Using the iomanip C++ library

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Using the iomanip C++ library

The library provides features for handling output formatting, such as the base used when formatting integers and the precision of floating point values .

It has the following functions:

  • setiosflags - Defines flags format
  • resetiosflags - Resets flags format
  • setbase - Sets the base of the flags
  • setfill - Defines the filling of characters
  • setprecision - Sets the precision of decimals
  • setw - Sets the width of a field
  • get_money - Gets the monetary value
  • put_money - Insert the monetary value
  • get_time - Get the time and date
  • put_time - Get the time and date

Some examples

setfill and setw

Fill with a certain character on the left side of the number(int, float, double,…)

#include <iostream>
#include <iomanip>

int main(){
  std::cout << std::setfill (' ') << std::setw( 10 );
  std::cout << 3.69f << '\n';
  return 0;
}

Filled with 6 blank spaces(10 minus the number of characters), so the output will be: 3.69. If we change std::setfill (' ') to std::setfill ('-') the output will be: ——3.69 .

setprecision

Defines how many digits you want to appear if there is no more after the comma ( in this case, point . ), it will fill with 0 zeros if we use std::fixed .

#include <iostream>
#include <iomanip>

int main(){
  double pi = 3.14159;
  std::cout << std::setprecision( 3 ) << pi << '\n';
  return 0;
}

Output: 3.14

#include <iostream>
#include <iomanip>

int main(){
  double pi = 3.14159;
  std::cout << std::setprecision( 4 ) << pi << '\n';
  return 0;
}

Output: 3.142 because the next significant number is equal to or greater than 5, so it approximates the previous one.

And if you change it to std::setprecision( 9 ), the output will be: 3.14159 will not complete with zeros, but using std::fixed

#include <iostream>
#include <iomanip>

int main(){
  double pi = 3.14159;
  std::cout << std::fixed << std::setprecision( 9 ) << pi << '\n';
  return 0;
}

The output will be: 3.141590000

get_money and put_money

Informs the value in integer, in other words transforms: float, double,… in integer. They are useful for use with data entry. Example:

#include <iomanip>
#include <iostream>

int main() {
  long double preco;
  std::cout << "Enter the price: ";
  std::cin >> std::get_money( preco );

  if( std::cin.fail() ){
    std::cerr << "Failed to read price." << '\n';
    return 1;
  }else{
    std::cout << "The price is: " << std::put_money( preco ) << '\n';
  }

  return 0;
}

get_time

Formats the time entry, example:

#include <iostream> 
#include <iomanip> 
#include <ctime> // struct std::tm

int main(){
  struct std::tm dthr;
  std::cout << "Enter the time: ";
  std::cin >> std::get_time(&dthr,"%R");   //extract the hours in 24H format 

  std::cout << "It's " << 
               dthr.tm_hour << " hours and " 
               << dthr.tm_min << " minutes\n";

  return 0;
}

Output:

Enter the time: 12:23
It's 12 hours and 23 minutes

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