如何访问C中数组内部的指针的索引？

Question

If I have the following code:

char test[3] = {11,22,33};
char *ptr, *ptr2;
char *array[2] = {ptr,ptr2};

How do I access the number 22 through the array?

I've tried the following with no success:

array[0][1]

But if I access the pointer variable through the the write() function, like:

write(file, array[0], 3)

It will write 112233 to the file no problem. I just want to access the 1 index though.

Answer 1

in

  char *ptr, *ptr2; char *array[2] = {ptr,ptr2}; 

you missed to initialize ptr and ptr2

note also ptr and ptr2 are not constant initializer element

do

char test[3] = {11,22,33};
char *ptr = test, *ptr2 = NULL; /* ptr2 initialized even though not important for array[0][1] */
char *array[2];

array[0] = ptr;
array[1] = ptr2;

and array[0][1] will be 22

---

I encourage you to compile with options to produce warning/error, and of course to take them into account to finally have no warning/error indicated by the compiler.

If I compile your code with options I get :

pi@raspberrypi:~ $ cat a.c
int main()
{
  char test[3] = {11,22,33};
  char *ptr, *ptr2;
  char *array[2] = {ptr,ptr2};

  return array[0][1];
}
pi@raspberrypi:~ $ gcc -pedantic -Wextra a.c
a.c: In function ‘main’:
a.c:5:9: warning: ‘ptr’ is used uninitialized in this function [-Wuninitialized]
   char *array[2] = {ptr,ptr2};
         ^~~~~
a.c:5:9: warning: ‘ptr2’ is used uninitialized in this function [-Wuninitialized]

But :

pi@raspberrypi:~ $ cat aa.c
#include <stdio.h>

int main()
{
  char test[3] = {11,22,33};
  char *ptr = test, *ptr2 = NULL; /* ptr2 initialized even not important for array[0][1] */
  char *array[2];

  array[0] = ptr;
  array[1] = ptr2;

  printf("%u\n", (unsigned char) array[0][1]);
}
pi@raspberrypi:~ $ gcc -pedantic -Wextra aa.c
pi@raspberrypi:~ $ ./a.out
22
pi@raspberrypi:~ $