Operators, Specifiers and Pointers

Operators represent mathematical, relational, bitwise, conditional, or logical data manipulations. There are many operators in the C/C++ language. In this chapter, the most important are presented. Logical operators will be shown in the next chapter as they are used together with conditional statements.

• Assignment operator ( = ) – the operator that assigns the value on the right to the variable on the assignment operator's left. The left side should represent the variable (must be able to be modified), and the right side can be the number (constant), variable, or expression. In the case of an expression, its value is first calculated, and then the result is assigned to the variable on the left. The work of an assignment operator can be seen in any of the following operation examples.

Arithmetic operations are used to do mathematical calculations with numbers or numerical variables. The arithmetic operators are the following.

• Addition ( + ) – one of the four primary arithmetic operations used to add numbers. The addition operator can add only numbers, numeric variables, or a mix of both. The following example shows the use of the addition operator.

int result = 1 + 2; //The result of the addition operation will be 3

• Subtraction ( - ) – the operation that subtracts one number from another where the result is the difference between these numbers.

int result = 3 - 2; //The result of the subtraction operation will be 1

• Multiplication ( * ) – the operation that multiplies numbers and gives the result.

int result = 2 * 3; //The result of the multiplication operation will be 6

• Division ( / ) – the operation that divides one number by another. If the result variable has the integer type, the result will always be the whole part of the division result without the fraction behind it. If the precise division is necessary, using the float type of variable for this purpose is important.

//The result of the division operation will be 3 
//(Only the whole part of the division result)
int result = 7 / 2;        
//The result of the division operation will be 3.5
float result2 = 7.0 / 2.0; 

• Modulo ( % ) – the operation that finds the remainder of the division of two numbers.

//The result of the modulo operation will be 1, 
//Because if 7 is divided by 3, the remaining is 1
int result = 7 % 3; 

Bitwise operators perform operations on bits in the variable. Among them, there exist bitwise logic operations. It means the same logic function is applied to every pair of bits in two arguments. Bitwise or ( | ) means that if at least one bit is “1” at the chosen bit position, the resulting bit will also be “1”. Bitwise and ( & ) means that if at least one bit is “0”, the resulting bit is “0”. Bitwise operators shouldn't be confused with Logic Operators ( || ), ( && ), which operate on a single boolean logic value.

byte result = 5 | 8;
; //The operation in numbers gives the result of 13
; //in bits can be shown as follows
; // 00000101b
; // 00001000b
; // ---------
; // 00001101b

byte result = 5 & 1;
; //The operation in numbers gives the result of 1
; //in bits can be shown as follows
; // 00000101b
; // 00000001b
; // ---------
; // 00000001b

Bitwise operators also allow shifting data left ( « ) or right ( » ) chosen number of bit positions. Shifting is often used in embedded programming to access the bit at a specific position. Shifting data one bit left gives the result of multiplication by 2, while shifting one bit right gives the effect of dividing by 2.

byte result = 5 << 1;
; //The operation in numbers gives the result of 10
; //in bits can be shown as follows
; // 00000101b
; // 00001010b

Compound operators in C/C++ are a short way of writing down the arithmetic operations with variables. All of these operations are done on integer variables. These operands are often used in the loops when it is necessary to manipulate the same variable in each cycle iteration. The compound operators are the following.

• Increment ( ++ ) – increases the value of integer variable by one.

int a = 5;
a++; //The operation a = a + 1; the result will be 6

• Decrement ( - - ) – decreases the value of the integer variable by one.

int a = 5;
a--; //The operation a = a – 1; the result will be 4

• Compound addition ( += ) – adds the right operand to the left operand and assigns the result to the left operand.

int a = 5;
a+=2; //The operation a = a + 2; the result will be 7

• Compound subtraction ( -= ) – subtracts the right operand from the left operand and assigns the result to the left operand.

int a = 5;
a-=3; //The operation a = a – 3; the result will be 2

• Compound multiplication ( *= ) – multiplies the left operand by the right operand and assigns the result to the left operand.

int a = 5;
a*=3; //The operation a = a × 3; the result will be 15

• Compound division ( /= ) – divides the left operand with the right operand and assigns the result to the left operand.

int a = 6;
a/=3; //The operation a = a / 3; the result will be 2

• Compound modulo ( %= ) – takes modulus using two operands and assigns the result to the left operand.

int a = 5;
//The result will be the remaining 
//Part of the operation a/2; it results in 1
a%=2; 

• Compound bitwise OR ( |= ) – bitwise OR operator that assigns the value to the operand on the left.

int a = 5;
a|=2; //The operation a=a|2; the result will be 7

• Compound bitwise AND ( &= ) – bitwise AND operator that assigns the value to the operand on the left.

int a = 6;
a&=; //The operation a=a&2; the result will be 2

Simple and complex types can be referred to with pointer variables. A pointer is a variable that holds the address of the variable. The length of the pointer is equivalent to the length of the memory address (usually 16, 32 or 64 bits). A pointer does not contain a value but instead points to the variable (a memory) where the value is stored. A pointer variable must be initialised and dereferenced with Address-Of and Dereferencing operators.
The following example presents a simple type declaration and the use of a pointer variable.

& operator returns an address of a variable.
* operator dereferences a variable (it provides access to a value that the pointer variable points to).

int n = 10;     //Declare a variable of type int and initialise it with 10
int *ptr;       //Declare a pointer variable.
                //At this point, *ptr does not contain any address yet, 
                //rather some random address or null.
ptr = &n;       //Assign to the pointer ptr an address of the variable n
                //ptr contains now an address of the memory where 
                //variable n is located, not a value 10
int k;          //Declare another variable
k = *ptr;       //Assign k a value that is pointed by ptr

Simple type variables such as int, double, float and so on are passed to the function arguments as values, so the original value is copied, and a copy is presented to the function code (more on functions one can find in the Sub-programs, Functions). Modifications to the argument do not change the original value but just a copy. This is not the case when passing a complex type, such as an array, as an argument. The importance of pointers is not to be underestimated in this case: you declare a pointer pointing to the array's first element and pass it to the function. Then, by modifying the pointer value (an address), it is possible to refer to the following elements of the array. In this case, any modification to the referred array element modifies an original one, so the change in the value is instant. It does not need a return variable from a function.