Double precision is different in different languages

The differences in output are due to differences in converting the floating-point number to a numeral. (By numeral, I mean a character string or other text that represents a number. “20”, “20.0”, “2e+1”, and “2•102” are different numerals for the same number.)

For reference, I show the exact values of `i` in notes below.

In C, the `%.17lf` conversion specification you use requested 17 digits after the decimal point, so 17 digits after the decimal point are produced. However, the C standard allows some slack in this. It only requires calculation of enough digits that the actual internal value can be distinguished.1 The rest can be filled in with zeros (or other “incorrect” digits). It appears the C standard library you are using only fully calculates 17 significant digits and fills the rest you request with zeros. This explains why you got “2.90000000000000120” instead of “2.90000000000000124”. (Note that “2.90000000000000120” has 18 digits: 1 before the decimal point, 16 significant digits after it, and 1 non-significant “0”. “0.10000000000000001” has an aesthetic “0” before the decimal point and 17 significant digits after it. The requirement for 17 significant digits is why ““0.10000000000000001” must have the “1” at the end but “2.90000000000000120” may have a “0”.)

In contrast, it appears your C++ standard library does the full calculations, or at least more (which may be due to a rule in the C++ standard2), so you get “2.90000000000000124”.

Python 3.1 added an algorithm to convert with the same result as Java (see below). Prior to that was lax about the conversion for display. (To my knowledge, it is still lax about the floating-point format used and conformance to IEEE-754 in arithmetic operations; specific Python implementations may differ in behavior.)

Java requires that the default conversion from `double` to string produce just as many digits as are required to distinguish the number from neighboring `double` values (also here). So it produces “.2” instead of “0.20000000000000001” because the the double nearest .2 is the value that `i` had in that iteration. In contrast, in the next iteration, the rounding errors in arithmetic gave `i` a value slightly different from the double nearest .3, so Java produced “0.30000000000000004” for it. In the next iteration, the new rounding error happened to partially cancel the accumulated error, so it was back to “0.4”.

Notes

The exact values of `i` when IEEE-754 binary64 is used are:

```0
0.1000000000000000055511151231257827021181583404541015625
0.200000000000000011102230246251565404236316680908203125
0.3000000000000000444089209850062616169452667236328125
0.40000000000000002220446049250313080847263336181640625
0.5
0.59999999999999997779553950749686919152736663818359375
0.6999999999999999555910790149937383830547332763671875
0.79999999999999993338661852249060757458209991455078125
0.899999999999999911182158029987476766109466552734375
0.99999999999999988897769753748434595763683319091796875
1.0999999999999998667732370449812151491641998291015625
1.1999999999999999555910790149937383830547332763671875
1.3000000000000000444089209850062616169452667236328125
1.4000000000000001332267629550187848508358001708984375
1.5000000000000002220446049250313080847263336181640625
1.6000000000000003108624468950438313186168670654296875
1.7000000000000003996802888650563545525074005126953125
1.8000000000000004884981308350688777863979339599609375
1.9000000000000005773159728050814010202884674072265625
2.000000000000000444089209850062616169452667236328125
2.10000000000000053290705182007513940334320068359375
2.200000000000000621724893790087662637233734130859375
2.300000000000000710542735760100185871124267578125
2.400000000000000799360577730112709105014801025390625
2.50000000000000088817841970012523233890533447265625
2.600000000000000976996261670137755572795867919921875
2.7000000000000010658141036401502788066864013671875
2.800000000000001154631945610162802040576934814453125
2.90000000000000124344978758017532527446746826171875
```

These are not all the same values you would get by converting 0, .1, .2, .3,… 2.9 from decimal to binary64 because they are produced by arithmetic, so there are multiple rounding errors from the initial conversions and the consecutive additions.

Footnotes

1 C 2018 7.21.6.1 only requires that the resulting numeral be accurate to `DECIMAL_DIG` digits in a specified sense. `DECIMAL_DIG` is the number of digits such that, for any number in any floating-point format in the implementation, converting it to a decimal number with `DECIMAL_DIG` significant digits and then back to floating-point yields the original value. If IEEE-754 binary64 is the most precise format your implementation supports, then its `DECIMAL_DIG` is at least 17.

2 I do not see such a rule in the C++ standard, other than incorporation of the C standard, so it may be that your C++ library is simply using a different method from your C library as a matter of choice.