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Practice: a hiking trip planner

The Kettle Ridge trail is 12.4 km, and every time somebody plans a day on it they do the same sums on the back of an envelope: how long it takes, how much water to carry, what the permits come to. You are going to write the planner.

Everything here uses only Chapters 1 and 2: variables, IO.readln, Integer.parseInt and Double.parseDouble, and arithmetic. No String.format and no if—those are Chapters 3 and 4, so every line you print is built from IO.println and +.

The numbers the planner works from:

Quantity	Value
Trail length	12.4 km
Water	0.75 liters per hour of walking
Permit	4.50 per person

Walking pace varies by person, so that one is always asked for, in minutes per kilometer. A steady walker does about 11.

Steps

Step 1. Put the trail name Kettle Ridge and its length 12.4 into two variables, then print Kettle Ridge is 12.4 km from them. Join the pieces with +, which adds no spaces of its own, so the spaces go inside the strings.

void main() {
    String trail = "Kettle Ridge";
    // Then the length, and the line.
}

Show a solution

void main() {
    String trail = "Kettle Ridge";
    double distanceKm = 12.4;
    IO.println(trail + " is " + distanceKm + " km");
}

+ puts nothing between the pieces it joins, so the spaces are typed inside the strings, " is " and " km".

Step 2. Ask for a walking pace in minutes per kilometer and print how many minutes the trail takes. Remember that IO.readln gives back a String: multiply 12.4 by that string and javac will refuse, so convert it first.

void main() {
    double distanceKm = 12.4;
    // Ask for the pace, then print the minutes.
}

Show a solution

void main() {
    double distanceKm = 12.4;
    String answer = IO.readln("Pace in minutes per km? ");
    double pace = Double.parseDouble(answer.strip());
    double minutes = distanceKm * pace;
    IO.println("That takes " + minutes + " minutes");
}

Double.parseDouble and not Integer.parseInt, because a pace can be 10.5. Without the conversion, javac refuses to multiply a number by a String: bad operand types for binary operator '*'.

Step 3. Minutes are hard to picture. Print the time in hours as well as minutes, by dividing.

void main() {
    double distanceKm = 12.4;
    // Pace in; minutes, hours out.
}

Show a solution

void main() {
    double distanceKm = 12.4;
    String answer = IO.readln("Pace in minutes per km? ");
    double pace = Double.parseDouble(answer.strip());
    double minutes = distanceKm * pace;
    double hours = minutes / 60;
    IO.println(minutes + " minutes, or " + hours + " hours");
}

Sixty minutes in an hour, so the hours are the minutes divided by 60. The answer is a long decimal for now; Chapter 3 shows how to round it.

Step 4. Work out the water. At 0.75 liters for every hour walked, print how many liters one person needs—and note that it is liters per hour, not per minute.

void main() {
    double distanceKm = 12.4;
    // Ask for the pace; print one person's water.
}

Show a solution

void main() {
    double distanceKm = 12.4;
    String answer = IO.readln("Pace in minutes per km? ");
    double pace = Double.parseDouble(answer.strip());
    double minutes = distanceKm * pace;
    double hours = minutes / 60;
    double water = hours * 0.75;
    IO.println("Water per person: " + water + " liters");
}

Per hour, so it multiplies the hours. Multiplying the minutes asks for 102 liters, which is a clue that the units were wrong.

Step 5. Ask how many people are going, and print the water the group needs altogether.

void main() {
    double distanceKm = 12.4;
    // Pace and group in; water out.
}

Show a solution

void main() {
    double distanceKm = 12.4;
    String answer = IO.readln("Pace in minutes per km? ");
    double pace = Double.parseDouble(answer.strip());
    double minutes = distanceKm * pace;
    double hours = minutes / 60;
    double water = hours * 0.75;
    int people = Integer.parseInt(IO.readln("How many people? ").strip());
    IO.println("Water for the group: " + (water * people) + " liters");
}

Integer.parseInt for a head count, since half a walker does not turn up.

Step 6. Ask how many people are going, and print what the permits cost, at 4.50 each.

void main() {
    // People in; permit total out.
}

Show a solution

void main() {
    int people = Integer.parseInt(IO.readln("How many people? ").strip());
    IO.println("Permits: " + (4.50 * people));
}

Without Integer.parseInt, the program would multiply 4.50 by a String, and javac refuses to.

Step 7. Put the planner together: ask for the trail length, the pace, and the group size, then print the length, the time in minutes, the time in hours, the water for one person, the water for the group, and the permit total.

void main() {
    // Three questions, then the whole plan.
}

Show a solution

// Plans a day on a trail: time, water and permits, from three answers.

void main() {
    String lengthAnswer = IO.readln("Trail length in km? ");
    double distanceKm = Double.parseDouble(lengthAnswer.strip());
    String paceAnswer = IO.readln("Pace in minutes per km? ");
    double pace = Double.parseDouble(paceAnswer.strip());
    int people = Integer.parseInt(IO.readln("How many people? ").strip());

    double minutes = distanceKm * pace;
    double hours = minutes / 60;
    double water = hours * 0.75;

    IO.println("Length: " + distanceKm + " km");
    IO.println("Time: " + minutes + " minutes");
    IO.println("Time: " + hours + " hours");
    IO.println("Water each: " + water + " liters");
    IO.println("Water total: " + (water * people) + " liters");
    IO.println("Permits: " + (4.50 * people));
}

Every printed number comes from the three that were typed in. Change the trail length and the whole plan follows.