Historical Perspective: FORTRAN I

The first version of FORTRAN violates many notions of good programming language (PL) design. Here are some highlights:

• Variables can be upto 5 chars long and must begin with a letter. Variables beginning with i, j, k, l, m and n are assumed to be integer types. Some PL experts say this is why the letters i and j are so popular even now as loop variable names.
  
• Variable declarations are not required, except for arrays.  Arrays are limited to 3 dimensions.
  
• The control structures consisted of:
  
• unconditional branch: GOTO statement_number
  
• conditional branch: GOTO (30, 40, 50) I1 where I1 has a value of 1, 2 or 3 to indicate which position in the list to jump to.
• IF statement: IF (A+B-1.2) 7, 6, 9 transfers control to statement labeled 7, 6, or 9  depending on whether (A+B-1.2) is negative, zero or positive. 
  
• DO statement: DO 17 I = 1, N, 2 which specifies that the set of statements following, up to and including that labeled 17, is to be executed with I first assigned value 1,  incrementing by 2, up through N. 
  
• No user-defined subroutines allowed, although several built-ins were provided including  READ and WRITE for I/O.
  
• Blanks are ignored in all statements, e.g., all the following are equivalent:
  
• DIMENSION IN DATA(10000) 
  
• DIMENSIONINDATA(10000) 
  
• D I M E N S I O N I N D A T A ( 1 0 0 0 0 ) 
  
• Whitespace was relevant in other cases: Code started on a particular column number.
• Reserved words are valid variable names, e.g., DIMENSION IF(100) declares an array called IF. 
  
• FORTRAN had two basic stages in compiling:
• Lexical analysis, parsing, and code generation in one big loop and switch program.
• The other stages of the compiler were dealing with code optimization steps.