The Arduino For Loop: How you can use it the Right Way and easily repeat blocks of code saving processor memory and simplifying access to array data. Avoid off by one errors and how you can create an infinite for loop.

• Save rewriting code with a for-loop.
  
  
• Avoid off by one errors - very easy to miss.
  
  
• Count up or down (and use the loop variable inside code).
  
  
• Find out where you can use for-loops.
  
  
• Access array data with a for-loop.
  

The 'for loop' is one of those bread-and butter functions that you will use all the time in your code. Microcontrollers are good at repetitive tasks but instead writing out the same code many times, the for-loop repeats a section of code a specific number of times.

Where to use for-loops

You can use for loops anywhere inside a function, so you can use them in the standard Arduino setup(), and loop() functions, and within your own functions. You can even use for loops inside other for loops which is typically used for multidimensional array access.

The following example shows a for-loop that loops round printing the numbers 0 to 9. It is placed into the setup() function since you only want to perform the for-loop action one time - otherwise if you place it within the loop() function it repeatedly outputs information - making you head spin (try it and see)!

void setup (void) {

   Serial.begin(9600);

   Serial.println("Arduino for loop");
   for (int i=0; i<10; i++) Serial.println(i);
}

void loop(void) {
}

How the Arduino for loop works

The Arduino for loop provides a mechanism to repeat a section of code depending on the value of a variable. So you set the initial value of the variable, the condition to exit the loop (testing the variable), and the action on the variable each time around the loop.

These three elements are parameters to the for loop and are separated by semicolons:

Arduino Loop Counter Control Variable

The other important point about for-loops is that they need a loop control variable - in the example this is the variable i. This is used as a loop counter and is used to decide when to exit the loop.

for (int i=0; i<10; i++) Serial.println(i);

Parameters of the Arduino for loop

Initialise

int i = 0;

First of all the variable 'i' is created (if it does not already exist) and set to zero.

This section of the for-loop is only executed once at the beginning.

Test

i < 10;

Next the loop variable is tested. Here, if it is smaller than 10 then carry on - otherwise the loop is exited.

This is executed every time around the loop.

Iterator

Arduino For loop Output

So the for-loop executes the code "Serial.print(i)" 10 times with value of 'i' going through 0 to 9.

And this is the output:

Arduino for loop
0
1
2
3
4
5
6
7
8
9

for (int i=1; i<=10 ; i++) Serial.println(i);

At the end of the loop code (outside curly braces) the value of 'i' will be 11, because when 'i' reaches 11 the statement 'i<=10' is false so the loop is exited.

Arduino for-loop body code

void setup (void) {

   Serial.begin(9600);

   Serial.println("Arduino for loop");
   for (int i=0; i<7; i++) {
      Serial.print("i is : ");
      Serial.println(i);
   }
}

void loop(void) {
}

Notice how there are two statements - ending in semicolons and both are enclosed by curly braces. Compare this to the previous example that used only one statement ending in a semi-colon.

Arduino for loop
i is : 0
i is : 1
i is : 2
i is : 3
i is : 4
i is : 5
i is : 6

Test Yourself

Q: Here's a question for you. What is the value of i after the loop has finished executing?

Q: Another question. Can you use i later on in the program?

How to setup a count down for loop

void setup (void) {

   Serial.begin(9600);

   Serial.println("Arduino count down for loop");
   for (int i=10; i>=1; i--) {
      Serial.print("i is : ");
      Serial.println(i);
   }
}

void loop(void) {
}

Arduino count down for loop
i is : 10
i is : 9
i is : 8
i is : 7
i is : 6
i is : 5
i is : 4
i is : 3
i is : 2
i is : 1

Test Yourself

Q: Here's the same question for you. What is the value of i after the loop has finished executing?

Arduino For loop Array Example

The reason that you start with the control loop variable at zero is that arrays are defined starting from index zero. If you don't use zero as an initial starting point then you can get an off by one error - meaning you can write to a array location that does not exist. For an array of 10 values only indices 0..9 are valid.

Here's an example of setting up an array initialising each element with a random number. Then printing out the array values in turn.

int myarray[10];

void setup (void) {

   Serial.begin(9600);

   for (int i=0; i<10; i++) myarray[i]= random(100);

   Serial.println("Arduino for loop");
   for (int i=0; i<10; i++) {
      Serial.print("i is : ");
      Serial.print(i);
      Serial.print(" array value is ");
      Serial.println(myarray[i]);
   }
}

void loop(void) {
}

Standard form of a for loop

The standard form of the for loop initialises the control variable to 0. For exiting the loop the condition tests whether the variable is smaller than the number of loops needed. You don't have to use this but it makes life easier when dealing with arrays.

The value of 'i' goes from 0 to 9 for the code within the curly braces. So it traverses 10 values. The reason it is easier for array use is that arrays start from index zero.

The standard form is this:

   for (int i=0; i<10; i++) {
      <block of code to execute>
   }

At the end of the loop code (outside curly braces) the value of 'i' will be 10, because when 'i' reaches 10 the statement 'i<10' is false so the loop is exited.

Initialising the For Loop

int i=0;

void myFunction(void) {
int i;

   for(i=0;i<10;i++) {
      Serial.println(i);
   }

   for(i=10;i>0;i++) {
      Serial.println(i);
   }
}

FAQ

How many times the loop function runs

One frequently asked question is:

    "How many times loop function runs in Arduino IDE:"

You can see from the previous examples exactly how the loop variables are controlled to set the number of times a loop will execute.

The loop function in Arduino code is what is known as an infinite loop which is can either be written using a while structure or a for loop structure:

Infinite While loop

The while loop is used slightly differently to the for loop and does not have an iterator.

   while(1) {
      // Do something forever
   }

Infinite For loop

   for(;;) {
      // Do something forever
   }

Either of these looping structures could be used in the Arduino code. The are known as infinite loops but this really is just qualified by the lifetime of the program. This is from the start time, when you either apply power or press reset. It obviously end when power is removed or rest is pressed again.

So to answer the question

    "How many times the main loop function runs in the Arduino IDE"

The answer is - "it depends!"

It depends on the crystal speed of the Arduino (16MHz). This defines how many instructions per seconds can be executed.  And it depends on how long the power is applied. The last factor is how much code is executed within the main loop().

So it depends on:

1. Processor clock speed,
2. How long power is applied,
3. How much code is within loop().
   

When you add more instructions inside loop(), mode code must be executed so the loop time increases. 

Measuring Loop Time

You can measure the loop time e.g. using microseconds timer but collect the data in a variable and then compute the average. If you use a Serial.print statement within the measurement it will give a larger the time period - as that code itself will take a long time.

To do it add a condition that prints out the data after 100 loops. However, even the test itself will take time but it will be a small proportion of the time if the main code is substantial.

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