Lesson 7 · 7 of 20

Comparisons and logic · C++

Compare values in C++ with ==, !=, < and >, combine conditions with &&, || and !, and print bools as true and false with std::boolalpha.

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

A comparison gives a `bool`: age >= 18 is false at 16. The operators are == (equal), != (not equal), <, >, <= and >=. Careful: = assigns and == compares.

By default, std::cout prints a bool as 0 or 1. After `std::boolalpha` it prints false and true, and keeps doing so until the program ends. Inside a chain of <<, comparisons go in brackets.

`&&` (and) needs both sides to be true, `||` (or) needs at least one and `!` (not) flips the value. That is why a minor can see the film with an adult, and 31.5 degrees is a hot day: it lies between 30 and 45.

C++ uses short-circuit evaluation: if the left side of && is false, the right side is never worked out. Thanks to that, boxes != 0 && items / boxes > 5 never divides by zero.

std::string values can be compared too: < follows alphabetical order, and "apple" < "banana" is true.

Code · main.cpp

#include <iostream>#include <string>int main() {    int age = 16;    bool withAdult = true;    double temperature = 31.5;    // A comparison gives a bool: true or false.    bool adult = age >= 18;    std::cout << "adult = " << adult << '\n';  // by default, 0 or 1    std::cout << std::boolalpha;               // from now on, false or true    std::cout << "adult = " << adult << '\n';    std::cout << "age == 16: " << (age == 16) << ", age != 16: " << (age != 16) << '\n';    // && (and): both must be true. || (or): at least one. ! (not) reverses.    bool canEnter = adult || withAdult;    std::cout << "Can see the film: " << canEnter << '\n';    bool hotDay = temperature > 30 && temperature <= 45;    std::cout << "Hot day: " << hotDay << ", not hot: " << !hotDay << '\n';    // Short-circuit: if the left side decides, the right one is not evaluated.    int boxes = 0, items = 12;    if (boxes != 0 && items / boxes > 5) std::cout << "The boxes are full\n";    else std::cout << "No boxes to check (and no division by zero)\n";    // Strings are compared in alphabetical order.    std::string a = "apple", b = "banana";    std::cout << a << " < " << b << ": " << (a < b) << '\n';    return 0;}

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

adult = 0
adult = false
age == 16: true, age != 16: false
Can see the film: true
Hot day: true, not hot: false
No boxes to check (and no division by zero)
apple < banana: true

Key points

A comparison gives true or false.
== compares; = assigns.
&& (and), || (or) and ! (not) combine conditions.
With && and ||, the right side is only evaluated when needed.
std::boolalpha prints true and false instead of 1 and 0.

Exercise

Write the condition for a special offer: there is a discount if the customer is a member (bool member) or spends over 100 euros, but never on a Sunday (bool sunday). Try it with several combinations.