Fortran Identifiers

A Fortran identifier must satisfy the following rules:

It has no more than 31 characters
The first character must be a letter,
The remaining characters, if any, may be letters, digits, or underscores,
Fortran identifiers are case insensitive. That is, Smith, smith, sMiTh, SMiTH, smitH are all identical identifiers.
Correct Examples:
- MTU, MI, John, Count
- I, X
- I1025, a1b2C3, X9900g
- R2_D2, R2D2_, A__
Incorrect Examples:
- M.T.U.: only letters, digits, and underscores can be used
- R2-D2: same as above
- 6feet: the first character must be a letter
- _System: same as above
Use meaningful names
- Good names: Total, Rate, length
- Not so good names: ThisIsALongFORTRANname, X321, A_B_012cm, OPQ
Fortran has many keywords such as INTEGER, REAL, PARAMETER, PROGRAM, END, IF, THEN, ELSE, DO, just name a few; however, Fortran does not have any reserved words. More precisely, a programmer can use these keywords as identifiers. Therefore, END, PROGRAM, DO are perfectly legal Fortran identifiers. However, this is definitely not a good practice.

Except for strings, Fortran 90 is not case sensitive. Therefore, identifier Name is identical to name, nAmE, NAme, NamE and namE. Similarly, PROGRAM is identical to program, PROgram, and progRAM. In this course, all keywords such as PROGRAM, READ, WRITE and END are in upper case and other identifiers use mixed cases.