Thread: Having trouble understanding 'for' loop parameters

i am working through a fundamental C book and i thought i understood for loops but this has me completely stumped. maybe someone here can explain the difference of the below snippets of code.

Code:

int c = 0;
int i = 0;

for (c = getchar(); c != EOF;)
{
if (c == ' ')
    i++;
}
printf("%d", i);

and

Code:

int c;
int i = 0;

for ( c = 0; (c = getchar()) != EOF;)
{
if(c == ' ')
    i++;
}
printf("%d", i);

they both compile and execute but the first one will not exit when i press ctrl + d and give the print out. the second one will work just fine giving the print out i want.

the only difference i can see is in the for loops but they are the same, right? obviously not but i can't see how. can someone please explain the difference here?

Code:

for( initialization [optional] ;   conditional [optional] evaluate to TRUE if no cond ; increment/decrement [optiona] )   semicolon are not optional!

for( i = 0; i < 10; i++)
statement_list;
In this loop, i is initialized to zero the first time. if the condition i < 10 is true, statement_list is executed, then i++ is executed at the end. Then go on to checking condition, and so on.
i = 0;
for(; i < 10;i++) // opt init eg

for( ; ; ) // infinite loop

A for loop has three fields, an init field done once, a condition, and a update field.

Code:

for( first initial state; condition; update )

In your first example you set the initial value of c to what ever getchar() returns, then you run the loop until c is EOF, which it will never be unless it's the first thing that getchar() gets in the init of c.

In your second example you init c to 0, then you continuously get new values for c in each iteration until c is EOF.

The first parameter is only executed ONCE...when the loop is first entered.

The condition is now:

Code:

while(c)

Originally Posted by zacs7

No, it's still...

Code:

while(c = getchar())

They are by no means equivalent.

Sure, but what is evaluated is c. How c get's it's value is irrelevant to the evaluation, in this case it's by getchar().

Originally Posted by zacs7

Maybe so, but the time c gets it value is relevant.

What do you mean? Shall the loop continue or not? If c.

Code:

for(;c = getchar(); c != EOF)

The c!=EOF does nothing in that position. It would just be eexecuted as:

c!=EOF;

at the end of each iteration. This would evaluate to 1 or 0 depending on if it is true or not, but it would have no effect whatsoever on the loop.

If you are trying to pecify the stopping condition for a loop, it has to go in the second of the three slots.