Reference

How to read code

Reading code means running it in your head before the computer does. The one rule: predict first, then reveal — you cannot run code you have not predicted. Here are the six reading moves the lab drills. Ready to try one? Open the trace lab →

• Read top to bottom

  Run each line in order, one at a time — the same way the computer does. Never jump ahead.

  let a = 6
  let b = 4
  print(a + b)

  prints 10

• A var is a box you can refill

  A var can be given a new value. Always keep the NEWEST value, not the first one.

  var n = 3
  n = n + 5
  print(n)

  prints 8

• Whole-number division drops the rest

  Dividing two whole numbers throws away the remainder toward zero — 7 / 2 is 3, not 3.5.

  print(7 / 2)

  prints 3

• ..< stops before · ... includes

  A range with ..< never reaches the last number; ... does include it. This is the #1 off-by-one to check.

  for i in 1..<4 {
    print(i)
  }

  prints 1 ⏎ 2 ⏎ 3

• Arrays count from 0

  The first item is at index [0], the second at [1]. And .count is how many there are.

  let p = ["cat","dog"]
  print(p[0])
  print(p.count)

  prints cat ⏎ 2

• \( … ) fills in a value

  Inside a string, \( name ) is replaced by that value. The name becomes what it holds.

  let x = 9
  print("age \(x)")

  prints age 9

Every program in TraceForge is written in a teaching subset of the Swift language — the same rules real programmers use, kept small so you can trace it by hand. Try a puzzle →