Magic number (programming)

In computer programming, a magic number is a special constant used for some specific purpose.

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Unnamed magic numbers are bad programming practice because:

• they make the program harder to read. For example, if the below pseudocode snippet is supposed to iterate through the months of the year

for n = 1 to 12

is more difficult to elucidate the meaning of than

for n = 1 to MonthsPerYear

• they make changing the value of the magic number globally throughout the code much more difficult. If the magic number is named beforehand, the definition need merely be changed to affect the rest of the named constants. For example,

for n = 1 to 12

print "There are " 12-n " more months to go"

print "This is month " n " of " 12

each 12 must be changed if we wish to use a different calendar, for example. However, if we say

define MonthsPerYear = 12

for n = 1 to MonthsPerYear

print "There are " MonthsPerYear-n " more months to go"

print "This is month " n " of " MonthsPerYear

When we wish to change the constant, we merely change its definition.