Implicit None in Fortran

implicit none is a directive from Fortran that instructs the compiler to ignore a default in the Fortran language:

• Variables starting with i, j, k, l, m or n (range i-n) are understood as INTEGER
• Variables starting in any other character are floating-point numbers, that is, of type REAL

This is done unless there is an explicit declaration of the variable.

This behaviour comes from the first version of Fortran, but it has long lost its advantages. Declaring variables explicitly is common practice in any static programming language, and Fortran is no different.

Not using this statement means that variables with typos in their names will be assigned one of these types and the programmer will not know it!