pointer math

[automagp68]

I have been stuck on a simple problem for 10 hours, can someone please help


all im doing is the following.

I have a function that is being passed a pointer.
The pointer points to an int array.

I wrote a function to find the largest value. For some reason it is failing the condition after it runs 6 times. The array is 10 long therefore it should run 10 times. Can someone please help. I have to write it this was, so yes im sure there are easier way but i need to write it the way i have it. For some other odd reason if i change 10 to 12 it works. but the array is not 12 long its 10 long.

//*a points to &myarray[0] inside of main. Then a is passed to the //function 
//&a+10 should of failed when it reaches address 9 of the array

for(;a<&a+10;a++){

printf("This is the address of a right now [%p]\n",a);//just for          

//test,prints the address im currently at.

count++;//count just keeps track of how man times the loop goes off

if (*a>*largest){

*largest=*a;
}
}


a=&a-10;

for(;a<&a+10;a++){

if(*a<*largest&&*second_largest<*a){

*second_largest=*a;


}


}

Im sorry its so messy i tried to clean it up but the forum editor will not coperate

Thanks for the help

[MK27]

Why don't you just use for (int i = 0; i < 10; i++) a++, etc? Maybe you are wasting your time trying to be clever ? It's not going to accomplish anything.

Here, it would seem you are comparing a (pointer) value to the address the pointer is stored at. That does not make much sense.

a<&a+10

When/if this condition fails depends where, in memory, a is in relation to to what it points to. Probably not a good thing to rely on, but since you did not post all the code (where a is declared, where the array is declared, etc) it's hard to say. In any case, this is not at all what you want to do.

[Subsonics]

//*a points to &myarray[0] inside of main. Then a is passed to the //function 
//&a+10 should of failed when it reaches address 9 of the array

for(;a<&a+10;a++)

There's a couple of problems with this, if *a = &myarray[0] then &a is the same as **a.

Secondly, since you increment a you can not compare with a. Let's say that a is initially 1 you then test if a is less than 1 + 10, second iteration a i 2 and you test if a is less than 2 + 10 and so on..

You always push the condition ahead 10 steps, you need a copy of a and increment that.

for(p = a; p < a + 10; p++)

[automagp68]

There's a couple of problems with this, if *a = &myarray[0] then &a is the same as **a.

Secondly, since you increment a you can not compare with a. Let's say that a is initially 1 you then test if a is less than 1 + 10, second iteration a i 2 and you test if a is less than 2 + 10 and so on..

You always push the condition ahead 10 steps, you need a copy of a and increment that.

for(p = a; p < a + 10; p++)

Hi i tried your example exactly like that yesterday and it crashes.

Just for kicks i just tried it again. still crashes.

@mk27
Thanks for the idea. I know how to use subscripts very well. As my original problem said, it has to be written this way.

[Subsonics]

Hi i tried your example exactly like that yesterday and it crashes.

Just for kicks i just tried it again. still crashes.

Then you obviously have other issues, that approach works.

[automagp68]

no

you are not understanding my issue correctly.

a is a pointer to the begining of an array

of you create a pointer inside of the function and point them to each toher then a never gets incremeted. so how does that work?

[Subsonics]

no

you are not understanding my issue correctly.

a is a pointer to the begining of an array

of you create a pointer inside of the function and point them to each toher then a never gets incremeted. so how does that work?

Sure I do, a is initialized to p. Example:

void foo(char *s, size_t size) {
    char *p;
    for(p = s; p < s + size; p++) {
        printf("%c", *p);
    }
}

But if you must increment a, here is an alternative.

void foo2(char *s, size_t size) {
    char *p = s + size;

    for(; s < p; s++) {
        printf("%c", *s);
    }
}

[automagp68]

sigh! ugh, thanks

Somedays i feel like i wil never be a good programmer. i need a martini and its not even noon

check this out. I changed it to this and now the top part works but not the smallest

for(ptr=a;ptr<a+10;ptr++){


printf("This is the address of a right now [%p]\n",a);

count++;//count just keeps track of how man times the loop goes off

if (*ptr>*largest){

*largest=*ptr;
}
}




for(ptr2=a;ptr2<a+10;ptr++){

if(*ptr2<*largest&&*second_largest<*ptr2){

*second_largest=*ptr2;


}


}

[automagp68]

I just found the mistake!

Thank you!

it works now!

well i guess i should of posted this 10 hours ago instead of pulling my hair out
Thanks man have a good day