This is a bug that dates back to the very earliest versions. Well I've found it. over to one of you guys to propose and test a fix.

[KT819GM]

Gosh, started to look at it and it was solved like 15 min before :D Just for the record:

if (prec == 0)
        prec = 6;

to

if (!prec)
   {
       prec;
   }

Works also. Used this as reference: Reference

[vsky279]

There's is a ++ operator missing in your comment in front of prec.
Anyway it seems that this modification applies to the %g formatting string

/* a precision of 0 is treated as a precision of 1. */

As per comment this is probably not the right fix for the %f.

[TerryE]

@KT819GM @vsky279 You guys have a ping-pong and ring the bell when you've got a solution. :+1: to you both. Thanks

[KT819GM]

There's is a ++ operator missing in your comment in front of prec.
Anyway it seems that this modification applies to the %g formatting string

/* a precision of 0 is treated as a precision of 1. */

As per comment this is probably not the right fix for the %f.

Hi, yes, ++ is missing, for a reason - 1. and 1. I've tested only one string from your example

for i=0,8 do print(("%."..i.."f"):format(pi)) end

and both variants return:

3
3.1
3.14
3.142
3.1416
3.14159
3.141593
3.1415927
3.14159274

BUT, it does not mean that it's same in all situations and I suspect that you've tested your solution far better than me, so I see it as correct bugfix.