code teaser

[Gurm]

Originally posted by cjolley
The word may be "for", but c "for" loops are realy "while" constructs.

Ok, they DO accomplish much the same thing. The for loop is more compact, though.

In fact, unless I'm not remembering correctly*,
for(i=x; i<j;--i)
would decrement i even if j<i , at least on old compilers*Like that never happens.

You're remembering incorrectly.

- Gurm

[Duty]

Originally posted by cjolley
The word may be "for", but c "for" loops are realy "while" constructs.

Basically for and while loops are the same logic construct. One is logically aimed at a known iterations while the other is logically aimed at an unknown number of iterations. But as a construct they are basically the same.

Sorry if I have some syntax errors I haven't touched c in 10 years.

Example 1: Know iterations...

int i, n = -20;
for (i = 0; i + n; i--)
printf("-");

or

int i = 0 , n = -20;
while( i + n)
{
printf("-");
i--;
}

Example 2: unknown iterations

Assumption that there is an int arry of [n] positions.


int x = 0;
while (intNumbers[x]!=null)
{
printf(intNumbers[x]);
x++;
}

int x;
for (x = 0 ; intNumbers[x]!=null ; x++)
{
printf(intNumbers[x]);
}


Example 3: unknows interations (ie listining to a port)...

while (true)
{
...code...
}

or

for ( ; true ; )
{
...code...
}

Switching between for and while loops was one of my standard lectures.


Jeff

[Duty]

Originally posted by cjolley
In fact, unless I'm not remembering correctly*,
for(i=x; i<j;--i)
would decrement i even if j<i , at least on old compilers

++x = Pre-increment - this happens before the variable is interpreted (or used)

x++ = Post-increment - this happens after the variable is interpreted (or used)

So for(x=0 ; x < y ; x++) is the same as for(x=0 ; x < y ; ++x) because the variable is only incremented. Pre and post increment only become important when the variable is going to be interpreted and incremented (ie. int z=someFunction(x++) or int z=someFunction(++x)).

I think this has been the standard since the beginning of ANSI C.

Remember in C you can ++x--

Have fun.

Jeff

[cjolley]

Originally posted by Gurm
Ok, they DO accomplish much the same thing. The for loop is more compact, though.
- Gurm

Well, I would say the difference between a "real" for loop and a c for loop is this.

In PL/SQL, the Oracle programming language based on ada, a for loop is more like this:

SQL> @connect chajol@tax_test
Enter password:
Connected.
[email]chajol@tax.expe[/email]rior> --a real for loop
[email]chajol@tax.expe[/email]rior> declare
  2     i number := -1;
  3  begin
  4     for i in 1..3 loop
  5        dbms_output.put_line('i='||to_char(i));
  6     end loop;
  7     dbms_output.put_line('i='||to_char(i));
  8  end;
  9  /
i=1
i=2
i=3
i=-1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.16
[email]chajol@tax.expe[/email]rior>

i still = -1 because the i in the loop is in it's own name space and has nothing to do with the declared i

This property makes it realy different from while.

c's for is just a convenient construct for incriment driven whiles.

chuck

[Duty]

Originally posted by cjolley
Well, I would say the difference between a "real" for loop and a c for loop is this.

In PL/SQL, the Oracle programming language based on ada, a for loop is more like this:

SQL> @connect chajol@tax_test
Enter password:
Connected.
[email]chajol@tax.expe[/email]rior> --a real for loop
[email]chajol@tax.expe[/email]rior> declare
  2     i number := -1;
  3  begin
  4     for i in 1..3 loop
  5        dbms_output.put_line('i='||to_char(i));
  6     end loop;
  7     dbms_output.put_line('i='||to_char(i));
  8  end;
  9  /
i=1
i=2
i=3
i=-1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.16
[email]chajol@tax.expe[/email]rior>

i still = -1 because the i in the loop is in it's own name space and has nothing to do with the declared i

This property makes it realy different from while.

c's for is just a convenient construct for incriment driven whiles.

chuck

chuck,

In Gurms defense, you’re wrong (sorry). You’re comparing syntax and variable scope, not logic. PL/SQL is not the right way, it is just a “BASIC syntax” derived way of building a FOR loop. The ADA syntax is a “BASIC derivative” just like VB, and COBOL. Where as C, C++, Smalltalk and Java are all derived from FORTRAN syntax. As to the variable, C has stronger variable naming constraints then PL/SQL, thats all.

Have fun,
Jeff

[cjolley]

Originally posted by Duty
chuck,

In Gurms defense, you’re wrong (sorry). You’re comparing syntax and variable scope, not logic. PL/SQL is not the right way, it is just a “BASIC syntax” derived way of building a FOR loop. The ADA syntax is a “BASIC derivative” just like VB, and COBOL. Where as C, C++, Smalltalk and Java are all derived from FORTRAN syntax. As to the variable, C has stronger variable naming constraints then PL/SQL, thats all.

Have fun,
Jeff

Apparently I didn't make myself clear.
I don't disagree with GURM.
ALL for and while constructs are logicaly interchangeable.
And I am not saying the PL/SQL for loop is "right".
I think the code I gave should produce a duplicate declaration compile time error. That would save a lot of newbie heartach in the Oracle world.
What I'm talking about is that the for in c has so few constraints on it that I would call it just a convenient syntax for while to a much larger extent than most other languages.
I like the way the c for loop works and think it is great for clarifying while logic when the loop is controlled by a counter.
But it is deadly if placed in the wrong hands. (or, I suppose, the right hands if you are a fan of obfuscated c like I am )
The fact that you can do things like:
for(printf("hello world/n"); random(1, 10000) > 9000; printf(""))
and/or things like that means that the counter aspect that makes for different from while is simply not enforced at all in c.

chuck

PS, I am shocked to find that the closest thing I have at home now to an ansi c compiler is VS6
My quick and turbo c disks are gone!?
So much for including cute examples....

[Duty]

Originally posted by cjolley
(or, I suppose, the right hands if you are a fan of obfuscated c like I am )
The fact that you can do things like:
for(printf("hello world/n"); random(1, 10000) > 9000; printf(""))
and/or things like that means that the counter aspect that makes for different from while is simply not enforced at all in c.

Yeah, C really expects that you know what you are doing. You can do some serious wacked code with pointers.


Jeff