使用 fmtlib，当值为负时，零填充数值更短，我可以调整这种行为吗？

Question

I am using fmtlib to format strings and numeric values but I have problem with negative integers. When I pad the value with zeroes, I expect a consistent number of zero no matter the sign of the value.

For instance, using a padding of 4, I would like the following:

• 2 to be returned as "0002"
• -2 to be returned as "-0002"

fmtlib's default behaviour is to take into account a prefix lenght (ie the sign "-") into the padded length which means that -2 is returned as "-002"

Here is an example:

#include <iostream>
#include "fmt/format.h"

int main()
{
    std::cout << fmt::format("{:04}", -2) << std::endl;
}

will output: -002

Is there a way to toggle this behaviour or a different way to zero-fill values to get my expected result?

Thanks for any help,

Answer 1

There's certainly nothing in the documentation for either fmt or Python's str.format (which fmt's syntax is based on). Both only state that padding is "sign-aware".

This question asks for the same functionality for Python's str.format . The accepted answer there is to move the length to an argument, and make it one larger if the number is negative. Translating that to C++:

for (auto x : { -2, 2 }) {
    fmt::print("{0:0{1}}\n", x, x < 0 ? 5 : 4 ); // prints -0002 and 0002
}

To break the format syntax down:

{0:0{1}}
 │ │ └ position of the argument with the length
 │ └── "pad with zeros"
 └──── position of the argument with the value

https://godbolt.org/z/5xz7T9