Program Control Statements, Logical operators

It is essential to understand that if no statements change the normal program flow, the microcontroller executes instructions one by one in the order they appear in the source code (from the top - to the down direction). Control statements modify normal program flow by skipping or repeating parts of the code. Often, to decide if the part of the code should be executed or to choose one of the number of possible execution paths, conditional statements are used. For repeating the part of the code, loop statements can be used.

if is a statement that checks the condition and executes the following statement if the condition is TRUE. There are multiple ways to write down the if statement:

//1st example
if (condition) statement;     
 
//2nd example
if (condition)
statement;                    
 
//3rd example
if (condition) { statement; } 
 
//4th example
if (condition)
{
  statement;
}                             

The version with curly braces is used when there is a need to execute part of the code that consists of more than a single statement. Many statements taken together with a pair of curly braces are treated as a single statement in such cases. When both TRUE and FALSE cases of the condition should be viewed, the else part should be added to the if statement in the following ways:

if (condition) {
  statement1;    //Executes when the condition is true
}
else {
  statement2;    //Executes when the condition is false
}

If more conditions should be viewed, the else if part is added to the if statement:

if (condition1) {
  statement1;    //Executes when the condition1 is true
}
else if (condition2) {
  statement2;    //Executes when the condition2 is true
}
else {
  statement3;    //Executes in all other cases
}

For example, when the x variable is compared and when it is higher than 10, the digitalWrite() method executes.

if (x>10) 
{
  //Statement is executed if the x > 10 expression is true
  digitalWrite(LEDpin, HIGH)  
}

To allow checking different conditions, logical operators are widely used with the condition statement if described above.

Comparison Operators

There are multiple comparison operators used for comparing variables and values. All of these operators compare the variable's value on the left to the value on the right. Comparison operators are the following:

• == (equal to) – if they are equal, the result is TRUE, otherwise FALSE,
• != (not equal to) – if they are not equal, the result is TRUE, otherwise FALSE,
• < (less than) – if the value of the variable on the left is less than the value of the variable on the right, the result is TRUE, otherwise FALSE,
• < = (less than or equal to) – if the value of the variable on the left is less than or equal to the value of the variable on the right, the result is TRUE, otherwise FALSE,
• > (greater than) – if the value of the variable on the left is greater than the value of the variable on the right, the result is TRUE, otherwise FALSE,
• > = (greater than or equal to) – if the value of the variable on the left is greater than or equal to the value of the variable on the right, the result is TRUE, otherwise FALSE.

Examples:

if (x==y){ //Equal
  //Statement
}
 
if (x!=y){ //Not equal
  //Statement
}
 
if (x<y){ //Less than
  //Statement
}
 
if (x<=y){ //Less than or equal
  //statement
}
 
if (x>y){ //Greater than
  //Statement
}
 
if (x>=y){ //Greater than or equal
  //Statement
}

Boolean Operators

The Boolean logical operators in C/C++ are the following:

• ! (logical NOT) – reverses the logical state of the operand. If a condition is TRUE the logical NOT operator will turn it to FALSE and the other way around,
• && (logical AND) – the result is TRUE when both operands on the operator's left and right are TRUE. If even one of them is FALSE the result is FALSE,
• || (logical OR) – the result is TRUE when at least one of the operands on the operator's left and right is TRUE. If both of them are FALSE, the result is FALSE.

Examples:

//Logical NOT
if (!a) { //The statement inside if will execute when the a is FALSE
  b = !a; //The reverse logical value of a is assigned to the variable b
}
 
//Logical AND
//The statement inside if will execute when the 
//Values both of the a and b are TRUE
if (a && b){  
  //Statement
}
 
//Logical OR
//The statement inside if will execute when at least one of the 
//a and b values are TRUE
if (a || b){  
  //Statement
}

A switch statement similar to the if statement controls the flow of a program. The code inside switch is executed in various conditions. A switch statement compares the values of a variable to the specified values in the case statements. Allowed data types of the variable are int and char. The break keyword exits the switch statement.

Examples:

switch (x) { 
   case 0:  //Executes when the value of x is 0
   // statements
   break;   //Goes out of the switch statement
 
   case 1:  //Executes when the value of x is 1
   // statements
   break;   //Goes out of the switch statement
 
   default:  //Executes when none of the cases above is true
   // statements
   break;   //Goes out of the switch statement
}