Exercise 4 of 10 · For loops

Bus Timetable

What you will make

A program that lists five upcoming bus departure times, spaced out by a fixed interval.

The one new idea: Counting for loops (for i := 0; i < n; i++) that compute a new value on each pass

Repeating a calculation a fixed number of times — and getting the loop bound exactly right — is one of the most common patterns in everyday programming, from timetables to batch processing.

Go straight to the code ↓

A for i := 0; i < n; i++ loop is Go's standard way to repeat something a known number of times, and getting the middle condition right is the whole trick: i < n runs the body exactly n times (for i equal to 0, 1, ..., n-1), while i <= n sneaks in one extra pass. Inside the loop, you're free to use i in a formula — just remember Go evaluates * before + unless you use parentheses to say otherwise, same as in ordinary math.

Worked example

Here's the same timetable idea with a shorter list and a different interval:

package main

import "fmt"

func main() {
	numBuses := 3
	intervalMinutes := 15
	firstDeparture := 4

	fmt.Println("Bus timetable (minutes from now):")
	for i := 0; i < numBuses; i++ {
		departure := firstDeparture + i*intervalMinutes
		fmt.Printf("Bus %d: %d min\n", i+1, departure)
	}
}

Output

Bus timetable (minutes from now):
Bus 1: 4 min
Bus 2: 19 min
Bus 3: 34 min

Three buses print because the loop stops as soon as i reaches 3, and each departure adds one more full 15-minute interval to the first one.

Your turn

The starter program is supposed to print exactly five buses, spaced 12 minutes apart, starting 6 minutes from now — but the loop runs one time too many, and the departure formula groups its terms incorrectly. Fix both marked lines so the timetable matches what the comments describe.

If something goes wrong

If you see six buses instead of five, the loop bound is still off by one. If the numbers grow much faster than expected (or shrink), check the parentheses in the departure formula — multiplying the whole (firstDeparture + i) by the interval is a very different calculation from adding i*intervalMinutes to firstDeparture. Try tracing the first two iterations by hand if the output still looks wrong.

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Things that often go wrong here

Using <= instead of < as a loop bound when counting from 0.: A loop from i := 0 with i <= n runs n+1 times, one more than intended, because it includes both 0 and n.
Wrapping the wrong terms in parentheses in a mixed +/* expression.: Go follows normal operator precedence (* before +), so (a + i) * b is a different calculation than a + i*b — parentheses change which operation happens first.
Printing the loop variable i directly as a 1-based label.: i starts at 0, so a human-facing label like 'Bus 1' needs i+1, not i itself.
Re-declaring a loop-body variable with := every iteration by accident and expecting it to persist.: A variable declared inside a for loop's body is a fresh variable each pass — it doesn't carry its value into the next iteration the way a variable declared before the loop does.

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