A for loop is used to repeat a block of statements enclosed in curly braces. for loops are useful for performing repetitive operations, and are often used in combination with arrays to operate on collections of data or multiple pins. A for loop is composed of two parts: first, a header, which sets up the for loop, and then a body, which is made up of lines of code enclosed in curly braces.
Contents
There are three parts to the for loop header: an initialization expression, loop condition expression, and a post-loop expression. The general syntax looks like this:
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for (initialization; condition; post-loop) {
// all of these lines inside the curly braces are part
// of the loop body.
statement 1;
statement 2;
...
}
(Note that there is no semicolon after the post-loop). The initialization happens first and exactly once, before the loop begins. Each time through the loop, the condition is tested. The condition is a boolean expression. If it is true, then the list of statements inside the curly braces are executed. Next, the post-loop is executed. The loop then begins again by evaluating the condition again, entering the loop body if it is true. This proceeds until the condition becomes false.
Here’s an example:
// Dim an LED using a PWM pin
int pwmPin = 9; // LED in series with 470 ohm resistor on pin 9
void setup() {
pinMode(pwmPin, PWM);
}
void loop() {
for (int i=0; i <= 65535; i++) {
pwmWrite(pwmPin, i);
delay(1);
}
}
There is a for loop In the loop() function of the above example. This loop starts by declaring an int variable named i, whose value starts out at zero. The loop proceeds by checking if i is less than or equal to 65535. Since i is zero, this is true, and so the calls to pwmWrite() and delay() happen next. At this point, the post-loop expression i++ is evaluated, which increments i, so that i becomes one. That concludes the first time through the loop. Each “time through the loop” is referred to as an iteration.
The loop then jumps back to the beginning, checking the condition as the beginning of its second iteration (initialization is skipped, since this only happens once, before the first iteration). One is less than 65535, so the loop statements are executed again. This proceeds over and over until the iteration when i finally reaches 65536. At that point, the condition is no longer true, so the loop stops executing, and the loop() function returns.
Here’s another example, using a for loop to brighten and fade an LED (see the pwmWrite() reference for more information):
int pwmPin = 9; // hook up the LED to pin 9
void loop() {
int x = 1;
for (int i = 0; i >= 0; i += x) {
analogWrite(pwmPin, i); // controls the brightness of the LED
if (i == 65535) {
x = -1; // switch direction, so i starts decreasing
}
delay(1);
}
}
The C for loop is more flexible than for loops found in some other computer languages, including BASIC. Any or all of the three header elements may be left blank, although the semicolons are required. Also the statements for initialization, condition, and post-loop can be any valid C statements, and use any C datatypes, including floating point numbers. These types of unusual for loops sometimes provide solutions to less-common programming problems.
For example, using a multiplication in the post-loop line will generate a geometric progression:
for(int x = 1; x <= 100; x = x * 2) {
SerialUSB.println(x);
}
This loop prints out the numbers 1, 2, 4, 8, ..., 64. Check your understanding of for loops by answering the following two questions (answers are in footnote [1]):
Footnotes
| [1] |
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License and Attribution
Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.