在 C 数组中编程

Question

I'm reading Programming in C by Kochan, 3rd ed.

In the introduction to arrays (program 7.1) he gives an example:

#include <stdio.h>


int main(void)
{
    int values[10];
    int index;

    values[0] = 197;
    values[2] = -100;
    values[5] = 350;
    values[3] = values[0] + values[5];
    values[9] =
    values[5] / 10;
    --values[2];

    for ( index = 0; index < 10; ++index )
        printf ("values[%i] = %i\n", index, values[index]);


    return 0;
}

which should give output

values[0] = 197
values[1] = 0
values[2] = -101
values[3] = 547
values[4] = 0
values[5] = 350
values[6] = 0
values[7] = 0
values[8] = 0
values[9] = 35

And with my quadriple-checking that i'm using the exact same code it only works halfway and gives me some weird extra numbers a [1], [4]

values[0] = 197
values[1] = 3210052
values[2] = -101
values[3] = 547
values[4] = 17704192
values[5] = 350
values[6] = 0
values[7] = 0
values[8] = 0
values[9] = 35

I'm using cl arrays.c -o arrays to compile.

Answer 1

Local variables are not initialized. That means the entries in the array that you do not explicitly initialize will contain indeterminate values (values that will be seen as random).

Actually, using uninitialized variables (or uninitialized entries of an array) is undefined behavior .

Answer 2

they're not initialized, so what you see in consolle output is what there was in the same memory locations

you may wonder why the first example gives '0' for each uninitialized value and "wierd numbers" in your implementation....maybe the first example was allocated into a memory area never used before, or maybe some compiler decides to set at 0x0000000 every memory register referred but not initialized

Answer 3

If you look at the cod provided in the book it never sets the value of array entries 1 or 4. They are therefore undefined and could be any value. Some compilers may go ahead and initialize them to zero but this is not part of the C specification and should never be relied upon.

Answer 4

I read the text that explain this program in the book which clearly says that (pg no. 100):

Because you never assigned values to five of the elements in the array—elements 1, 4, and 6 through 8—the values that are displayed for them are meaningless. Even though the program's output shows these values as zero, the value of any uninitialized variable or array element is not defined. For this reason, no assumption should ever be made as to the value of an uninitialized variable or array element.

And this is what standard says that the uninitialized automatic local variable invokes undefined behavior.