A C++ control statement redirects the flow of a program in order to execute additional code. These statements come in the form of conditionals (if-else, switch) and loops (for, while, do-while). Each of them relies on a logical condition that evaluates to a boolean value in order to run one piece of code over another.
Boolean values on their own do not appear to be very useful, however when combined with control statements they become one of the most powerful tools that any programing language has to offer. As stated in the data types notes, boolean variables (bool) can be either true or false. There are also binary operators that return a boolean value depending on specific conditions. Some operators compare, such as ==, !=, >, etc., the values on either side of it and return true if the entire statement is true. Others, work with boolean values to return another boolean, such as && and ||.
| Operator | Symbol | Usage | Example |
| Equals | == | Returns true if the statements on either side of the operator are the same value, otherwise it returns false. Can be used with int, double/float, char, boolean, and string. | x == 'A' |
| Not Equal | != | Returns true if the statements on either side of the operator are not the same, otherwise it returns false. | x != 2 |
| Greater Than | > | Returns true if the first term is greater than the second, otherwise it returns false. Note: If both terms are equal it returns false. | x > 0 |
| Less Than | < | Returns true if the first term is less than the second, otherwise it returns false. | x < 10 |
| Greater Than or Equal | >= | Returns true if the first term is greater than or equal to the second, otherwise it returns false. | x >= 0 |
| Less Than or Equal | <= | Returns true if the first term is less than or equal to the second, otherwise it returns false. | x <= -2 |
| And | && | Returns true if both terms on either side of the operator are true, otherwise it returns false. It is commonly used to concatenate statements together. |
x && True (x == 2) && (y >= 0) |
| Or | || | Returns true if any of the terms on either side of the operator are true, otherwise it returns false. | x || y |
| Not | ! | Negates the value of the statement that follows it. | !(x > 2) |
If-Else statements allow the program to execute different blocks of code depending on conditionals. All If statements have the following form:
if ( condition ) {
//body
}
An If statement executes the code in the body section if the condition evaluates to True, otherwise it skips the body. When the condition evaluates to False, the program will either test another condition with a following Else-if, run the code inside an Else statement, or continue as normal if neither an Else-if nor Else block exist.
An Else statement acts as a default case for If statements. In the case that an If is directly followed by an Else and the condition is false, the code in the Else is executed. Else statements do not have a condition themselves.
if ( condition ) {
//body
} else {
// else body
}
An If statement can followed by any number of Else-if blocks. Each Else-if has its own conditional statement and body of code. If an Else-if conditional is False, then its body of code is skipped and the program will check the next Else-if in order that they appear. An If followed by a long list of Else-IF's is usually referred to as an If-Else ladder.
if ( condition ) {
//body
} else if ( 2nd condition ) {
// else if body
} else {
// else body
}
A for loop allows for a block of code to be executed until a conditional becomes false. For loops are usually used when a block of code needs to executed a fixed number of times. Each loop consists of 3 parts, an initialization step, the conditional, and an iteration step. The initialization is run before entering the loop, the condition is checked at the beginning of each run through the loop ( including the first run ), and the iteration step executes at the end of each pass through the loop, but before the condition is rechecked. It is usual practice to have the iteration step move the loop on step closer to making the condition false, thus ending the loop, but this does not need to be the case.
for( initialization ; conditional ; iteration ) {
// loop body
}
A while loop is a simple loop that will run the same code over and over as long as a given conditional is true. The condition is checked at the beginning of each run through the loop ( including the first one ). If the conditional is false for the beginning, the while loop will be skipped all together.
while ( conditional ) {
// loop body
}
A do-while loop acts just like a while loop, except the condition is checked at the end of each pass through the loop body. This means a do-while loop will execute at least once.
do {
// loop body
} while ( condition );
Break is a useful keyword that allows the program to exit a loop or switch statement before the expected end of a that code block. This is useful in error checking or if the outcome of a loop is not certain. For example, the following code will break out of the for loop if a user asks to leave.
string inputs[10];
string input;
for (int i = 0; i < 10; i++ ) {
cin >> input;
if ( input == "quit" ) {
break;
}
inputs[i] = input;
}