for loop compile error

[EBaller55]

I am using Visual C++ 2005 with SP1

why does the following code generate a compiler error (error C2062: type 'int' unexpected)?

for(int i=0,int j=0;i<5;i++,j++){}

Note the following two variations generate no compile error

int i,j;
for(i=0,j=0;i<5;i++,j++){}

int j;
for(int i=0,j=0;i<5;i++,j++){}

Is this just a bug in the Visual C++ compiler or is this the way the standard is defined?

Thanks.

[Ancient Dragon]

You can only specify one type of loop counter.

>>Is this just a bug in the Visual C++ compiler
Not a bug. That type a statement is never allowed

int a, int b; // error
int a,b; // ok

[Denniz]

It should be:

for(int i=0, j=0;i<5;i++,j++){}

[EBaller55]

OK thanks. All these years of programming and I never knew i++,j++; was a valid C++ line of code. I would always just write two lines of code. I thought it was just a special case in the header of a for loop and assumed the int i=0, int j=0 would work the same way. Interestingly, a google search for "for(int i=0,int j=0" will give some hits, one of which is from books.google.com Do some compilers support this? It would be nice because then I could do the following:

for(int i=0, double d=1;i<5;i++,d*=1.5){cout<<"1.5^"<<i<<"="<<d<<endl;}

This would make for cleaner code as it is clear that i and d are used only inside the for loop.

[ArkM]

The for statement syntax:

for (for-init-statement condition(opt); expression)
for-init-statement::
    expression-statement
    simple-declaration
expression-statement::
    expression(opt) ;

Informally simple-declaration is well-known type names-with-possible-initializators construct. The semicolon token terminates construct in C++.

Obviously, int i = 0, int j = 0; and especially int i = 0, double d = 1; are not valid C++ declarations.

About i++, j++ - that's another matter. It's an expression with comma operator (evaluate left part and discard its result then evaluate right part - it's an expression result).

[dmanw100]

Using nested loops would be one solution

[WaltP]

It would be nice because then I could do the following:

for(int i=0, double d=1;i<5;i++,d*=1.5){cout<<"1.5^"<<i<<"="<<d<<endl;}

This would make for cleaner code as it is clear that i and d are used only inside the for loop.

I disagree. That to me is confusing and unclean.

On the other hand:

for (int i = 0; i < 5; i++)
{
    double d = 1;
    cout << "1.5^" << i << "=" << d << endl;
    d *= 1.5;
}

This is much easier to read because
1) i is the only variable important to the for statement itself
2) d is defined to be used only in the loop
3) formatting and whitespace makes the code much easier to read

[ArkM]

Regrettably, it does not work (d is local auto var in the loop body block):

for (int i = 0; i < 5; i++)
{
    double d = 1; // new incarnations for 0, 1, 2, 3, 4...
    cout << "1.5^" << i << "=" << d << endl;
    d *= 1.5;
}

Right solution:

double d = 1.0;
for (int i = 0; i < 5; ++i, d*=1.5)
{
    cout << "1.5^" << i << "=" << d << endl;
}