Some Remarks About C

[Since so many people in this class already know C, the following remarks are 
included more for completeness than to indicate what was discussed in class.]
		
		The C programming language has been used extensively in
		class examples.  However, not everyone is familiar with C.
		One advantage of learning C is that programming constructs in
		the AWK language are very much like C.

		One good think about C is that it is a "strong typing" 
		language.  This means you must declare all your variables.  
		The program lint can be used to help you find bugs in your
		code even before you try to compile it.  It checks that 
		variables have been declared and that they are not used before
		being initialized.

		C declares variables with statments like:
		int i;
		float r;
		However, C also has some special variables called pointers.
		int *j_ptr;
		float *s_ptr;
		declare that j_ptr is the address of an integer, i.e., j_ptr
		points to an integer.  s_ptr points to a floating point 
		variable.

		In Fortran a subroutine can be used to change many of its
		arguments, but in C, there are only functions.  A function
		can only return one value.  However, if the arguments to the
		C function are pointers to variables you want changed, then the
		function can reach out and touch those variables!  If you have
		a pointer to a variable, you can use the operation of 
		dereferencing to get the value stored at the address.  For
		example
		float y;
		y = *s_ptr;
		sets the variable y to the value stored in the address s_ptr.
		So, *s_ptr is a floating point value.
		However, a statment like
		y = s_ptr;
		is NOT valid because y is a float and s_ptr is a pointer
		or address.  To get the address of a variable, you just put
		an ampersand, &, in front of the variable name.  Here is
		an example from ~sg/chap3/euler_cromer.c:

		euler_2d(&y,&v,&accel,&t,g,dt,nsteps);

		Down in the euler_2d subroutine, we have derefencing is in:
		*v_ptr = (*v_ptr) + (*accel_ptr)*dt;

		Control structures in C

		The for loop is used extensively in C.  The word for is
		followed by three statements within parentheses.  The
		statment that follows is executed under control of the loop.
		If you want more than one statement, enclose them in curly
		braces { and }.  Consider as an example:
		for (i=1; i <= nsteps; ++i) printf ("%d\n",i);

		Within the parentheses after for is first an initialization
		statement i=1.  This sets i to 1 before the loop starts
		executing.  One can actually have more than one initialization
		if they are separated by commas.  The next part i <= nsteps
		is a logical condition that is tested at the beginning of 
		the loop.  If it is true, the loop is executed.  At the end
		of the loop, the third part within the for statement is
		executed.  ++i is short for i = i + 1.

		Format statment in C

		%d	integer variable
		%f	floating point variable 6.3
		%e	exponential or scientific format 1.03e-10
		%le	double precision exponential format
		%s	string variable