Conditional statements in C++ are identical to conditional statements in C as explained below.

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1 Decisions Conditional statements in C++ are identical to conditional statements in C as explained below. In C++ an if statement structure or a switch statement structure are used to perform a decision making statement. The following is a table with the equality operators used by C++ for decision making: Algebraic equality or relational operator Equality operators Relational operators C equality or relational operator C example = == x == y x is equal to y Meaning!= x!= y x is not equal to y > > x > y x is greater than y < < x < y x is less than y >= x >= y x is greater than or equal to y <= x <= y x is less than or equal to y Making decisions with the if and if..else structures The if statement has this form, where condition is evaluated to either true or false. if ( _condition_ ) {... // Do this clause if the condition is true. -- OR -- if ( _condition_ ) {... // Do this clause if the condition is true else {... // Do this clause if the condition is false For example, the following program, after asking the user to enter his age, checks whether the user can drive a car or not. The decision is based on the age of the user. If the user is 18 years old or older, then he can drive a car, otherwise he cannot drive a car. Copy and execute this program to understand the functionality of an if..else statement: Matthew Xuereb 2014/2015 Page 17

2 From a technical perspective, a conditional statement changes the flow of the program. The following flowchart illustrates this concept: Matthew Xuereb 2014/2015 Page 18

3 Note that the flow of the program can take different routes It is common to make a series of tests on a value, where the else part contains only another if statement as shown in the example below. Copy and execute this program to understand the functionality of having another if statement as within the else part of a conditional statement: Matthew Xuereb 2014/2015 Page 19

4 Sometimes it is required to use more than one condition in a selection statement. For example to check whether a number is in range or not, the AND operator is required in an if statement. In C a logical AND is written as && while a logical OR is written as. Copy and execute the following program to understand this technique: Matthew Xuereb 2014/2015 Page 20

5 When multiple conditions are required in an if statement as shown in the example above, make sure that you use the appropriate && and logical operators. If you use the other operators (& and ), although you can get a valid answer you are using the bit-wise operators and not the logical operators. Use the bitwise operators only when you are doing bitwise arithmetic. Bitwise operators will be covered in another topic. Making decisions with switch structure The if statement allows you to select one of two sections of code to execute based on a boolean value (only two possible values). The switch statement allows you to choose from many statements based only on integers (including char and enum values). The syntax of a switch statement is as follows: switch ( _variable_ ) { case c1: statements // do these if _variable_ == c1 break; case c2: statements // do these if _variable_ == c2 break; case c2: case c3: case c4: // Cases can simply fall thru. statements // do these if _variable_ == any of c's break;... default: statements // do these if _variable_!= any above The switch keyword is followed by a parenthesized variable, which is followed by the cases, all enclosed in braces. The switch statement executes the case corresponding to the value of the expression. Normally the code in a case clause ends with a break statement, which exits the switch statement and continues with the statement following the switch. If there is no corresponding case value, the default clause is executed. If no case matched and there is no default clause, execution continues after the end of the switch statement. The case keyword is followed by an integer constant and a colon. This begins the statements that are executed when the switch expression has that case value. Matthew Xuereb 2014/2015 Page 21

6 If no case value matches the switch expression value, execution continues at the default clause. This is the equivalent of the else for the switch statement. It is written after the last case by convention, and typically isn't followed by break because execution just continues out the bottom of switch if this is the last clause. The break statement causes execution to exit to the statement after the end of the switch. If there is no break, execution flows thru into the next case. Flowing directly into the next case is almost always an error. Copy and execute the following program to understand the switch statement: Remove the break keywords from the above example and see what will happen. Matthew Xuereb 2014/2015 Page 22

7 Repetitions A repetition (or loop or iteration) is a sequence of statements which is specified once but which may be carried out several times in succession. There are three main kinds of loop statements in C++, exactly the same as in the C programming language. These are: 1. for loop 2. while loop 3. do..while loop for loop A for loop is a loop that repeats a group of statements for a defined number of times. Thus it can be defined as an iteration statement that has a counter. This type is loop is usually used when you now before hand the amount of times that is required to loop. In fact a variable is used as a counter. The following is the structure of a for loop: for(_counter_ ; _condition_ ; _increment_ ){.... As it can be noted in the above code structure, in a for loop there are three parts: The counter initialisation (Starting value) The stopping condition (Repeat until condition is false) The Increment mechanism (Statement to increment the value) Consider the following example: Matthew Xuereb 2014/2015 Page 23

8 The above for loop is repeating the instruction cout << "Hello" << endl;. Copy the above program and execute it to understand better how a for loop works. Note that in the above example, the loop is incrementing the counter variable by 1. In fact i++ is used as a shortcut for i = i + 1. To decrement by 1 i-- can be used. To increment or decrement using a different value rather than 1, an assignment statement should be written instead as shown in the example below: Copy and execute the above program. while loop A while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. The condition is first evaluated - if the condition is true the code within the block is then executed. This type of loop is used when you do not know the exact number of repetitions you want to perform and you may not want to perform the loop at all. The structure of a while loop is shown below: while( _condition_ ){.... Consider the following example: Matthew Xuereb 2014/2015 Page 24

9 Copy the above code at execute it to understand better how a while loop works. do..while loop A do..while loop is similar to the while loop in the sense that it is used when you do not know the exact number of repetitions you want to perform. However, unlike the while this loop will at least perform the loop once. The structure of a do..while loop is shown below: do {.... while( _condition_ ); Consider the following example: Matthew Xuereb 2014/2015 Page 25

10 Copy the above code at execute it to understand better how a while loop works. Comparison between the while loop and the do..while loop The only difference between the while loop and the do..while loop is that the latter has the stopping condition at the end of the loop and therefore at least the looping content is executed once. This is graphically explained in the following two flow charts: Matthew Xuereb 2014/2015 Page 26

11 Final note on loops and selection statements In these notes, all the selection and loop statements included the {. However note that if you want to loop only one statement or execute only one statement after an if or an else, the { is not required. It is however suggested that you always should use { even if only one statement is required to be selected/repeated. Matthew Xuereb 2014/2015 Page 27

12 Unconditional branching Unconditional branching is when the programmer forces the execution of a program to jump to another part of the program. Theoretically, this can be done using a good combination of loops and if statements. In fact, as a programmer, you should always try to avoid such unconditional branching and use this technique only when it is very difficult to use a loop. Consider the following example: The above example illustrates a label LAB1 and the keyword goto that is used to change the direction of the program flow. The following example illustrates a different way of looping 10 times using a goto statement instead of a for loop: Matthew Xuereb 2014/2015 Page 28

13 Although the above program will do exactly the same thing as a for loop does, it is highly recommended that you DO NOT USE this technique. A lot of goto statements in a C++ program will make the program unstructured and difficult to trace. Sometimes this is called spaghetti programming. Matthew Xuereb 2014/2015 Page 29