In C/C++ Why Does the Do While(Expression); Need a Semi Colon

In C/C++ why does the do while(expression); need a semi colon?

Because you're ending the statement. A statement ends either with a block (delimited by curly braces), or with a semicolon. "do this while this" is a single statement, and can't end with a block (because it ends with the "while"), so it needs a semicolon just like any other statement.

Why do { } while(condition); needs semicolon at the end of it but while(condition) {} doesn't?

You put semicolon after all statements, except the block statement. This is the reason that you place it after the while in do while, but not after the block in the while {...}.

You also use it to terminate almost all declarations. The only exceptions I can think about at the moment is function bodies, and namespace bodies in C++.

why there is semicolon after loop while();

The ; is just a null statement, it is a no op but it it the body of the while loop. From the draft C99 standard section 6.8.3 Expression and null statements:

A null statement (consisting of just a semicolon) performs no operations.

and a while statement is defined as follows from section 6.8.5 Iteration statements:

while ( expression ) statement

So in this case the statement of the while loop is ;.

The main effect of the while loop is here:

string1[i++] == string2[j++]
        ^^^             ^^^

So each iteration of the loop increments i and j until the whole condition:

string1[i++] == string2[j++] &&string1[i-1] != 0 && string2[j-1] != 0

evaluates to false.

When is a semicolon after } mandated in c/c++?

int a[2] = {1,2}, j = 5;

When initialization of array or structure are done with {} all the subsequent variables are declared after ,.

Edit: As you changed your question; ; is mandatory after the class, enum, initialization syntax declarations.

class A {};  // same for `struct
enum E {};   // enum class (c++0x)
int a[] = {1,2};  // array or object initialization

And as per comment by @rubenvb:

do {} while(condition);

Effect of using a comma instead of a semi-colon in C and C++

It doesn't make a difference in the code you posted. In general, the comma separates expressions just like a semicolon, however, if you take the whole as an expression, then the comma operator means that the expression evaluates to the last argument.

Here's an example:

b = (3, 5);

Will evaluate 3, then 5 and assign the latter to b. So b = 5. Note that the brackets are important here:

b = 3, 5;

Will evaluate b = 3, then 5 and the result of the whole expression is 5, nevertheless b == 3.

The comma operator is especially helpful in for-loops when your iterator code is not a simple i++, but you need to do multiple commands. In that case a semicolon doesn't work well with the for-loop syntax.

Semi-Colon placed before opening bracket in a while loop

You can rewrite your code using a better formatting, which probably would make the problem clearer to you:

while(condition)
  ;

{ 
    statement;
}

If condition is true (i.e. non-0), you iterate an infinite loop.

If condition is false (i.e. 0), execution moves forward to the following statement.

why does semicolon in c++ does not show any error when inserted at the begining of code?

You don't get an error because it isn't one.
You are terminating an empty statement. That's legal in C++ (and has been in C).

So you're example consists of 2 statements (not one):

<empty> ;
string a = "5.558" ;

This can actually be useful. For instance, the for loop needs 3 expressions (init, condidtion and increment/step). Sometimes you don't want to pass all 3, so this is a legal code as well:

for(;;) {  /* do something in this endless loop */ }

The 2 semicolons inside for are therefore separators for 3 (empty) expressions

---

Related Topics