使用 C++ output 运算符打印前导零？

Question

How can I format my output in C++? In other words, what is the C++ equivalent to the use of printf like this:

printf("%05d", zipCode);

I know I could just use printf in C++, but I would prefer the output operator << .

Would you just use the following?

std::cout << "ZIP code: " << sprintf("%05d", zipCode) << std::endl;

Answer 1

This will do the trick, at least for non-negative numbers ^(a) such as the ZIP codes ^(b) mentioned in your question.

#include <iostream>
#include <iomanip>

using namespace std;
cout << setw(5) << setfill('0') << zipCode << endl;

// or use this if you don't like 'using namespace std;'
std::cout << std::setw(5) << std::setfill('0') << zipCode << std::endl;

The most common IO manipulators that control padding are:

• std::setw(width) sets the width of the field.
• std::setfill(fillchar) sets the fill character.
• std::setiosflags(align) sets the alignment, where align is ios::left or ios::right.

And just on your preference for using << , I'd strongly suggest you look into the fmt library (see https://github.com/fmtlib/fmt ). This has been a great addition to our toolkit for formatting stuff and is much nicer than massively length stream pipelines, allowing you to do things like:

cout << fmt::format("{:05d}", zipCode);

And it's currently being targeted by LEWG toward C++20 as well, meaning it will hopefully be a base part of the language at that point (or almost certainly later if it doesn't quite sneak in).

---

^(a) If you do need to handle negative numbers, you can use std::internal as follows:

cout << internal << setw(5) << setfill('0') << zipCode << endl;

This places the fill character between the sign and the magnitude.

---

^(b) This ("all ZIP codes are non-negative") is an assumption on my part but a reasonably safe one, I'd warrant :-)

Answer 2

使用setw 和 setfill调用：

std::cout << std::setw(5) << std::setfill('0') << zipCode << std::endl;

Answer 3

In C++20 you'll be able to do:

std::cout << std::format("{:05}", zipCode);

In the meantime you can use the {fmt} library , std::format is based on.

Disclaimer : I'm the author of {fmt} and C++20 std::format .

Answer 4

cout << setw(4) << setfill('0') << n << endl;

from:

http://www.fredosaurus.com/notes-cpp/io/omanipulators.html

Answer 5

or,

char t[32];
sprintf_s(t, "%05d", 1);

will output 00001 as the OP already wanted to do

Answer 6

Simple answer but it works!

ostream &operator<<(ostream &os, const Clock &c){
// format the output - if single digit, then needs to be padded with a 0
int hours = c.getHour();

// if hour is 1 digit, then pad with a 0, otherwise just print the hour
(hours < 10) ? os << '0' << hours : os << hours;

return os; // return the stream
}

I'm using a ternary operator but it can be translated into an if/else statement as follows

if(c.hour < 10){
 os << '0' << hours;
}
 else{
  os  << hours;
 }