if, else if and else

Everything so far has run every line, every time, in order. if is where a sketch starts to choose.

if (something is true) {
  do this
}

The condition in the brackets is a yes-or-no question — exactly the kind you built in the boolean lesson. If the answer is yes, the block runs. If not, it is skipped entirely.

else catches everything the if did not. else if lets you test a second question, and a third, until one fits. Only one branch of the whole chain ever runs: the first one whose condition is true. That is the part people get wrong, and it is the part that makes a chain of else if different from several separate ifs.

The traps are small and famous. = where you meant ==. A stray semicolon after the closing bracket. And a missing else on the last branch, so a case you did not think about quietly does nothing at all.

No wiring. Read everything in the Serial Monitor at 9600 baud.

→ if / else — the same ground with an LED and a button.

if

int temperature = 31;

if (temperature > 30) {
  Serial.println("Hot");
}

The block runs only when the condition is true. Otherwise the sketch carries straight on.

Note what is not there: no semicolon after if (...), and none after the closing brace.

else

if (temperature > 30) {
  Serial.println("Hot");
} else {
  Serial.println("Not hot");
}

Exactly one of the two runs. Always. else needs no condition — it is "in every other case".

else if

if (temperature > 35) {
  Serial.println("Very hot");
} else if (temperature > 30) {
  Serial.println("Hot");
} else if (temperature > 25) {
  Serial.println("Warm");
} else {
  Serial.println("Cool");
}

Tested top to bottom. The first true one wins and the rest are skipped.

With temperature = 31: the first is false, the second is true, so it prints and never looks at the third.

A dice game.

Roll a dice every two seconds and react:

• a 6 prints Six! Roll again
• a 1 prints Snake eyes
• an even number prints Even: n
• anything else prints Odd: n

Only one message per roll. Think carefully about the order — 6 is also even, and 1 is also odd.

Keep a running score too: add the roll to a total, except that a 1 resets the total to zero. Print the total each time.

Think about it: your chain has four branches. If the game grew to twenty different reactions, would twenty else if branches still be readable? That is the question switch answers in the next lesson but one.