Avoid the use of numerical values in code ("magic numbers") when possible. Reasons for this include, appropriately named symbolic constants make code more readable rather than checks against a specific number. For portability reasons also if a specific number needs to be changed reassigning a symbolic value is much easier than replacing a specific number in the code since each case has to be checked specifically.
Non Compliant Code:
if(age>=18)
{
printf("Of legal voting age");
}
Compliant Code:
if(age>=VOTING_AGE)
{
printf("Of legal voting age");
}
Source: http://www.doc.ic.ac.uk/lab/cplus/c++.rules/chap10.html