3.2: Selection Statement- if-statement

General structure of an "if" statement in C++:

if (relational statement) {

statement1;
statement2; <----------- body of the if statement
.........
.........

}

Statement (s)

Following flow chart demonstrates the logic of an "if" statement:

When a computer executes an if statement in a program, it will first evaluate the value of the relational statement in the if statement, if the value of the vlaue of the relational statement is true, then the computer will execute the statements in the body of the if statement, otherwise, the computer will skip the statements in the body of the if statement and move on the next statement that immediately follows the close brace } in the program.

Example

When we first see this programming problem, it seems intimidating. We just don't know what it is talking about and, of course, we don't know how to get started.

Don't worry, at least we can create an empty project and open a blank source file and that is a good start!

Read the question, the first statement of the question says: "Write a program that asks a user to enter two integers and save them into two variables". So, that means we will need two variables in this program. Since all the variables used in C++ program need to be declared, we should put a line of code that declares two integer variables. Yes, that is exactly what we need to do in this program:

We made some progress. It may not be perfect correct, but it is better than simply stare at screen and do nothing! In the first statement of the question, it also says: "Ask a user to enter two integers", this tells us we should put a line of code like following:

cout<<"Please enter two integer:"<<endl;

because this line of code will display the message "Please enter two integers:" on the output screen and it can be seen by a user.

The first statement of the question also says to save the data a user entered into those variables we declared in line 6. Recall the statement we used in the example of calculating the area of a rectangle. There, we use

cin>>length;
cin>>width;

to store the data user entered in the variables length and width. Now, we use similar statements to store the integer user entered into the variable a and b:

cin>>a;
cin>>b;

Alternatively, those two statements can be written equivalently as following: (which uses a single line of the code instead of two)

cin>>a>>b;

we can use either one of them in our program to store the integers the user entered into the variable a and b:

We are making more progress!

Now, continue to read the question. Next phrase is "Then switch the values of two variables" Well, this may be a little bit difficult for you to grasp.

There are two data storage used in this program, one is represented by the variable a and the other is represented by the variable b, they are current used for storing the first and second integer entered by the user. We can't use following two lines of code for switching the values:

b = a;
a = b;

The reason is when the program executes the first statement, the the value of b is replaced by the value of a and the original value of b is gone. The second statement just put the same data that was in a back to a. Thus, after above two statements, the values of both variables are the same. In other words, after above two lines of code, both a and b are holding the the data that user entered into a.

To switch the values between the variables a and b, we will need a third variable, say t. We will need to copy the data in a and save it in the variable t by following statement:

t = a;

then copy the value of b and put it into a:

a = b;

Now, the data in a is gone and replaced by a copy of the data in b. Since the data in the variable a has been saved in the variable t, we now copy the data in t, which is original data in a, and put it into the variable b by using following statement:

b = t;

So, by following three statements:

t = a; //copy the data in a to t

a = b; //copy the data in b to a

b = t; //copy the data in t to b

we successfully swapped the values of the variables a and b. However, when you read the question carefully, you will realize that we don't want to do the swapping for any integers user enter unconditionally. We only do the swapping when the first number a user entered is greater than the second number the user entered. That being said, we only need to execute above three statements under the condition: (a > b). Therefore, we will use an if statement to control the execution flow as following:

The code shown above controls the execution of the switching under the condition (a > b) with an if statement beginning at line 10. Since we added an additional variable t in the program, we will need to go back to the line 7 and add it to the declaration statement. Now, the last thing we need to do in this program is to use a "cout" statement at line 15 to display the values of a and b.

Now, we have completed the coding of this program. When we run this program, for any two numbers a user entered, the program will always display those two numbers in an ascending order. Please test run this program on your computer.

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Exercise

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