This chapter is from the book
This chapter is from the book
This chapter is from the book
Statements
You have already been exposed to some of what this section covers. Here you'll look at statements and how they relate to the Visual Basic code with which you are familiar.
This section covers assignment statements, control, flow, and looping statements.
Assignment
Visual Basic offers only the simple assignment; however, in C you have what are termed simple assignments (x=1;) and compound assignments (x+=1;).
Simple assignments in C are shown here:
x = 2; y = x;C also offers compound assignments, like the one shown here:
x += 2;In Visual Basic, this compound assignment must be expressed in a different way.
x = x + 2The Visual Basic form is also allowed in C:
x = x + 2;
However, x+=2 can more easily be optimized by the C/C++ compiler.
Control and Flow
The C statements involved with flow control include goto, switch, case, default, break, if, and else. These types of statements fall into two general categories, if statements and case statements. Both C and Visual Basic have these two types of statements, but there are some elementary differences between the two languages.
Goto
The goto and label statements fall outside these two general categories (if and case), but a goto is flow control. A goto is also one of the basic constructs of programming.
Note
It is well accepted that certain other programming elements do a good job of hiding the ugliness of jumps or goto. Loop constructs and logical constructs make the goto almost extinct. However, all of these programming tasks are constructed by using jumps which is a goto in its rawest form.
The goto in C statement works just like the goto statement in Visual Basic. This statement shouldn't show up in your code very much, if at all. goto makes the code difficult to read and maintain and is considered bad form.
if
Let's take a look at the if statement now. In Visual Basic there are five statements involved with the if construct: If, Then, ElseIf, Else, and End If. Only the if and else exist in C, while the Then and End If statements are implied in the way the language is used. The ElseIf feature is actually lost. The C if statement uses parenthesis to enclose the logical expression used within the if statement.
In C if statements typically use blocks, or compound statements as they are sometimes called. The braces that are used with the if block can impact the way the if statement is evaluated and so these blocks should be constructed with care. Nested if statements can be especially tricky. It is wise to use braces to make sure the statements work as you expect them to work. Look at Listing 3.14.
Listing 3.14 if Blocks
//c nested if
if (x>1)
if (x<10)
y=1;
else
y=0;
'Equivalent Basic code
If x > 1 Then
If x < 10 Then
y = 1
Else
y = 0
End If
End If
//c nested If with braces
if (x>1)
{
if (x<10)
y=1;
}
else
y=0;
'Equivalent Basic code
If x > 1 Then
If x < 10 Then
y = 1
End If
Else
y = 0
End If
I've indented the else statement to make it clear which if statement it goes with; in the first nested if statement, the else is paired with the nested if. Except for the braces, the two C listings are identical. When the braces are added, the else is paired with the first if statement, completely changing the logic of the statement.
If you use block If statements in Visual Basic there is little ambiguity. C is more difficult to read and easier to make mistakes in grouping the statements.
Tip
I recommend giving the braces their own line. It increases the readability. If you are unsure, use the braces to force the code to behave as you want it to. However, a good editor should make the program flow more obvious by auto indenting and brace matching.
Case
The other type of flow control is a case statement. In Visual Basic this consists of Select Case, Case Else, and End Select. C uses switch, case, and default. Again the braces take the place of the closing or End Select statement in VB.
There are some differences to look for in the C switch statement. The biggest is that control will fall through the case statements. The break statement must be used to exit a case statement in C. This offers some interesting ways to structure code within the switch–case statement. Listing 3.15 shows a switch–case statement and the equivalent Visual Basic code.
Listing 3.15 case Statements
switch (x)
{
case 0:
y0++;
break;
case 1:
y1++;
break;
case 2:
y2++;
default:
y++;
}
'Equivalent VB case statement
Select Case x
case 0:
y0=y0 + 1
case 1:
y1 = y1 + 1
case 2:
y2 = y2 + 1
Y = y + 1
Case Else:
y = y + 1
End Select
Notice the break statements in the C code. These are required to keep the flow from dropping through to the next case statement.
Note
Visual Basic does by default what the break statement adds to a case statement in C.
If you look closer at the case 2 statement, you see that the C code falls through to the default and both y2 and y are incremented.
To get this functionality in Visual Basic, you place the code in both the case 2 and the Case Else in order. This doesn't seem too bad, but it depends upon how much code you are talking about.
Looping
There are three basic types of looping provided within Visual Basic and C: For–Next, Do–Loop and While–Wend. Again C does away with the statement in VB used to close these loop statements, relying instead upon the braces to close the loop statement.
For
For loops share a very similar structure between Visual Basic and C, but as usual the C syntax is somewhat more cryptic. Let's take a look at the for statements in Listing 3.16.
Listing 3.16 for Loop
'C loop, 1 to 10, but code exits at 5
for (x=1;x<10;x++)
{
if (1==x)
continue;
if (5==x)
break;
y++;
}
'VB equivalent
For x=1 To 9 Step 1
If x = 1 Then
x = x + 1
End if
If x = 5 Then
Exit For
End If
y = y + 1
Next
Again you use the braces to define the beginning and end of the set of statements. You should be starting to see a pattern with the braces by now. C typically doesn't use some kind of closing statement like Visual Basic does.
Note
I have demonstrated what I consider to be poorly structured code here to demonstrate the continue statement. As a matter of fact, I discourage the use of a continue statement because it destroys the structure of the loop. The Visual Basic code I've written here doesn't really duplicate the continue statement, just allows the code to act the same. As a Visual Basic programmer, you never had the continue statement and you can probably do without it in C.
Notice too in Listing 3.16 that the break statement does the job of the Exit For. You saw the break statement before in the switch case. Break does double and triple duty. Usually the break statement means exit from whatever block statement you are in. It will exit from switch, for, while, and do blocks.
One more thing to notice before you leave this listing is the comparison that you are doing. The if statement compares x to 5 and exits the for if they are equal. The constant is placed first and the x is placed after the double equals. Note that if you place the x first and follow with a single equal sign (x=5) that this is a legal statement, will assign 5 to x and return true. The loop will exit the first time through. Depending on warning levels, the compiler may (but not necessarily) warn you about this. Getting in the habit of placing the constant first will help you guard against this misstep. A constant cannot be an l-value so if you follow the constant with a single equal, you will receive an error during compile.
Do
The other type of loop provided by C is the do–while, or the while by itself. In Visual Basic you typically use While Wend if you test at the front of the loop, or Do–Until if you test at the end. In reality the while or Until will test at either end of the loop in Visual Basic; the Until is just another way to do the test. The Until is not really required and C doesn't have it.
Listing 3.17 shows you both types of loops and the Visual Basic equivalents.
Listing 3.17 do and while Loops
do
{
x++;
}
while (x < 10);
'VB equivalent
Do
x = x + 1
Loop While x < 10
while (x < 10)
{
x++;
}
'VB equivalent
While x < 10
x = x + 1
Wend
What are the differences in these two styles of loops? Mostly, the do loop executes the contents of the loop at least one time. The While loop tests the expression and if the test fails the loop is never executed. Sound familiar? It should, it works the same as Visual Basic and logic dictates that it should.
Remember break? It gets you out of the loop early, just like Exit Do or Exit While. Continue can be used to go to the next iteration of the loop.