Centering

Centering
A Programming Pattern

Computer Science Department

bernstdh@jmu.edu

Motivation

The Idea:
- Many programs need to center content of some kind in a container of some kind
An Example:
- Text within a line (so it needs to find the column in where the text should start)

Review

A Limitation of Console Output:
- You can't specify the column
A Work-Around:
- Create a String that has the appropriate number of spaces before the text

Thinking About the Problem

A Specific Example:
- The line is 9 characters wide and 0-based
- The text is 5 characters wide and 0-based
Some Implications:
- The middle character in the line has index 4
- The middle character of the text has index 2
A Solution to this Specific Example:
- Character 2 of the text has to be at position 4 of the line, meaning character 0 of the text must be at position 2 of the line (i.e., there must be two leading spaces)

The Pattern

Notation:
- \(C\) denotes the container (a generalization of the line)
- \(c\) denotes the content (a generalization of the text)
- \(E\) denotes the extent (a generalization of the width)
- \(M\) denotes the mid-point
- \(R\) denotes the reference (a generalization of the starting character)
The Equations (for Each Dimension):
- \(C^M = C^R + (C^E / 2)\)
- \(c^M = (c^E / 2)\)
- \(c^R = c^M - (c^E / 2)\)

The Pattern (cont.)

In One Dimension

javaexamples/programmingpatterns/Centering.java (Fragment: one)

The Pattern (cont.)

In Multiple Dimensions

javaexamples/programmingpatterns/Centering.java (Fragment: multi)

Examples

In One Dimension

Examples (cont.)

In Two Dimensions

Some Warnings

Coordinate Systems:
- These examples use Euclidean coordinates but computer displays use screen coordinates
Discrete Sets:
- If the content/container are discrete (i.e., use integer sizes and positions) then the content may have to "lean" to the left or right
Clipping:
- The container may not be large enough to hold the content in which case it may need to be clipped

There's Always More to Learn