unwanted element popping up in int list - c

Question

I have written a C-program to generate random numbers and then sort them with the bubblesort-algorithm.
However, printing out the numbers (by an array, see the code) will show that the first element is 63, no matter what the randomly generated numbers were.
What causes this? And is there a more elegant way to bypass this than just skipping the first element of the list?
int list[10];
int cache_num;
srand((unsigned) (time(NULL))); //Gives the rand() function a new seed from the current time
for (int i = 0; i < 10; i++) {
list[i] = rand();
printf("\n%d", list[i]);
}
for (int i = 0; i < 10; i++) {
for (int j = 0; j < (10 - i); j++) {
if (list[j] > list[j + 1]) {
cache_num = list[j];
list[j] = list[j + 1];
list[j + 1] = cache_num;
}
}
}
for (int i = 0; i < 10; i++) {
printf("\n%d", list[i]);
}

Answer 1

In the inner loop of these loops
for (int i = 0; i < 10; i++) {
for (int j = 0; j < (10 - i); j++) {
if (list[j] > list[j + 1]) {
cache_num = list[j];
list[j] = list[j + 1];
list[j + 1] = cache_num;
}
}
}
there is an access to the non-existent element of the array with the index 10
list[j] = list[j + 1];
^^^^^^
when j is equal to 9.
It si better to write the loop like
for (int j = 1; j < (10 - i); j++) {
if (list[j - 1] > list[j]) {
cache_num = list[j];
list[j] = list[j - 1];
list[j - 1] = cache_num;
}

Answer 2

You are going one past the array when the variable j reaches the value of 9. Use (10 - i - 1) to resolve the issue:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void) {
int list[10];
int cache_num;
srand((unsigned) (time(NULL))); //Gives the rand() function a new seed from the current time
for (int i = 0; i < 10; i++) {
list[i] = rand();
printf("%d\n", list[i]);
}
for (int i = 0; i < 10; i++) {
for (int j = 0; j < (10 - i - 1); j++) { // note here ...
if (list[j] > list[j + 1]) {
cache_num = list[j];
list[j] = list[j + 1];
list[j + 1] = cache_num;
}
}
}
putchar('\n');
for (int i = 0; i < 10; i++) {
printf("%d\n", list[i]);
}
return 0;
}

Answer 3

Here in the following line you are going out of bounds because of which your are getting garbage value and thus altering your final result. Ex: when i=0 and j=9, this loops terminating condition will be is 9 < 10-0 which is true and it will execute. But for list [ j + 1 ] it will go out of bounds because there is no list [ 10 ].
for (int j = 0; j < (10 - i); j++){ ... }
It should have condition with -1 , (10-i-1)
for (int j = 0; j < (10 - i - 1); j++) { ... }

Answer 4

Instead of your implementation of the bubble sort to sort the data of 10 elements, you can use the below implementation.
The idea behind is that when you analyze (which you should) how the bubble-sort algorithm basically sorts the data, you will see a pattern i.e. in each pass of the inner-loop, the largest element (in-case of sorting the data in increasing order) will automatically go to its right location so, you don't need to consider that element again as it is already in its right position.
#include <stdio.h> // For basic I/O functions.
#include <stdlib.h> // For srand() & rand() functions.
#include <time.h> // For time(NULL) functions.
#define MAX_ARRAY_SIZE 10
int main(void) {
int list[10];
srand(time(NULL));
for(int i = 0; i < MAX_ARRAY_SIZE; ++i) {
list[i] = rand();
}
// Implementation of Bubble-Sort
for(int i = (MAX_ARRAY_SIZE - 1); i >=0; --i) {
bool is_sorted = true;
for(int j = 0; j < i; ++j) {
if(list[j] > list[j + 1]) {
int cache_num = list[j];
list[j] = list[j + 1];
list[j + 1] = cache_num;
is_sorted = false;
}
}
if(is_sorted) {
break;
}
}
for(int i = 0; i < MAX_ARRAY_SIZE; ++i) {
printf("%d ", list[i]);
}
printf("\n");
return 0;
}
P.S: As you are sorting only 10 elements, using bubble-sort is not going to make any difference, but I suggest you use insertion-sort instead of bubble-sort because insertion-sort is faster for small-size arrays.
Refer: Comparison Between Bubble-Sort/Selection-Sort/Insertion-Sort.
For Some Fun: Watch this bubble sort algorithm dance. 😁