DAY19
Monday, 25 June 2012

Today we will talk about:

• some errors that occur with array programming: uninitialized data; referencing an "out of bounds" entry.
• character arrays, which are related to strings.
• variable length arrays, which allow us to choose the size of an array as the program is running.
• how vectors from linear algebra can be defined as an array in C; what simple vector operations look like when implemented in C, including the vector norm, dot product, and angle.

Programs and functions we might talk about:

• array_bounds.c shows that, even if you define an array A to be of size 5, the expression "A[6]" or "A[-4]" actually is legal, and you might be able to tell what it means. This suggests how C thinks of variable names as "addresses" in memory; then indexes just tell you how far to walk from the basic address to get to the value you want.
• char_array.c sets up an array of characters. Although an array of characters is similar to a string, we will have to wait until chapter 10 to see how to handle strings in a general way.
• grades.c uses an array to store up to 100 grades. It computes the average, and prints a summary of the grades by letter. My first version of the program printed out nonsense, because I did not initialize one of the arrays!
• grades.txt a file containing 53 grades, followed by a "-1" to signal the end. The grading program can read this data, if you use a command like ./a.out < grades.txt.
• how_big.c, shows how an array, which is normally given a numeric dimension, can be declared with a variable dimension that is entered by the user.
• spring_ode2.c, the source code.
• spring_ode2_output.txt, the output file, which contains just the graphics data.
• vector_2d.c allows the user to specify two vectors in 2D, and computes the norms, dot and cross product, cosine and angle.
• vector_3d.c considers three vectors in 3D space, and finds their lengths, dot products, pairwise cosines and angles.
• xv_time.png, a time plot, created by gnuplot, of the spring displacement x and velocity v.
• xv_phase.png, a phase plot, created by gnuplot, of the spring displacement x and velocity v.