Bool-urns
Roy: (Singing) We don’t need no education.
Moss: Yes you do. You’ve just used a double negative.
— The IT Crowd, season 1, episode 4, 2006
Do you like samurai? Do you like pizza? Do you like cats? If you answer yes to all three questions, you might like Samurai Pizza Cats. Do you also like banana? Perhaps you might be interested in Bananya. Questions such as the above often require a yes/no answer. Yes means true, you prefer something. No means false, you dislike something. Questions or statements that can be answered with yes/no or true/false are said to have boolean values. The two boolean values in JavaScript are true and false.
What else can you do with boolean values? A simple operation is to use the boolean operator ! (an exclamation mark) to negate a boolean value, thus resulting in the opposite value. The result of !true is false because the opposite of true is false. As you might have already guessed, the result of !false is true.
Given a bunch of boolean values, you can use the JavaScript boolean operators || and && to calculate boolean results. The operator || means “or”, i.e. logical disjunction. In everyday English usage, the word “or” means “either this or that”. In computer programming, “or” means “this or that or both”. The following table should help to clarify the meaning of || and its effect when given two boolean values. The table below uses OR instead of || because Markdown cannot properly render || when used within a table.
OR | false | true |
|---|---|---|
false | false | true |
true | true | true |
From the above table, the result of false || true is true, so is true || true. The one and only time when || returns false is when both operands are false. Take a moment to use the above table and work through the output of the following program. Note that in each template literal below, the result of a boolean expression is first calculated, then substituted into the appropriate location within the output string.
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/**
* Boolean or.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const coffee = true;
const tea = false;
const water = true;
const beer = false;
ns.tprintf(`Coffee or tea? ${coffee || tea}`);
ns.tprintf(`Tea or water? ${tea || water}`);
ns.tprintf(`Tea or beer? ${tea || beer}`);
}
The boolean operator && means “and”, i.e. logical conjunction. Its result is true provided that both operands are true. Its result is false for all other cases. The table below helps to clarify the effect of &&.
&& | false | true |
|---|---|---|
false | false | false |
true | false | true |
Unlike the expression true || false, the result of true && false is false. The only time when && returns true is the expression true && true. Again, take some time to work through the boolean results of the following program.
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/**
* Boolean and.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const coffee = true;
const tea = false;
const water = true;
const beer = false;
ns.tprintf(`Coffee and tea? ${coffee && tea}`);
ns.tprintf(`Coffee and water? ${coffee && water}`);
ns.tprintf(`Tea and beer? ${tea && beer}`);
}
Exercises
Exercise 1. Simplify the statement: “I will not not read anymore.”
Exercise 2. What’s the output of the following program? Explain why you got such output.
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/**
* What's my type?
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
ns.tprint(typeof true);
ns.tprint(typeof false);
}
Exercise 3. Run the following program. Modify the program so the expression likeCat && likeTiger returns false.
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/**
* Sam's pets.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const likeCat = true;
const likeDog = true;
const likeTiger = true;
const likeWolf = false;
ns.tprintf(`Sam likes cats and dogs? ${likeCat && likeDog}`);
ns.tprintf(`Sam likes tiger or wolf? ${likeTiger || likeWolf}`);
ns.tprintf(`Sam likes cat or tiger? ${likeCat || likeTiger}`);
ns.tprintf(`Sam likes cat and tiger? ${likeCat && likeTiger}`);
}
Exercise 4. What is the back of your back?
Exercise 5. Determine the output of the following program.
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/**
* Negate and negate again.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
const likeCat = true;
ns.tprintf(`${!!likeCat}`);
}
Exercise 6. Examine the following image. Determine which food Tabby dislikes. Write a program that uses boolean operators to achieve the same output as in the image.
Food for Tabby
Exercise 7. In JavaScript, the number 0 also means false. Similarly, the number 1 means true. When using boolean operators on these two numbers, the result is either 0 or 1, not false or true. For example, the result of the expression 0 || 1 is 1, which we know means true. You can use double negation to convert 0 or 1 to their corresponding boolean values. The expression !!0 now evaluates to false and !!1 evaluates to true. In the program bool-or.js above, replace false and true with 0 and 1, respectively. Use double negation to convert your results to their boolean values. Repeat the exercise for the program bool-and.js .
Exercise 8. If the double negation operator !! converts an integer to its corresponding boolean value, how would you do it the other way around? Use the function Number(). Write a program that uses the function Number() to determine the integer values of true and false, respectively.
Exercise 9. You know that true and false are like 1 and 0, respectively. In fact, you can use the arithmetic operators +, -, /, and * with boolean values. What’s the output of each statement in the following program? Why do you get such output? Why would the result of true / false be Infinity?
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/**
* Arithmetics with boolean values.
*
* @param {NS} ns The Netscript API.
*/
export async function main(ns) {
ns.tprint(true + true);
ns.tprint(true - false);
ns.tprint(false / true);
ns.tprint(true / false);
ns.tprint(true * false);
}
Exercise 10. The word “or” in everyday English means, “Either this or that, but not both.” In computer programming, this meaning of “or” is called exclusive or, often abbreviated as XOR. JavaScript does not have a boolean operator for XOR per se. However, you can achieve the same effect by combining || with &&. The XOR of two boolean variables a and b is defined as the expression
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(a || b) && !(a && b);
Fortunately, you do not need to use the above expression whenever you want to calculate the XOR of two boolean values. JavaScript has the bitwise operator ^, which is XOR defined for the binary values 0 and 1. Write a program that uses the bitwise XOR operator ^ to achieve the same output as shown in the image below.
Sam’s pet