Lesson 17 · 17 of 20

Formatting output · C++

Line up columns and control decimals in C++ with setw, left, right, setfill, fixed and setprecision from <iomanip>: a café menu.

C++ · Formatting output in DMarlon Studio: the example's code and its output in the terminal

`<iomanip>` brings the manipulators, which you send to std::cout with << to change how whatever follows is printed. With them, the café menu is laid out in columns.

`std::setw(14)` reserves 14 characters for the next value only, which is why it is repeated before each column. `std::left` and `std::right` align the text within that width, and they stay in force until you change them.

`std::setfill('-')` changes the padding character: with setw(22) and an empty string, it draws a line of 22 dashes. Then std::setfill(' ') goes back to spaces. The same trick with '0' prints the order number 00042.

`std::fixed` together with `std::setprecision(2)` always prints two decimals: 3 comes out as 3.00, perfect for prices. Without fixed (std::defaultfloat), setprecision(3) counts significant digits, so π becomes 3.14.

`std::hex` and `std::oct` print integers in hexadecimal and octal; std::dec goes back to decimal. The pairs in menu are walked with structured bindings, as in the previous lesson.

Code · main.cpp

#include <iomanip>#include <iostream>#include <string>#include <utility>#include <vector>int main() {    std::vector<std::pair<std::string, double>> menu = {        {"Coffee", 1.5}, {"Croissant", 2.25}, {"Orange juice", 3}, {"Toast", 1.8}};    // setw sets the width of the NEXT value; left and right align it.    std::cout << std::left << std::setw(14) << "Item" << std::right << std::setw(8) << "Price" << '\n';    // setfill changes the character that fills the gap.    std::cout << std::setfill('-') << std::setw(22) << "" << std::setfill(' ') << '\n';    // fixed + setprecision(2): always two decimals, until changed.    std::cout << std::fixed << std::setprecision(2);    double total = 0;    for (const auto& [name, price] : menu) {        std::cout << std::left << std::setw(14) << name << std::right << std::setw(8) << price << '\n';        total += price;    }    std::cout << std::left << std::setw(14) << "TOTAL" << std::right << std::setw(8) << total << '\n';    // Leading zeros for an order number.    std::cout << "Order #" << std::setfill('0') << std::setw(5) << 42 << std::setfill(' ') << '\n';    // Without fixed, setprecision counts significant digits.    std::cout << std::defaultfloat << std::setprecision(3) << "Pi is about " << 3.14159 << '\n';    // Integers in other bases.    std::cout << "255 in hex: " << std::hex << 255 << ", in octal: " << std::oct << 255 << std::dec << '\n';    return 0;}

Output · c++ -std=c++17 main.cpp -o main && ./main

Item             Price
----------------------
Coffee            1.50
Croissant         2.25
Orange juice      3.00
Toast             1.80
TOTAL             8.55
Order #00042
Pi is about 3.14
255 in hex: ff, in octal: 377

Key points

std::setw(n) sets the width of the next value.
std::left and std::right align, and last until changed.
std::setfill(c) changes the padding character.
std::fixed << std::setprecision(2): always two decimals.
std::hex, std::oct and std::dec change the base.

Exercise

Add a Qty column to the menu with the units ordered of each item and another with the amount (units times price), all right-aligned, and the total at the bottom.