Thread: Pointers

Originally Posted by jazzglenn421

I'm just new to the C World. I just only know a few and coded some successfully in school. I'm better of reading a code than making one.

That probably applies to nearly everyone. Just like I can read some legal documents in English (or Swedish which is my native language), but I could not write Legal English.

Now, in school we have tackled about "Pointers" and its uses. My teachers said that in higher years pointers will be very useful, but most of the times i dont think that pointers are not important. Please show me the light

Well, that would be a fairly large undertaking. Most books that deal with pointers would do so in about 15 pages or so.

Pointers are important to some things, and less important in other types of code. The first instance of pointers is probably when you need to pass a single variable (say an int) to a function to be filled in, e.g.

Code:

#include <stdio.h>
void getCoord(int *px, int *py)
{
   *px = 47;
   *py = 11;
}

int main()
{
   int x = 0, y = 0;
   getCoord(&x, &y);
   printf("x = %d, y = %d\n", x, y);
   return 0;
}

If you remove the * signs in the getCoord and the & from the calling code, you will not get x and y to change.

But pointers are used for many, many other things.

--
Mats

If you need to pass a large array of data into a function, you simply pass a pointer to the array. Without pointers you'd have to pass a whole new copy of the array into the function, which is obviously far less efficient and wasteful of memory.

Originally Posted by jazzglenn421

I was just wondering why cant firstvalue be like "firstvalue=10"? instead of "*p1=10". and like "secondvalue=firstvalue" and "firstvalue=20"?

It can, but presumably the purpose of the exercise is to practice working with pointer syntax and the address of operator.

Originally Posted by jazzglenn421

just wondering thought.... through that thought i think that pointers are quite useless. maybe someone could show a program that pointers are really really important.

Write a program that reads in an arbitrary number of integers from standard input and prints them to standard output in sorted order. You may freely use the C standard library (but you may not "pass the buck" to an external program/script).

Pointers are highly useful in many ways:

1)
Assume you have a large amount of data and you want a function to analyze it, e.g. compute its size. The naive approach is to simply pass all data as a function argument, which makes a local copy of the data. For an int, this is ok. For anything larger, it's probably not. So you don't pass the data itself, but merely a pointer to the data, i.e. you tell the function "go there and have a look".

2)
Assume you have a function that needs more than one return value, e.g. one for the actual result and one for the error condition (think of converting a string to an int; what do you return if the string doesn't contain the textual representation of a number?). In cases like this, the actual result is often stored at a memory location which is passed with a pointer, so the return value can be used to tell the caller whether it worked or not.

3)
Pointers allow for data structures which are more complex than arrays, e.g. linked lists, trees, graphs. This is useful because if you have a large set of items, your memory probably can't hold it contiguously. With pointers, you can store each item somewhere in memory and use a pointer to tell where the "next" item can be found. The items are distributed all over your physical memory, but the pointers glue them together logically.


There are many more examples, but these three have analogies in the real world. I'd recommend reading about pointers without trying to understand everything at first glance. Once you're used to using pointers, go back to the pointer chapter and find out why it's working the way you're used to doing it.

Greets,
Philip

Code:

#include<stdio.h>
#define p printf

trying to avoid typing four letters twice by typing #define p printf seems a lot of work to me, and it's naturally obfuscating your code.

And #include <stdio.h> should be before main() definition.

The example shown is definitely an example of "no need for pointers" here. But to make sensible and yet simple examples can be quite difficult. This might be one:

Code:

#include <stdio.h>
int main()
{
    int array[5];
    int *pMax, *pMin;
    int i;
    int sum;
    printf("Enter 5 numbers with space between them:\n");

    for(i = 0; i < 5; i++)
    {
      scanf("%d", &arr[i]);
    }
    // Now, find the max and min values:
    pMax = &array[0];
    pMin = &array[0];
    for(i = 0; i < 5; i++)
    {
       if (*pMax > array[i])
           pMax = &array[i];
       if (*pMin < array[i])
           pMin = &array[i];
       sum += array[i];
    }
    sum -= *pMax + *pMin;
    printf("Average of 5 items, less biggest and smallest: %5.2f\n", 
              sum / 3.0);
    printf("Max = %d, min = %d\n", *pMax, *pMin);
    return 0;
}

--
Mats