Egg Boxes

Sums the way a keyboard writes them

TypeScript works sums out much as you would on paper, with a couple of symbols swapped for ones a keyboard has:

• + adds and - subtracts
• * multiplies, a star in place of a times sign
• / divides
• round brackets group part of a sum so it is worked out first

console.log(9 - 1 * 5);
console.log((9 - 1) * 5);
console.log(45 / 6);

4
40
7.5

Multiplying happens before subtracting unless brackets say otherwise, as in school maths. The third line is the one to look at. TypeScript's everyday type for numbers is called number, and it holds whole numbers and decimals alike. Nothing about a number says it has to be whole, so / keeps the decimals: 45 eggs split into sixes is 7.5.

A report about egg boxes needs two narrower answers instead.

How many whole groups fit? Math.floor(...) takes a number and rounds it down to a whole number, so for a count like this one it drops everything after the decimal point. Math.floor(45 / 6) is 7. It is written with a capital M, a dot, then floor, with the brackets holding what you hand over.

What is left over? % gives the remainder of a division by itself: 45 % 6 is 3, the eggs that seven full boxes of six do not use. On a keyboard it is the percent sign; in a sum it means remainder.

A worked example

Pencils and packs rather than eggs and boxes, so the report in the editor stays yours to finish:

const pencils = 50;
const perPack = 12;

console.log("Exact share ...", pencils / perPack);
console.log("Full packs ....", Math.floor(pencils / perPack));
console.log("Spare pencils .", pencils % perPack);

Exact share ... 4.166666666666667
Full packs .... 4
Spare pencils . 2

Three answers from the same two numbers. The long decimal is the share with its decimals kept, 4 is how many packs you can fill, and 2 is what is still on the desk. The rows of dots are only text inside the quotes; they line the numbers up so the report is easy to read.

The compiler checks sums too

Arithmetic only makes sense on numbers, and the compiler knows which of your values are numbers. Put quotes around one and it becomes text, and the division is refused before the program runs:

const eggs = "45";

console.log(eggs / 6);

main.ts(3,13): error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number', 'bigint' or an enum type.

In that message, the left-hand side means whatever sits to the left of the /, which is eggs here. Of the types the message lists, number is the only one this path uses. Types get an exercise of their own next.

Your turn

Nine hens each laid five eggs, and a box holds six. The program already multiplies the hens by the eggs each to get the total, and it already prints the report. Two lines are unfinished:

const fullBoxes = 0;   // whole boxes - change this
const leftOver = 0;    // eggs past those - and this

Replace each 0 with a sum built from eggs and perBox: the number of full boxes for fullBoxes, and the eggs that do not make up a box for leftOver. Press Run and read the report back. Seven full boxes and three eggs spare means it worked.

If something goes wrong

A decimal in the box count, as in Full boxes ....... 7.5, means the division is right but Math.floor is not wrapped around it yet.

A box count of 45 with no message at all means two slashes where one was meant. Two slashes begin a comment, so everything after them on that line is ignored, and fullBoxes quietly takes the whole egg count.

If nothing prints and the compiler reports Cannot find name 'math'. Did you mean 'Math'?, the capital M went missing.

Nothing here can break. Change the number of hens and run it again: the whole report works itself out afresh. The check is looking for nine hens, so put 9 back to see it pass again.