Array printing out -858993460

Question

I've been trying to work out why my array is printing out -858993460 with the following code

int myInt = 0;
int myArray[10]; // I know this is undefined, but the same results occur if I put {} to initialize it

while(myInt <= 10)
{
    myArray[myInt] = myInt;
    myInt++;
    std::cout << "Test " << myArray[myInt] << std::endl;
    std::cout << "Counter " << myInt << std::endl;
}

When print this out, myInt increments fine. However, myArray is printing out -858993460. If I insert a break point and step over the while loop with each iteration, I can see the numbers are being fed into the array, but it only prints out that random number (assuming it's a random number from the stack/heap?).

Now, if I swap around the while loop so it's now

while(myInt <= 10)
{
    myInt++;
    myArray[myInt] = myInt;
}

it prints out the numbers correctly. I can't seem to work out what's going on here...

`-858993460` on msvc means uninitialized stack memory. In hex its `0xCCCCCCCC`. Related: [https://stackoverflow.com/questions/127386/what-are-the-debug-memory-fill-patterns-in-visual-studio-c-and-windows](https://stackoverflow.com/questions/127386/what-are-the-debug-memory-fill-patterns-in-visual-studio-c-and-windows) — drescherjm, Jun 01 '22 at 19:49
valid indices for an array of 10 is 0-9, use < 10 *not* <= 10 — Borgleader, Jun 01 '22 at 19:50
When you get a bizzarro number, convert it to hex (-858993460 -> CCCCCCCC) and see what it looks like. If it's a word or something highly repetitious, it's probably the program trying to tell you something. — user4581301, Jun 01 '22 at 19:50
In general when you see a `<=` in the exit condition of a loop iterating any sort of container, you're probably looking at a bug and need to inspect the range of the index variable more closely. Even if it is correct, consider rewriting the condition or commenting heavily otherwise for the rest of the code's existence people are going to waste time double-checking the loop. — user4581301, Jun 01 '22 at 21:24

Spencer · Accepted Answer · 2022-06-01T20:32:19.810

Arrays in C (and by extension C++) are zero based, so an array of ten elements is indexed 0 through 9.

But your loop is trying to access elements 0 through 10. myArray[10] is actually the eleventh element of the array, which doesn't actually exist. Trying to change it is undefined behavior. Your original code was actually closer to being correct; your revised version only made it worse. What you want is:

int myInt = 0;
int myArray[10]; 
while(myInt < 10)  // <- strictly LESS THAN
{
   myArray[myInt] = myInt;
   ++myInt; // <- increment AFTER accessing/assigning
}

In comments, you added an additional requirement that myArray[0] should contain 1 and so forth. To get this, you need to access the element before modifying the index. You could do this with two variables:

int myInt = 0;
int myArray[10]; 
while(myInt < 10)  
{
   myArray[myInt] = myInt+1;
   ++myInt; 
}

You can't combine the two, unfortunately:

int myInt = 0;
int myArray[10]; 
while(myInt < 10) 
{
   myArray[myInt] = ++myInt;  // <- NOPE: wrong order
}

In the revised code where you try to print out the values, you tried to access myArray[MyInt] after you changed MyInt. So you're now looking at a different element than the one you just set. No surprise it didn't print out what you want! Just move the increment to the end:

while(myInt <= 10)
{
    myArray[myInt] = myInt+1;
    std::cout << "myArray [" << myInt << "] = " << myArray[myInt] << '\n';
    ++myInt;  <- ALWAYS INCREMENT THE LOOP INDEX LAST
}

score 0 · Answer 2 · answered Jun 01 '22 at 20:52

You print myArray[myInt] after you have incremented myInt. That array element hasn't been initialized yet.

More over at the end myInt is 10 and your myArray only goes from 0 to 9 leading to a buffer overflow.

And last while(myInt <= 10) goes even one step further, initializing myArray[10] and printing myArray[11], both outside the array.

Array printing out -858993460

2 Answers2