Formatted Printing

fmt.Println gets you started; fmt.Printf is how programs actually talk. It's Go's version of the C printf family — a format string with verb placeholders, filled by the arguments that follow — and since graders (and log parsers, and users) compare output byte-for-byte, fluency here pays off on literally every exercise that follows.

The verbs that matter

The working set:

Three of these are quiet superstars:

• %v prints any value sensibly — slices as [1 2 3], maps as map[a:1], structs as {Alice 30}. And %+v adds field names: {Name:Alice Age:30}. It's the universal debugging verb.
• %q wraps strings in quotes and shows escape characters — when output "looks right" but fails comparison, %q reveals the trailing \n or sneaky tab that %s hides. Debugging tool of the course.
• %T answers "what type is this, actually?" — worth its weight when type errors confuse.

Width and precision

Numbers between % and the verb control layout:

That's how tables line up and how "print with exactly two decimal places" — a constant refrain in graded exercises — is spelled: %.2f rounds (3.847 → 3.85), it doesn't truncate.

The family

Same verbs, three destinations:

Sprintf is the one people forget exists — any time you're tempted to build a string with + and conversions, Sprintf is cleaner: label := fmt.Sprintf("(%d, %d)", x, y).

The two classic mistakes

1. Printf does not add a newline. Println does. Forgetting \n in a format string glues your output to the next line — and fails byte-exact tests. When output is "correct but on one line," this is why.
2. Verb/argument mismatch — %d with a string prints %!d(string=hi) right into your output. Go doesn't crash; it tells you inline, and go vet catches most cases at build time. Read the noise; it names the verb and the type.

Your exercise — the greeting card — is a Printf composition test: fixed text, interpolated values, exact spacing, exact newlines. Sketch the expected output literally, then transcribe it into a format string verb by verb. That transcription skill is the whole lesson.