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]

Try taking a look at boost::format(). It supports printf() type syntax for iostreams and is type safe. — Jon Trauntvein, Dec 14 '15 at 20:51
@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. — Jonathan Mee, Dec 16 '15 at 16:41

AlexD · Answer 1 · 2015-12-14T20:04:41.690

4

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.

edited Dec 14 '15 at 20:04

answered Dec 14 '15 at 20:01

AlexD

32,156
3
71
65

Yup, that got it, what's the meaning of fixed in this context? – Jonathan Mee Dec 14 '15 at 20:02
1

@JonathanMee It uses fixed notation. I updated the post. – AlexD Dec 14 '15 at 20:07

score 2 · Accepted Answer · answered Dec 14 '15 at 20:06

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.

Matching printf Formatting with iomanip

2 Answers2