Understanding how to properly declare and utilize variables is a key foundation for any successful Arduino programmer. Selecting appropriate data types determines how values are stored in memory and interpreted by the microcontroller hardware. Misusing data types can waste RAM, cause unexpected bugs, and lead to inaccurate measurements or results.
This comprehensive guide explores the built-in primitive types like integers, floats and booleans that Arduino provides out of the box. You‘ll also learn about more complex aggregated types such as arrays and custom structures for organizing related data.
Along the way we‘ll look at plenty of actionable coding examples demonstrating real-world usage of these core data types. By the end, you‘ll have all the knowledge needed to make optimal choices when handling the diverse data challenges faced in Arduino projects – from sensors and serial comms to displays and data logging.
Why Carefully Choosing Data Types Matters
Unlike desktops or smartphones, Arduino boards have highly limited RAM and flash storage – often just kilobytes in size. Similarly, the 8-bit AVR microcontrollers at their heart have restricted computational bandwidth for juggling multiple variables and data structures.
Using the proper data types matching your desired value sizes, signage, precision etc. is therefore crucial. Declaring an unsigned long variable to store a sensor reading that only requires an unsigned int, for instance, needlessly wastes 232 bits of RAM – nearly a tenth of total storage on an Arduino Uno!
Conversely, accidentally storing a value requiring 32-bits of range in a 16-bit integer risks unintended overflow or sign flipping. These types of mismatches between data types and actual usage are a common source of insidious bugs in Arduino programs.
Carefully selecting the smallest yet sufficient data type avoids such waste and unintended behaviors. This helps your precious RAM serve more useful purposes – whether holding an array of sensor measurements or the stack frames for nested function calls.
Primitive Data Types
These core types provided by Arduino handle common scenarios like storing sensor readings, boolean flags, text and timer values.
int and long – Handling Integer Values
The int data type is the workhorse for storing integer values in Arduino. An int occupies 16-bits and can store integers from -32,768 to 32,767. This range works well for most general purpose integer math and counters.
int sensorValue = 320;
int timerCountdown = 1000; For non-negative numbers, using Arduino‘s unsigned int variant instead doubles the range – up to 65,535. Just be cautious of unintended overflow.
If your application requires larger integers – such as epoch timestamps or cryptographic nonce values – the Arduino long data type exists. long variables use 32 bits, providing numeric ranges from −2,147,483,648 to 2,147,483,647.
Just watch that your total RAM usage doesn‘t exceed your particular Arduino board‘s available memory when declaring multiple long variables.
float and double – Storing Decimal Numbers
When working with analog sensors, positioning systems like GPS or advanced math, fractional numeric precision is often needed.
The Arduino float data type is perfect for such applications, able to store values with approximately 6-7 digits of decimal precision. Under the hood, floats use 32-bits and can represent content both extremely small – as low as 3.4028235E+38, and huge – up to 3.4028235E38.
float latitude = 40.7190;
float sensorVoltage = 3.456; For the rare cases requiring extras precision up to 15 decimal places, Arduino provides the double type. But beware – doubles chew up 64-bits per variable declared!
boolean – True or False Flags
The Arduino boolean data type leverages just 1 bit toTRACK store a simple true or false, on or off value.
Thanks to this compact size, booleans help simplify Arduino code when branch logic based on binary state or "flags" is needed.
boolean doorOpened = true;
if(doorOpened) {
// Take action
}No need for clumsy integrer equivalents like 1 or 0. Save those integers for numeric calculations instead!
char – One Text Character
The 8-bit char data type handles storage for a single text character – from ‘A‘ to ‘?‘ to ‘{‘. Special symbols and even custom characters are supported too making chars useful for displaying menu options, parsing input or embedding icons.
liquidCrystal.print(‘->‘); // prints arrow iconUnder the hood, Arduino stores char data using ASCII encoding – with each character getting a corresponding decimal number value.
One limitation is that only single-byte ASCII characters are supported natively, so languages requiring multi-byte Unicode encoding need special Arduino libraries.
Aggregate Data Types
These more complex, compound data types help group together related values – for example sensor readings over time or GPS latitude/longitude pairs.
Array
Array data structures enable storing a sequence of variables accessible by index rather than unique name. This provides convenience and efficiency benefits to Arduino programmers needing to track values that often change over the course of program execution – like sensor measurements, control output states or communication data packets.
The syntax for declaring arrays uses square brackets after the base data type:
int sensorReadings[100]; // array of 100 ints
sensorReadings[0] = 320; // first element
sensorReadings[99] = 270; // last elementWhen declared this way, all elements get default initialized to 0. The Arduino standard library also contains a Vector class providing a more full-featured, dynamic array implementation on top of the basics.
One Challenge with arrays is their tendency to quickly chew up RAM. For example, on an Arduino Uno, declaring a 100 element float array named sensorReadings uses:
100 size of float = 100 4 bytes = 400 bytes
So be careful not to exceed your board‘s available memory!
struct
The struct data type enables bundling together varied data types into a custom aggregate. Each related piece of data, like latitude or logitude for GPS data, becomes its own named member within the struct.
This helps organize complex, multidimensional data cleanly while keeping members tightly packed in memory – as opposed to using pointer linking.
struct GPS_position {
float latitude;
float longitude;
int altitude;
};
GPS_position myPosition;
myPosition.latitude = 40.7190;
myPosition.altitude = 331; Accessing members of your custom struct just uses dot notation after an instance name.
Efficient Serialization
A great benefit of custom structs is enabling easy serialization to formats like JSON or binary ProtoBuf – hugely useful for recording data locally or transmitting it remotely.
Key Considerations When Selecting Data Types
Here are vital tips to remember during your Arduino programming when working with the data types covered above.
Optimize for Smallest Size
Favor using the smallest data type possible – for example unsigned int instead of unsigned long when the value range allows it. This minimizes RAM usage.
Use Exact Size Integers
Performing math across integers of differing sizes like ints and longs risks unintended overflows or loss of precision.
Float vs. Double
Floating point types like float or double enable fractional decimal precision for math and measurements. But their substantial RAM footprint – from 32 to 64-bits per value – merits saving them only for when truly necessary vs integer types.
Cast Carefully
Directly converting integer data types to smaller sizes can truncate and lose precision. Pay close attention to value ranges when explicitly casting between types to avoid data loss.
Use Booleans for Conditionals
Leverage boolean true/false values over integers like 0 or 1 for representing conditional program logic. This enhances readability.
Watch Array Sizes
It‘s easy to unintentionally consume available RAM by declaring Arduino arrays at large sizes. Compute your total usage to prevent hitting memory limits.
Summing Up Effective Data Type Usage
Carefully selecting appropriate data types is one of the easiest ways to hugely enhance Arduino programs. Avoiding waste from outsized types like double or string is crucial given incredibly constrained memory on boards like the Uno or Nano…
