Line numbers are numerical identifiers assigned to individual lines of code in a program. This system was prominently used in early versions of the BASIC (Beginner’s All-purpose Symbolic Instruction Code) programming language and is indicative of the simpler, non-structured nature of early programming practices.
The Origin of Line Numbers
Line numbers originated in the 1960s with the advent of the BASIC programming language, developed by John G. Kemeny and Thomas E. Kurtz at Dartmouth College. These numbers were integral to writing and managing code, serving both as labels for lines of code and as a means of directing program flow in an era before structured programming constructs like loops and functions were common. An example of usage in BASIC might look like this:
110 PRINT "HELLO, WORLD!"
220 GOTO 10
In this example, “10” and “20” are the line numbers used to order and control the sequence of execution.
Significance in Non-Structured Programming
Direct Control Flow
Before modern control structures (like loops and conditionals) became prevalent, line numbers were used to manually direct the flow of a program. Commands such as GOTO or GOSUB would transfer control to the specified line number, making it possible to write loops and conditional logic, albeit in a less intuitive manner compared to structured programming.
Ease of Debugging
In early computing environments, debuggers and integrated development environments (IDEs) were rudimentary or non-existent. Line numbers facilitated manual debugging and made it easier to identify and fix errors by referencing specific lines of code.
Types of Line Numbers
- Sequential Line Numbers: Typically starting from 10 and increasing by increments of 10 (e.g., 10, 20, 30), allowing programmers to insert additional lines of code without renumbering the entire program.
- Non-sequential Line Numbers: Programmers could assign any number they wished, though sequential numbers became a convention due to their practicality.
Examples
A simple BASIC program with line numbers:
110 INPUT "What is your name?", A$
220 PRINT "Hello, "; A$
330 GOTO 10
Here, the program takes a user’s input and creates an infinite loop to repeatedly prompt and greet the user.
Historical Context
In the context of early computing, line numbers represented a compromise between ease of programming and the limitations of early hardware and software. They were a familiar sight in the early years of personal computing and educational programming environments. However, as more sophisticated languages and tools were developed, the limitations of line-numbered coding became apparent, leading to the adoption of more structured programming paradigms.
Applicability in Modern Programming
Line numbers are largely a relic of the past, preserved in modern programming only for historical significance or in certain niche applications related to legacy systems. Modern high-level programming languages and IDEs employ more advanced and flexible constructs for mapping and controlling program flow.
Comparisons and Related Terms
- Structured Programming: Utilizes control structures such as loops, conditionals, and functions to create clear, maintainable code without relying on line numbers.
- Legacy Code: Refers to software written in older programming languages or styles, often including line-numbered code.
- Debugger: A tool that helps programmers find and fix bugs, reducing the need for line numbers.
FAQs
Do modern programming languages use line numbers?
Can I still write programs with line numbers?
Why were line numbers used in BASIC?
References
- Grier, D. A. (2002). When Computers Were Human. Princeton, NJ: Princeton University Press.
- Campbell-Kelly, M. (2004). From Airline Reservations to Sonic the Hedgehog: A History of the Software Industry. Cambridge, MA: MIT Press.
Summary
Line numbers are a hallmark of early programming languages like BASIC, reflecting the non-structured nature of early software development. They played a crucial role in the manual control of program flow and facilitated debugging in the rudimentary computing environments of the past. While largely obsolete today, line numbers remain an important part of programming history.
