Arduino for loops and arrays — repeat without copy-paste

#The short answer

A for loop runs the same block of code a set number of times, with a counter that changes each time. An array is a list of values under one name, numbered from 0. Together:

const int PINS[] = { 2, 3, 4, 5 };

for (int i = 0; i < 4; i++) {
  pinMode(PINS[i], OUTPUT);   // i = 0, 1, 2, 3 → pins 2, 3, 4, 5
}

Four pinMode calls in three lines, and adding a fifth pin means changing one list, not copying a line.

#How a for loop counts

for (int i = 0; i < 4; i++) {
  Serial.println(i);
}

The round brackets hold three parts, separated by semicolons:

Step by step, the board does this:

It prints 0, 1, 2, 3: four numbers, starting at 0 and never reaching 4. That "start at 0, stop before the count" shape is the one you will write most often, because it matches how arrays are numbered.

#Arrays: many values, one name

const int NOTES[] = { 262, 294, 330, 349 };   // C D E F, in hertz

• The square brackets [] say "this is an array". The compiler counts the items for you.
• NOTES[0] is 262, NOTES[1] is 294, … NOTES[3] is 349. The first item is number 0, not 1.
• The number in brackets is called the index. An array of 4 items has indexes 0 to 3.

All items in one array have the same type: all int, or all float, and so on. See variables and data types for which type to pick.

#Together: play a tune

This is the core of our tune project. Two arrays line up side by side: the note and how many beats it lasts.

const int BUZZER = 8;
// "Ode to Joy" (Beethoven): E E F G G F E D C C D E E D D
const int NOTES[] = { 330, 330, 349, 392, 392, 349, 330, 294, 262, 262, 294, 330, 330, 294, 294 };
const int BEATS[] = { 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2 };
const int COUNT = 15;
const int BEAT = 300; // ms

void loop() {
  for (int i = 0; i < COUNT; i++) {
    int ms = BEATS[i] * BEAT;
    tone(BUZZER, NOTES[i], ms - 30);
    delay(ms);
  }
  delay(1500);
}

• i goes 0 to 14, one turn per note.
• NOTES[i] and BEATS[i] use the same index, so note number i always gets its own length.
• ms - 30 stops each note 30 ms early, so two equal notes in a row sound like two notes and not one long one.

Want a new song? Change the two lists and COUNT. The loop stays the same.

Play in 3DRun the code · press the parts · turn it around

#Together: read eight keys

The mini piano has eight buttons on pins 2 to 9. Instead of eight digitalRead lines, it scans them:

const int KEYS[] = { 2, 3, 4, 5, 6, 7, 8, 9 };

void setup() {
  for (int i = 0; i < 8; i++) pinMode(KEYS[i], INPUT_PULLUP);
}

void loop() {
  int down = -1;                      // -1 = no key
  for (int i = 0; i < 8; i++) {
    if (digitalRead(KEYS[i]) == LOW) down = i;   // pressed keys read LOW
  }
  // down is now 0…7 for the key that is pressed, or -1
}

The loop asks every key "are you pressed?" and remembers which number answered yes. That index then picks the note from a second array, NOTES[down]. One loop, eight keys, and the if inside does the deciding.

#Counting the items for you

Writing COUNT = 15 by hand works, but it is easy to forget to update. This line lets the compiler count:

const int COUNT = sizeof(NOTES) / sizeof(NOTES[0]);

sizeof(NOTES) is the size of the whole array in bytes (15 × 2 = 30 on an UNO, where an int is 2 bytes). Divide by the size of one item and you get 15.

#Common mistakes

• Going one past the end. for (int i = 0; i <= 4; i++) on a 4-item array reads NOTES[4], which does not exist. The board does not warn you: it reads whatever memory comes next. Use <, not <=.
• Starting at 1. NOTES[1] is the second item. Arrays start at 0.
• A semicolon after the ). for (int i = 0; i < 8; i++); loops eight times doing nothing, then runs the block once.
• Arrays of different lengths. If NOTES has 15 items and BEATS has 14, the last note reads a random length. Keep paired arrays the same size.
• A long delay() inside a scanning loop. The piano scans all eight keys thousands of times a second. Add delay(500) inside and the keys feel dead. See millis() vs delay().

#Check yourself

#Questions people ask

#Can a for loop go on forever?

Only if its condition never becomes false, like for (;;). You rarely need that on an Arduino, because loop() already repeats forever.

#How big can an Arduino array be?

The UNO has 2 KB (2048 bytes) of RAM for all your variables. An int array of 100 items uses 200 bytes, which is fine; 1000 items would fill it. Fixed lists such as notes can be stored in flash memory with PROGMEM when they get large.

#What is the difference between a for loop and a while loop?

A for loop is best when you know how many times to repeat: 8 keys, 15 notes. A while loop repeats until something happens, such as "while the button is held". Each can do the other's job, but the right one reads more clearly.

#Can I change the values in an array while the sketch runs?

Yes, if you leave out const. int scores[3] = {0, 0, 0}; then scores[1] = 42; changes the second item. Keep const for lists that never change, like pins and notes.

#What's next

Build the mini piano and change its notes, then make every key press count exactly once with button debounce.

Last updated 2026-11-11