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for loop compile error

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EBaller55

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for loop compile error

Oct 3rd, 2008

I am using Visual C++ 2005 with SP1

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

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 C++ Syntax (Toggle Plain Text) 
for(int i=0,int j=0;i<5;i++,j++){}
for(int i=0,int j=0;i<5;i++,j++){}

Note the following two variations generate no compile error

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int i,j;
for(i=0,j=0;i<5;i++,j++){}
int i,j;
for(i=0,j=0;i<5;i++,j++){}

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 C++ Syntax (Toggle Plain Text) 
int j;
for(int 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.

Last edited by EBaller55; Oct 3rd, 2008 at 12:49 am.

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Re: for loop compile error

Oct 3rd, 2008

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

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

Last edited by Ancient Dragon; Oct 3rd, 2008 at 1:03 am.

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Re: for loop compile error

Oct 3rd, 2008

It should be:

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 C++ Syntax (Toggle Plain Text) 
for(int i=0, j=0;i<5;i++,j++){}
for(int i=0, j=0;i<5;i++,j++){}

Last edited by Denniz; Oct 3rd, 2008 at 2:49 am. Reason: Typo

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Join Date: Oct 2008

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EBaller55

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Re: for loop compile error

Oct 3rd, 2008

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:

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

Last edited by EBaller55; Oct 3rd, 2008 at 1:36 pm.

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Re: for loop compile error

Oct 3rd, 2008

The for statement syntax:

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

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Re: for loop compile error

Oct 3rd, 2008

Using nested loops would be one solution

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Re: for loop compile error

Oct 4th, 2008

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Originally Posted by EBaller55

It would be nice because then I could do the following:
Help with Code Tags

C++ Syntax (Toggle Plain Text)
for(int i=0, double d=1;i<5;i++,d*=1.5){cout<<"1.5^"<<i<<"="<<d<<endl;}
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:

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

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Re: for loop compile error

Oct 4th, 2008

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

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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;
}
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:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
double d = 1.0;
for (int i = 0; i < 5; ++i, d*=1.5)
{
    cout << "1.5^" << i << "=" << d << endl;
}
double d = 1.0;
for (int i = 0; i < 5; ++i, d*=1.5)
{
    cout << "1.5^" << i << "=" << d << endl;
}