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Practice: a savings goal tracker

The bike costs 240. You put something aside each week, and you want to know when you will have it—not roughly, but which week.

Everything here uses Chapters 1 to 5: variables, IO.readln and Double.parseDouble, arithmetic, joining text with +, if, and now while, for, break and continue. There are no arrays or lists yet, so the running total lives in a single variable that you add to as you go—the accumulator pattern from the chapter.

Each step is the same question with one thing changed, so the answer moves:

Saving 15 a week	Weeks to reach 240
Straightforwardly	16
Skipping every 5th week	19
With a 10 bonus every 4th week	14
With 1% interest added weekly	15

In every step that looks for the goal, reaching exactly 240 counts.

Steps

Step 1. Print Week 1 through Week 8, one per line, with a for loop. Eight lines, and one IO.println in your code.

void main() {
    // Week 1 to Week 8, one per line.
}

Show a solution

void main() {
    for (int week = 1; week <= 8; week++) {
        IO.println("Week " + week);
    }
}

week = 1 and week <= 8 give the eight weeks 1 to 8, from one IO.println. Starting at 0 would label the first week zero.

Step 2. Ask how much goes in each week, then print the running total at the end of each of the first 8 weeks. Keep one variable and add to it—do not recalculate from scratch each time.

void main() {
    // Eight running totals.
}

Show a solution

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;

    for (int week = 1; week <= 8; week++) {
        total += weekly;
        IO.println("Week " + week + ": " + total);
    }
}

total starts at 0 outside the loop and survives each pass. Created inside the loop, it would start again at 0 every week. A double prints with its decimal point, so 15 a week shows as 15.0.

Step 3. Ask the weekly amount again and use a while loop to find how many weeks it takes to reach 240. Print the number of weeks. Reaching exactly 240 counts.

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    // Then a while loop, up to 240.
}

Show a solution

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;
    int weeks = 0;

    while (total < 240) {
        total += weekly;
        weeks++;
    }

    IO.println("Weeks needed: " + weeks);
}

while (total < 240) stops the moment the goal is met. Using <= keeps going for one more week after it is already reached.

Step 4. A while loop that never reaches its goal runs forever. Do the same thing with a for loop over 52 weeks and a break when the goal is met. Report after the loop rather than inside it: print Reached in week N, or Not this year if the 52 weeks run out. The loop's own week is gone once the loop ends, so keep the week you reach the goal in a variable created before the loop. Reporting from inside the loop is the bug this step is about—without the break it announces success every week from the 16th on.

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    // Then a for loop over 52 weeks.
}

Show a solution

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;
    int reachedIn = 0;

    for (int week = 1; week <= 52; week++) {
        total += weekly;
        if (total >= 240) {
            reachedIn = week;
            break;
        }
    }

    if (reachedIn > 0) {
        IO.println("Reached in week " + reachedIn);
    } else {
        IO.println("Not this year");
    }
}

The break leaves the loop in the week the goal is met, and reachedIn keeps that week after the loop's own week is gone, so the report happens once, afterward. reachedIn stays 0 when the goal is never met, which is how the if after the loop tells the two endings apart.

Step 5. Every 5th week the money goes on something else. Using continue, skip weeks 5, 10, 15 and so on—nothing saved, nothing lost—and print which week the goal is reached.

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    // Skip every 5th week.
}

Show a solution

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;

    for (int week = 1; week < 200; week++) {
        if (week % 5 == 0) {
            continue;
        }
        total += weekly;
        if (total >= 240) {
            IO.println("Reached in week " + week);
            break;
        }
    }
}

continue skips the rest of this pass and moves on to the next week. break would end the loop entirely, which here means giving up in week 5.

Step 6. Instead, a 10 bonus lands every 4th week—weeks 4, 8, 12—on top of the usual amount. Print which week the goal is reached now. Be careful which weeks you mean: paying it on weeks 1, 5 and 9 instead gives the same answer for a lot of weekly amounts, and a different one for the rest.

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    // A 10 bonus every 4th week.
}

Show a solution

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;

    for (int week = 1; week < 200; week++) {
        total += weekly;
        if (week % 4 == 0) {
            total += 10;
        }
        if (total >= 240) {
            IO.println("Reached in week " + week);
            break;
        }
    }
}

week % 4 == 0 is every fourth week: 4, 8, 12. Comparing with 1 picks weeks 1, 5 and 9 instead, which pays the first bonus three weeks early.

Step 7. The account pays 1% a week, added after your deposit goes in. Print which week the goal is reached.

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    // 1% interest after each deposit.
}

Show a solution

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;

    for (int week = 1; week < 200; week++) {
        total += weekly;
        total *= 1.01;
        if (total >= 240) {
            IO.println("Reached in week " + week);
            break;
        }
    }
}

The deposit goes in first, then total *= 1.01 adds one percent of the new balance. Doing it the other way around pays nothing on the money just paid in.

Step 8. Put the tracker together: ask for the weekly amount, then print the balance at the end of each of the first 8 weeks as Step 2 did, with no bonus and no skipped weeks, and then the week the goal is reached, this time counting the bonus every 4th week and skipping every 5th.

void main() {
    // Eight balances, then the goal week.
}

Show a solution

// The savings tracker: eight weekly balances, then the week the goal is reached.

void main() {
    double weekly = Double.parseDouble(IO.readln("Saving how much a week? ").strip());
    double total = 0;

    for (int week = 1; week <= 8; week++) {
        total += weekly;
        IO.println("Week " + week + ": " + total);
    }

    total = 0;
    for (int week = 1; week < 200; week++) {
        if (week % 5 == 0) {
            continue;
        }
        total += weekly;
        if (week % 4 == 0) {
            total += 10;
        }
        if (total >= 240) {
            IO.println("Goal reached in week " + week);
            break;
        }
    }
}

Nothing new, only assembly: the first loop prints the eight balances, then total goes back to 0 for the second, which skips every fifth week and pays the bonus every fourth. The bonus and the skipped weeks pull in opposite directions, and together they land on week 17 rather than either answer on its own.