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In an Arduino sketch, the modulo operator % gives the remainder after integer division, and an if statement uses a true-or-false condition to decide whether code runs. Together, they let you test for even numbers, act every few loop passes, or cycle through array positions.
What does the modulo operator do?
Modulo, written as %, returns the remainder left after dividing one integer by another. For example, 7 % 5 is 2, 9 % 5 is 4, and 5 % 5 is 0. The Arduino modulo reference describes these examples at Arduino modulo reference.
When the remainder is zero, the number is evenly divisible by the divisor. That makes value % n == 0 a useful way to test whether an integer is a multiple of n. The examples below use integers: Arduino’s modulo reference notes that % does not work on floating-point values.
How does an if statement make a decision?
An if statement checks a condition and runs its following statement or block when the condition is true. Conditions commonly use comparison operators such as ==, !=, <, >, <=, and >=. The Arduino if reference explains the statement and its condition syntax.
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Use braces to make the statements controlled by the condition clear, especially when a block contains more than one statement:
if (temperature > 25) {
// Runs when temperature is greater than 25
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}Without braces, only the next statement belongs to the if. Braces make that boundary visible and help prevent later edits from accidentally leaving a statement outside the condition.
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Do not confuse assignment with comparison
= assigns a value; == compares two values. For example, if (x = 10) assigns 10 to x rather than testing whether x already equals ten. To make that test, write if (x == 10).
How do you use modulo in an if statement?
Put the modulo expression in the condition, then compare its remainder. This example runs the block when count is even:
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int count = 8;
if (count % 2 == 0) {
// count is even
}Here, count % 2 is 0, so the comparison is true. Change the divisor to 3 to test whether an integer is a multiple of three:
if (count % 3 == 0) {
// count is divisible by 3
}How can modulo run code every N loop passes?
Arduino repeatedly executes the contents of loop(), as described in its getting-started guide. If you increment an integer counter once per pass, a condition such as count % 10 == 0 is true whenever the count reaches a multiple of ten.
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int count = 0;
void setup() {}
void loop() {
count++;
if (count % 10 == 0) {
// Runs on every tenth loop pass
}
}This counts loop passes, not seconds. How much time passes between those checks depends on the work elsewhere in loop(), so this pattern is not a precise elapsed-time schedule.
How can modulo cycle through an array?
Use the array length as the divisor to wrap an index back to zero after its last valid position. In this example, i moves through indices 0 to 9 and then returns to 0:
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int values[10];
int i = 0;
void setup() {}
void loop() {
values[i] = analogRead(A0);
i = (i + 1) % 10;
}Keep the divisor positive and equal to the array length. This rollover pattern is not suitable for an empty array.
When should you use millis() instead of modulo?
Use modulo when the rule is based on a number of loop passes—for example, doing something every tenth iteration. Use elapsed-time logic with millis() when the rule is based on how many milliseconds have passed. Arduino’s getting-started guide presents millis() for scheduling events without pausing the program with delay().
| Approach | Measures | Useful when |
|---|---|---|
count % N == 0 |
Loop iterations | An action should follow a set number of passes, regardless of elapsed time. |
millis() interval |
Elapsed milliseconds | An action should follow elapsed time while the sketch continues other work. |
What if the value is a float?
The Arduino modulo reference says % does not work on floating-point values. If a sensor reading is stored as a float, do not apply % directly to it. Choose a comparison that fits the measurement, or convert to an integer only when discarding the fractional part is appropriate for your application.
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