Unexplained last item in array

Question

I have the following array:

int distances[10] = {10, 11, 14, 12, 11, 14, 9, 7, 10, 10};

When I ran this array through this for loop:

for (int j = 0; j <= 20; j++) { std::cout << distances[j] << " "; }

and printed all of the outputs, this was my output:

10, 11, 14, 12, 11, 14, 9, 7, 10, 10, 4196294, 0, -487465136, 30021, 0, 0, 0, 0, -359687355, 26470, 0

Why are the last 11 elements so varied in size and not in the rest of the array?

Hello and welcome to StackOverflow. Please take some time to read the [help page](https://stackoverflow.com/help), especially the sections named [How to Ask](https://stackoverflow.com/help/how-to-ask). You might also want to learn about [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example). You didn't include the loop you are actually using and that is probably the problem in your code. — Tarick Welling, Aug 21 '19 at 10:11
Your array only contains 10 elements (indexed 0 to 9) but your loop iterates 20 times (0 to 19). Iterating past the end of an array means you are reading undefined memory locations, that's why you get the random numbers. — Tony, Aug 21 '19 at 10:57
Read this: https://stackoverflow.com/questions/1239938/accessing-an-array-out-of-bounds-gives-no-error-why — chtz, Aug 21 '19 at 10:57

score 0 · Accepted Answer · answered Aug 22 '19 at 11:17

Your array contains 10 elements (indexed 0 to 9)

int distances[10] = {10, 11, 14, 12, 11, 14, 9, 7, 10, 10};
                     ^   ^   ^   ^   ^   ^   ^  ^  ^   ^
                     0   1   2   3   4   5   6  7  8   9

but your loop iterates 20 times (0 to 19)

int distances[10] = {10, 11, 14, 12, 11, 14, 9, 7, 10, 10}; ?      ?
                     ^   ^   ^   ^   ^   ^   ^  ^  ^   ^    ^      ^
                     0   1   2   3   4   5   6  7  8   9    10 ... 19

Iterating past the end of an array means you are reading undefined memory locations, which is why you get the random numbers - it's data being interpreted as integers.

You can calculate the number elements in the array using

sizeof(distances)/sizeof(distances[0])

but if you are using C++11 you should be able to use

for(int distance : distances)

Search for related questions here on stackoverflow, such as the Correct way of loop through the C++ arrays and Accessing an array out of bounds gives no error, why?

Unexplained last item in array

1 Answers1