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c++floating-pointprintfcoutmodifiers

Matching printf Formatting with iomanip

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]

Solution

  • 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.