When you do not want others (or yourself) to override existing variable values, use the const keyword (this will declare the variable as "constant", which means unchangeable and read-only):
Example
const int myNum = 15; // myNum will always be 15
myNum = 10; // error: assignment of read-only variable 'myNum'
myNum = 10; // error: assignment of read-only variable 'myNum'
You should always declare the variable as constant when you have values that are unlikely to change:
Example
const int minutesPerHour = 60;
const float PI = 3.14;
const float PI = 3.14;
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