Rainfall Week

Two values that have to survive the loop

There is almost no new syntax in this exercise. An array, a for...of loop, an if, a template literal, + — every piece of it has already been on screen. That is the point: holding five ideas separately is not the same as getting them to cooperate in one program, and that gap is where most people stall.

The one new sign is >, which is the >= from the umbrella check with its equals sign taken off: strictly bigger, so a tie does not count.

The idea that joins them up is small. A name made above a loop is still standing on every pass, so whatever one pass leaves in it, the next pass finds. Anything the loop must carry from day to day belongs above it, and two things here are worth carrying that way.

A running total. Start a name at zero above the loop, then inside it write total = total + mm. JavaScript works out the right-hand side first — whatever the total held a moment ago, plus this pass's measurement — and stores it back under the same name.

The best so far. Start a second name at zero, then inside the loop ask whether this pass beats it: if (mm > wettestMm). When it does, store the new number. By the final pass, the largest so far is simply the largest.

Keeping which day it was is one more line inside the same if: the amount and the day are one fact in two halves.

Why these are let and rain is const

You have met both words. const promises the name will never be pointed at a different value; let is for a name meant to move, like the countdown's counter or the scoreboard's player, score and bar. A running total is the clearest case of all — it exists in order to change — so it is declared with let, and so are the record holder and the day counter. Point a const at something new and JavaScript stops with TypeError: Assignment to constant variable.

rain stays const, because the measurements never change, and so does bar: each pass builds a brand new one rather than changing the old.

The word is half the story. Where the declaration sits is the other half:

for (const n of [1, 2, 3]) {
  let total = 0;
  total = total + n;
  console.log(`this pass: ${total}`);
}

this pass: 1
this pass: 2
this pass: 3

Nothing adds up, because the line setting it to zero runs again every time round. Worse, a let made inside braces belongs to those braces: after the loop that name does not exist at all, and asking for it gives ReferenceError: total is not defined.

A worked example

Four marks from a notebook, with the total printed on every pass so you can watch it climb:

const marks = [4, 7, 2, 7];
let total = 0;
let best = 0;

for (const mark of marks) {
  total = total + mark;
  if (mark > best) {
    best = mark;
  }
  console.log(`mark ${mark}  total ${total}`);
}

console.log(`best mark: ${best}`);

mark 4  total 4
mark 7  total 11
mark 2  total 13
mark 7  total 20
best mark: 7

total never drops back to zero, because the line that starts it sits above the loop and runs once. best ends on 7 and does not move on the last pass, even though that mark is a 7 as well: 7 > 7 is false, so a tie leaves the first one holding the record.

Your turn

The editor holds a week of rainfall. rain is an array of seven measurements in millimetres, and the loop prints a row for each: the day number, a bar of one # per millimetre, then the measurement. Day two was dry, so its bar is empty — correct, not broken.

Press Run first. The chart is right; the two lines beneath it are not. The total reads 0 mm, and the wettest day comes out as day 0, which is not one of the seven.

Both additions go inside the loop's braces.

1. Add this day's rain to total.
2. If this day beats wettestMm, store both the new amount and the day number.

All three names already exist above the loop, holding zero: you are changing them, not making them. And day = day + 1 is a running total too, so the shape of your first line is on screen already.

If something goes wrong

If the program stops with ReferenceError: mm is not defined, the new lines are below the loop's closing brace rather than inside it. mm is the loop's own name and is gone once the loop finishes, so move the lines up inside the braces.

If the total still reads 0 mm with no error, look for let in front of total inside the loop. That makes a second total belonging to the braces; the addition goes into it, and the one the last lines print never moves. Written as let total = total + mm, the same slip stops the program with ReferenceError: Cannot access 'total' before initialization. Either way, delete the let: the name exists already. A line reading only total + mm also leaves 0, because the sum is worked out and never stored.

If the amount is right but the day is wrong, look at the if. Either the line storing the day is missing, or the if has no braces, in which case only the first line belongs to it and the second runs every pass, leaving day 7 on the board. Tidy indentation counts for nothing; the braces decide.

A TypeError about a constant variable means a const is being changed: either a value that moves was made with const, or a single = has crept into the if and is trying to store into mm, which the loop made with const.

Once the week adds up, change a number in rain and run it again: the bar, the total and the wettest day should all move on their own.