Matching printf Formatting with iomanip

Question

I have some old C code I'm trying to replicate the behavior of in C++. It uses the printf modifiers: "%06.02f".

I naively thought that iomanip was just as capable, and did:

cout << setfill('0') << setw(6) << setprecision(2)

When I try to output the test number 123.456, printf yields:

123.46

But cout yields:

1.2+e02

Is there anything I can do in iomanip to replicate this, or must I go back to using printf?

[Live Example]

Alberich

Try taking a look at boost::format(). It supports printf() type syntax for iostreams and is type safe. – Kronick

@JonTrauntvein While I appreciate the tip (especially since tons of stuff in boost eventually makes it's way into the standard) I don't include boost wherever possible. And here it seems possible not to include it. – Alberich

Answer 1

The three C format specifiers map to corresponding format setting in C++ IOStreams:

• %f -> std::ios_base::fixed (fixed point notation) typically set using out << std::fixed.
• %e -> std::ios_base::scientific (scientific notation) typically set using out << std::scientific.
• %g -> the default setting, typically set using out.setf(std::fmtflags(), std::ios_base::floatfield) or with C++11 and later out << std::defaultfloat. The default formatting is trying to yield the "best" of the other formats assuming a fixed amount of digits to be used.

The precision, the width, and the fill character match the way you already stated.

Kimball

Answer 2

Try std::fixed:

std::cout << std::fixed;

Sets the floatfield format flag for the str stream to fixed.

When floatfield is set to fixed, floating-point values are written using fixed-point notation: the value is represented with exactly as many digits in the decimal part as specified by the precision field (precision) and with no exponent part.

Misbehave

Yup, that got it, what's the meaning of fixed in this context? – Alberich

@JonathanMee It uses fixed notation. I updated the post. – Misbehave