More about variables

Variables are independent:

int a;
int b;

a = 121;
b = a;

printf("a = %i, b = %i\n", a, b); /* prints a = 121, b = 121 */

a = 1001;

printf("a = %i, b = %i\n", a, b); /* prints a = 1001, b = 121 */

Even though the variable b initially contains the same value as the variable a, it is still a different variable. Variables are storage locations. Storing a value in one variable does not change the value of any other variable.

Updating a variable

The value stored in a variable can be updated whenever necessary. For example, adding 1 to a variable and storing the incremented value back into the same variable:

int a;

a = 10;

printf("a = %i\n", a);

a = a + 1;

printf("a = %i\n", a);

Remember, a variable is a storage location for a value. Because a new value can be stored in a variable at any time, overwriting the previous value, the values of variables can change as the program executes.

Increment and Decrement

Updating a variable to add 1 to or subtract 1 from the value stored in the variable is such a common operation in C that there are special operators, the increment and decrement operators, to perform these tasks.

Example:

int a;
int b;

a = 3;
b = 3;

a++;
b--;

printf("a = %i, b = %i\n", a, b);

This code produces the output

a = 4, b = 2

Compound Assignment

Another way to update the value of a variable using one of the compound assignment operators.

Example:

int a, b, c;

a = 3;
b = 3;
c = 3;

a += 2;
b -= 2;
c *= 2;

printf("a = %i, b = %i, c = %i\n", a, b, c);

This code produces the output

a = 5, b = 1, c = 6

A good way to think about how the compound assignment operators work is to consider them as combining the effect of a mathematical operator with an assignment. For example, the code

a += 2;

is exactly equivalent to the code

a = a + 2;

In both cases, the sum of a and 2 is computed, and then the sum is stored back into a, replacing the previous value.

<math.h>

So far, the operators we have seen are useful for simple arithmetic. More powerful mathematical functions are available through the inclusion of the <math.h> header file. To use these functions, simply add the line

#include <math.h>

to the top of your program, just below the line

#include <stdio.h>

Most of the functions in <math.h> operate on double values. Here are a few of the functions:

Function	Purpose
sqrt	Compute a square root.
sin	Compute the sine of an angle (measured in radians.)
cos	Compute the cosine of an angle (measured in radians.)

A few examples of using functions from <math.h>:

double x;
double theta;

printf("Enter a number: ");
scanf("%lf", &x);

printf("The square root of %f is %f\n", x, sqrt(x));

printf("Enter an angle in radians: ");
scanf("%lf", &theta);

printf("The sine of %f is %f\n", theta, sin(theta));
printf("The cosine of %f is %f\n", theta, cos(theta));

In addition to the functions available in <math.h>, the special constant value M_PI is available. This constant represents the double value closest to the value of π (pi).

Type casts

Sometimes you may want to convert one type of numeric value to another type of numeric value.

For example, you might want to convert int values into double values---for example, to avoid truncating the result of a division. This can be accomplished with a type cast. The general form of a type cast is

( type ) expression

A type cast converts the value computed by an expression to an "equivalent" value belonging to another type. For example, the int value 4, when cast to a double, is converted to the double value 4.0.

Example: dividing one integer value by another, retaining the fractional part of the answer:

int a;
int b;
double quotient;

a = 7;
b = 4;
quotient = (double) a / (double) b;

print("%i / %i = %f\n", a, b, quotient);

What's going on is that we are casting the int variables a and b to the type double, and then using the division operator on the resulting double values. This modified code fragment finally produces the output we want, 1.750000.

Mixed-type expressions

Sometimes you will want to perform a computation on two numeric values of different types. An expression that has two operands of different types is called a mixed-type expression.

To evaluate any expression with two operands, the operands must have the same type. So, in a mixed-type expression, C will convert one of the operands so that the types match. This is called an implicit conversion.

The question, of course, is which operand should be converted. The following general rules apply:

1. If one operand's type is integer and the other floating-point, the integer value is converted to floating-point
2. If one operand's type is more precise than the other, the value whose type is less precise is converted to the more precise type

A type is "more precise" than another when it has more bits in its representation. E.g., double is a more precise type than float because double values occupy 64 bits and float values occupy 32 bits.

Example:

int a;
double b;

scanf("%i", &a);
scanf("%lf", &b);

printf("%f\n", a * b);

The int value a is converted to an equivalent double value before the multiplication is performed.

Unary minus

The unary minus operator computes the negation of a numeric value.

Syntax:

- expression

Examples:

int i;
double d;

i = 17;
d = 43.3;

printf("%i\n", -i);
printf("%f\n", -d);