# publicJuliaLang/julia-tutorial

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# Grid of Resistors

## Description

The problem is to compute the voltages and the effective resistance of a 2n+1 by 2n+2 grid of 1 ohm resistors if a battery is connected to the two center points. This is a discrete version of finding the lines of force using iron filings for a magnet. The picture below describes the two dimensional problem.

The method of solution that we will use here is successive overrelaxation (SOR) with red-black ordering. This is certainly not the fastest way to solve the problem, but it does illustrate many important programming ideas.

It is not so important that you know the details of SOR. Some of the basic ideas may be found on pages 407-409 of Gil Strang's Introduction to Applied Mathematics. A somewhat more in-depth discussion may be found in any serious numerical analysis text such as Stoer and Bulirsch's Introduction to Numerical Analysis. What is important is that you see that the nodes are divided in half into red nodes and black nodes. During the first pass, the red nodes obtain the voltages as a weighted average of their original voltage, the input (if any) and the four surrounding black nodes. During the second pass, the black nodes obtain voltages from the four surrounding red nodes. The process converges in the limit to the correct answer for the finite grid.

A C program is provided in `sor2d.c` and a MATLAB version in `sor2d.m`. Implement the following versions in julia and compare the timings of the different approaches:

1. A vectorized version.
2. A devectorized version.
3. Create a stencil function and refactor the stencil part of the computation.
4. Use the stencil function and implement using comprehensions.
5. Use a macro for the node updates.

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