Using Struct Types with Functions

Struct types are data types (the same way that int, double, and the other built-in types are data types). They can be used with functions in the same way as the built-in types, both as parameter types (to send information to a function) and as a return type (to return information from a function.)

The following struct type will be used in the examples below:

struct Complex {
    double real;
    double imag;
};

An instance of the Complex type represents a complex number. The field real represents the magnitude of the real part of the complex number, and the field imag represents the magnitude of the imaginary part.

Struct Return Value

A function can return a struct instance. For example, we could create a function to return a Complex value with specified real and imaginary field values:

struct Complex makeComplex(double realValue, double imagValue) {
    struct Complex result;
    result.real = realValue;
    result.imag = imagValue;
    return result;
}

We can call this function as follows:

struct Complex origin;
origin = makeComplex(0.0, 0.0);
printf("real=%.1lf, imag=%.1lf\n", origin.real, origin.imag);

The code above would print the following output:

real=0.0, imag=0.0

Struct Parameters

Structs, by default are passed-by-value (similar to int, double, etc.) which means that when we pass a struct to a function, a copy is made of all its fields into the local parameter variable. This mechanism also means that a struct may be returned from a function and assigned to a similar type variable in the calling routine. Thus in this style of programming,

Because passing and returning struct instances by value involves copying the data (including array fields) of each struct instances passed to or returned from each function, this style is most appropriate when the struct types have a small number of fields and do not contain array fields which might contain a large number of values.

For example: complex numbers

A complex number has the form

a + bi

where i is the imaginary unit, such that

i2 = (-i)2 = -1

and a and b are real numbers which represent the magnitudes of the real and imaginary parts of the complex number, respectively.

Complex numbers are useful in a number of situations. For example, the roots of a quadratic equation may be complex. For example, the equation

x2 - 10x + 34

has the complex roots

x = 5 + 3i

and

x = 5 - 3i

[Example taken from regentsprep.org.]

Operations

What will make this a useful data type to use in programs are functions to perform operations on instances of the Complex type.

For example,

We might specify functions to perform these operations as follows:

struct Complex makeComplex(double real, double imag);
struct Complex addComplex(struct Complex left, struct Complex right);
struct Complex subtractComplex(struct Complex left, struct Complex right);
struct Complex mulitplyComplex(struct Complex left, struct Complex right);
struct Complex divideComplex(struct Complex left, struct Complex right);

Note: these are just the prototypes, we would also need to write a definition of each function which performs the appropriate computation.

For example, we might define the makeComplex function as follows:

struct Complex makeComplex(double real, double imag)
{
    struct Complex result;

    result.real = real;
    result.complex = imag;

    return result;
}

This function is useful because it allows us to “construct” an instance of Complex value from the magnitudes of the real and imaginary components.

Using the operations

Given the operations (functions) defined above, the following code would compute the product of the complex numbers

(3 + 4i)(-5 + 2i)

struct Complex first, second;

first = makeComplex(3, 4);
second = makeComplex(-5, 2);

struct Complex product;

product = multiplyComplex(first, second);