Structures

C has a notion similar to Pascal's record, called the struct, which allows you to keep various related pieces of information together, without having to declare lots of variables:

{ /* define the type `struct point' as convenient way of describing points in two-dimensions. This definition can occur at any place that a variable declaration would be legal */ struct point { double x; double y; }; struct point pt; /* declare pt as a `struct point' */ /* we want to make pt represent (0,1) */ pt.x = 0.0; /* . gives access to the fields of a struct */ pt.y = 1.0; }

A quicker way of initializing a structure resembles that for arrays:

struct point the_origin = { 0.0, 0.0 };

The type of pt is struct point, and like any type of known size (the size is known because the compiler knows the size of a double), you can declare arrays of them:

struct point { double x; double y; }; struct point box[4]; box[0] = { 0.0, 0.0 }; box[1] = { 1.0, 0.0 }; box[2] = { 1.0, 1.1 }; box[3] = { 0.0, 1.0 };

To use a struct in more than one part of a program, the definition must occur above the point where it is first used.

struct point { double x; double y; }; void display(struct point e) { printf("x = %f", e.x); printf("y = %f", e.y); } /* .. */

It is very common to pass pointers-to-structs rather than the structs themselves as arguments to a function. If x is a pointer-to-struct, C has a nice syntax for indirecting the pointer followed by selecting one of the fields, ->. The function display would more normally be written as

/* .. */ /* take a pointer-to-struct-point */ void display(struct point* e) { printf("x = %f", e->x); /* e->x means (*e).x */ printf("y = %f", e->y); } int main(void) { /* .. */ display(&box[0]); }

structs support assignment from variables of the same type, but do not support comparison operators, not even tests for equality/non-equality.