Difference between C++ stream and print() - fillers

Question

It has been a while since I posted any questions on StackOverflow; I hope I remember more or less the appropriate way to address a question (sorry in advance).

I was playing with C++ streams and the FMT library, which provides a preview to C++23 print. Streams are chill when setting fillers to complete the display width of an integer. Example:

const int y = 730;
cout << "y = " << setw(5) << y << endl;
cout << "y = " << setfill('0') << setw(5) << y << endl;
cout << "y = " << setfill('X') << setw(5) << y << endl;
cout << "y = " << setfill('*') << setw(5) << y << endl;

The output is

y =   730
y = 00730
y = XX730
y = **730

I was trying to set the same fillers (including 'X' and '*' chars) using fmt::print—something like:

print("y = {:5d}\n", y);
print("y = {:05d}\n", y);

Well, I failed to include 'X' and '*'. I am not claiming it is a good idea to have random chars as fillers; I am just curious as I need to explain the difference between these commands to students.

Thanks in advance. Vivian

Try adding the alignment specifier: https://godbolt.org/z/oaadTx5nY — Bob__, Mar 29 '23 at 18:31
The best thing you can do for such questions is to provide a piece of code we can run as is (with the right headers, with `main()`, etc). If there's also a godbolt link to quickly run it, that's a huge plus. — HolyBlackCat, Mar 29 '23 at 18:38

HolyBlackCat · Accepted Answer · 2023-03-29T18:43:34.627

6

The manual shows following grammar for integer format specifiers:

format_spec ::=  [[fill]align][sign]["#"]["0"][width]["." precision]["L"][type]
fill        ::=  <a character other than '{' or '}'>
align       ::=  "<" | ">" | "^"

Here format_spec is the part inside of {: }.

So specifying a custom filler (fill) also requires specifying alignment (you want >, i.e. align to the right):

fmt::print("y = {:X>5d}\n", y); // XX730
fmt::print("y = {:*>5d}\n", y); // **730

edited Mar 29 '23 at 18:43

answered Mar 29 '23 at 18:33

HolyBlackCat

78,603
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Awesome - one quick change is that you need to remove the 0 – Vivian Miranda Mar 29 '23 at 18:41
@VivianMiranda Thanks, fixed. Interesting that it worked with `0` too. – HolyBlackCat Mar 29 '23 at 18:43
it didn't worked with zero in my laptop (which is also interesting!) – Vivian Miranda Mar 29 '23 at 18:45
1

@VivianMiranda Hmm, my guess is that you use an older version of libfmt. – HolyBlackCat Mar 29 '23 at 18:45
1

@VivianMiranda The 0 (which *"enables sign-aware zero-padding for numeric types"*) in `{:X>05d}` is ignored by the trunk version, while up to 9.1 "dominates" over the fill char. I'm not sure what should be the expected behavior, here. – Bob__ Mar 29 '23 at 18:51
awesome - thank you so much for digging into this. – Vivian Miranda Mar 29 '23 at 18:53
2

@Bob__ The C++ standard says that if *align* is present, the 0 will be squeezed out and not show up. http://eel.is/c++draft/format.string.std#8 Likely that {fmt} wants to follow that rule too. – BoP Mar 29 '23 at 21:00

score 5 · Answer 2 · answered Mar 29 '23 at 18:35

5

The alignment is mandatory when a filling character is given.

print("y = {:*>5}\n", y);

answered Mar 29 '23 at 18:35

dalfaB

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Difference between C++ stream and print() - fillers

2 Answers2