Use a separate counter for each loop

Oct 13, 2025 by pinkandtrash.com 21 views

Introduction

Loops are a fundamental concept in programming, allowing us to execute a set of instructions repeatedly. In the context of Plain TeX, loops can be implemented using counters, which are variables that can be incremented or decremented. In this article, we will delve into the world of Plain loops with counters, exploring the basics, common pitfalls, and best practices.

What are Plain Loops?

Plain TeX is a typesetting system developed by Donald Knuth. It is a low-level, macro-based system that provides a high degree of control over the typesetting process. Plain loops are a way to execute a set of instructions repeatedly, using a counter to keep track of the number of iterations.

The Basics of Counters

In Plain TeX, counters are defined using the \newcount command. For example:

\newcount\countA

This defines a counter named countA. The \count command is used to access the value of a counter. For example:

\count0=\countA

This sets the value of count0 to the value of countA.

Defining a Loop

A loop in Plain TeX is defined using a macro. The macro takes one argument, which is the number of iterations. For example:

\def\loopmacro#1{
  \count0=0
  \loop
    \... % loop body
  \repeat
}

This defines a macro named loopmacro that takes one argument, #1. The loop body is executed #1 times.

Using a Counter in a Loop

To use a counter in a loop, we need to increment it at the end of each iteration. We can do this using the \advance command. For example:

\def\loopmacro#1{
  \count0=0
  \loop
    \... % loop body
    \advance\count0 by 1
  \repeat
}

This increments the counter count0 by 1 at the end of each iteration.

Common Pitfalls

When working with loops and counters in Plain TeX, there are several common pitfalls to watch out for:

Counter overflow: If the counter value exceeds the maximum value that can be represented by the counter, it will wrap around to a small value. This can cause unexpected behavior.
Counter underflow: If the counter value is decremented below 0, it will wrap around to a large value. This can also cause unexpected behavior.
Loop termination: If the loop termination condition is not met, the loop will run indefinitely.

Best Practices

To avoid common pitfalls and write efficient loops, follow these best practices:

Use a separate counter for each loop: This helps to avoid counter overflow and underflow.
Use a clear and concise loop termination condition: This helps to avoid infinite loops.
Use a consistent increment/decrement value: This helps to avoid unexpected behavior.

Example Use Case

Here is an example use case for a loop with a counter:

\def\printnumbers#1{
  \count0=1
  \loop
    \the\count0\space
    \advance\count0 by 1
  \repeat
  \count0=0
  \loop
    \the\count0\space
    \advance\count0 by -1
  \repeat
}
\printnumbers{10}

This defines a macro named printnumbers that takes one argument, #1. The macro prints the numbers from 1 to #1 and then from #1 to 1.

Conclusion

In conclusion, Plain loops with counters are a powerful tool for executing a set of instructions repeatedly. By understanding the basics of counters and loops, and following best practices, you can write efficient and effective loops. Remember to use a separate counter for each loop, use a clear and concise loop termination condition, and use a consistent increment/decrement value.

Common Issues and Solutions

Here are some common issues and solutions when working with loops and counters in Plain TeX:

Counter overflow: Use a larger counter or use a different data type, such as a token list.
Counter underflow: Use a smaller counter or use a different data type, such as a token list.
Loop termination: Use a clear and concise loop termination condition, such as a conditional statement.
Infinite loop: Use a loop termination condition that is always true, such as a conditional statement.

Conclusion

Q: What is the difference between a counter and a token list in Plain TeX?

A: A counter is a variable that can be incremented or decremented, whereas a token list is a collection of tokens that can be manipulated. In Plain TeX, counters are used to keep track of the number of iterations in a loop, while token lists are used to store and manipulate a collection of tokens.

Q: How do I define a counter in Plain TeX?

A: You can define a counter in Plain TeX using the \newcount command. For example:

\newcount\countA

This defines a counter named countA.

Q: How do I increment a counter in Plain TeX?

A: You can increment a counter in Plain TeX using the \advance command. For example:

\advance\countA by 1

This increments the value of countA by 1.

Q: How do I decrement a counter in Plain TeX?

A: You can decrement a counter in Plain TeX using the \advance command with a negative value. For example:

\advance\countA by -1

This decrements the value of countA by 1.

Q: What is the difference between a loop and a conditional statement in Plain TeX?

A: A loop is a set of instructions that is executed repeatedly, whereas a conditional statement is a set of instructions that is executed only if a certain condition is met. In Plain TeX, loops are used to execute a set of instructions repeatedly, while conditional statements are used to execute a set of instructions only if a certain condition is met.

Q: How do I define a loop in Plain TeX?

A: You can define a loop in Plain TeX using the \loop and \repeat commands. For example:

\def\loopmacro#1{
  \count0=0
  \loop
    \... % loop body
  \repeat
}

This defines a macro named loopmacro that takes one argument, #1. The loop body is executed #1 times.

Q: How do I terminate a loop in Plain TeX?

A: You can terminate a loop in Plain TeX using the \repeat command. For example:

\def\loopmacro#1{
  \count0=0
  \loop
    \... % loop body
  \repeat
}

This terminates the loop when the loop body is executed #1 times.

Q: What is the difference between a macro and a function in Plain TeX?

A: A macro is a set of instructions that is executed when a certain command is invoked, whereas a function is a set of instructions that is executed when a certain value is passed to it. In Plain TeX, macros are used to define a set of instructions that can be executed repeatedly, while functions are not used in the same way.

Q: How do I define a macro in Plain TeX?

A: You can define a macro in Plain TeX using the \def command. For example:

\def\macro#1{
  \... % macro body
}

This defines a macro named macro that takes one argument, #1. The macro body is executed when the macro is invoked.

Q: How do I invoke a macro in Plain TeX?

A: You can invoke a macro in Plain TeX by typing the name of the macro followed by the argument. For example:

\macro{argument}

This invokes the macro named macro with the argument argument.

Q: What is the difference between a token list and a string in Plain TeX?

A: A token list is a collection of tokens that can be manipulated, whereas a string is a sequence of characters that can be typeset. In Plain TeX, token lists are used to store and manipulate a collection of tokens, while strings are used to typeset a sequence of characters.

Q: How do I define a token list in Plain TeX?

A: You can define a token list in Plain TeX using the \def command. For example:

\def\tokenlist#1{
  \... % token list body
}

This defines a token list named tokenlist that takes one argument, #1. The token list body is executed when the token list is invoked.

Q: How do I invoke a token list in Plain TeX?

A: You can invoke a token list in Plain TeX by typing the name of the token list followed by the argument. For example:

\tokenlist{argument}

This invokes the token list named tokenlist with the argument argument.