A variable is a way of naming and storing a value for later use by the program, such as data from a sensor or an intermediate value used in a calculation.
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Before they are used, all variables have to be declared. Declaring a variable means defining its type, giving it a name, and (optionally) giving it an initial value (this is often referred to as initializing the variable). Variables do not have to be initialized (given a value) when they are declared, but it is good style to give them an initial value whenever possible.
Here is an example of declaring a variable named inputVariable1 with type int (the int type is used to store integers, like -2, -1, 0, 1, etc.):
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int inputVariable1;
In the above declaration, we did not give the variable an initial value. Here is another example, where we declare an int variable named inputVariable2, with an initial value of 0:
int inputVariable2 = 0;
The Maple environment comes ready to use with many useful types of variables. See the built-in types page for more information.
Here are a few examples of declaring variables of different types:
int lightSensVal;
char currentLetter;
unsigned long long speedOfLight = 186000ULL;
char errorMessage = {"choose another option"}; // see string
The rules for naming a variable are simple. Names for variables can contain letters, numbers, and underscores (the underscore is the _ character), and cannot begin with a number. So temperature_reading, tempReading, tempReading1, and tempReading2 are all valid variable names, but 4_temp_readings is not, because it begins with a number.
You cannot choose a name for a variable that is one of the C++ keywords.
Variable names are case-sensitive, so “tempreading” and “tempReading” are different variables. However, it is very bad style to write code that chooses variables which are the same up to case.
You should give your variables descriptive names, so as to make your code more readable. Variable names like tiltSensor or pushButton help you (and anyone else reading your code) understand what the variable represents. Variable names like var or value, on the other hand, do little to make your code readable.
An important choice that programmers face is where (in the program text) to declare variables. The specific place that variables are declared influences how various functions in a program will “see” the variable. This is called variable scope. See the scope reference for more information.
Variables may be initialized (assigned a starting value) when they are declared or not. It is always good programming practice however to double check that a variable has valid data in it before it is used. Using a variable before you give it a value is a common source of bugs.
Every (numeric) type has a valid range. The range of a type is the smallest and largest value that a variable of that type can store. For example, the int type has a range of -2,147,483,648 to 2,147,483,647 [1].
When variables are made to exceed their range’s maximum value, they “roll over” back to their minimum value. Note that this happens in both directions. It’s like in the game Pac-Man – when Pac-Man goes past the right edge of the screen, he reappears on the left, and when he goes past the left side of the screen, he reappears on the right:
int x;
x = -2,147,483,648;
x = x - 1; // x now contains -2,147,483,647; rolled over "left to right"
x = 2,147,483,647;
x = x + 1; // x now contains -2,147,483,648; rolled over "right to left"
Each numeric type’s reference page includes its range. See the built-in types reference for links to each type’s reference page.
Once variables have been declared, they are given values using the assignment operator, which is a single equals sign, =. The assignment operator tells the program to store the value on the right side of the equals sign into the variable on the left side:
inputVariable1 = 7; // sets variable named inputVariable1 to 7
inputVariable2 = analogRead(2); // sets variable named inputVariable2 to
// the (digitized) input voltage read from
// analog pin #2
Once a variable has been set (assigned a value), you can test its value to see if it meets certain conditions, or you can use its value directly. For instance, the following code tests whether the inputVariable2 is less than 100, then sets a delay based on inputVariable2 (which, at that point, is at least 100):
if (inputVariable2 < 100) {
inputVariable2 = 100;
}
delay(inputVariable2);
Footnotes
| [1] | This range might seem a little weird at first. The reasons for this range of values have to do with the fact that an int occupies 32 bits of memory, and the facts 2^31 = -2,147,483,648
2^31 - 1 = 2,147,483,647
Why 2^31 instead of 2^32? Well, that has to do with how ints are (usually) stored on computers. |
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.