Fix of string.format("%.0f")

[vsky279]

Fixes #3169.

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/*.

This PR fixes the string.format("%.0f") to output float number without decimals after decimal point. At this same time it preserves 6 decimals when no format is specified (%f).

The default value needs to be set in the vsprintf function rather than in dtoa function of app/libc/stdio.c.

The following testing code gives results as expected (also for %e and %g formatting strings).

local pi=3.14159265359
print(("%f"):format(pi)) 
for i=0,12 do print(("%"..i.."f"):format(pi)) end
for i=0,8 do print(("%."..i.."f"):format(pi)) end
for i=0,12 do print(("%"..i..".4f"):format(pi)) end

[TerryE]

BTW, did you see that I've added the % operator for strings, so

for i=0,12 do print(("%"..i.."f"):format(pi)) end

Can be rewritten:

for i=0,12 do print("%%%uf" % i % pi) end

[TerryE]

a=1.23456789  for _,f in pairs{"%%%uf","%%.%uf","%%%u.4f"} do 
for i=0,12 do print(f % i % a) end end

works fine now, so I will commit this.

[vsky279]

BTW, did you see that I've added the % operator for strings

@TerryE I missed that. It's quite compact way to write string format functionality. 👍
In term of efficiency of compiled code I think there is no advantage. Or am I wrong?

[TerryE]

Of the 3 ways of coding this, the modulus operator generates the least code. Try doing a luac.cross -l if you want to see:

                        ; a=string.format(f,1)
        3       [3]     GETTABUP        2 0 -2  ; _ENV "string"
        4       [3]     GETTABLE        2 2 -3  ; "format"
        5       [3]     MOVE            3 1
        6       [3]     LOADK           4 -4    ; 1
        7       [3]     CALL            2 3 2
        8       [3]     MOVE            0 2
                        ; a=f:format(1)
        9       [4]     SELF            2 1 -3  ; "format"
        10      [4]     LOADK           4 -4    ; 1
        11      [4]     CALL            2 3 2
        12      [4]     MOVE            0 2
                        ; a=f%1
        13      [5]     MOD             0 1 -4

It will also be slightly faster.

[vsky279]

I see. Very good. I did not realize the interpretation of the operator is done inside Lua and not in the compiled code :-)