Bubble sort replacing a value with another one that I haven't even input

Question

#include<stdio.h>
//input function taking inputs from the user.
void input(int *p)
{
    int i;
    printf("Enter 5 numbers : ");
    for(i=0; i<=4; i++)
    {
        scanf("%d", p+i);
    }
}

//display function used to display values of the array.
void display(int *p)
{
    int i;
    for(i=0; i<=4; i++)
    {
        printf("%d\t", *(p+i));
    }
    printf("\n");
}
//Bubble Sort
void sort(int *p)
{
    int rounds, temp, i;
    for(rounds=1; rounds<=4; rounds++)
    {
        for(i=0; i<=4; i++)
        {
            if(*(p+i)>*(p+i+1))
            {
                temp=*(p+i);
                *(p+i)=*(p+i+1);
                *(p+i+1)=temp;
            }
        }
    }

}

//Main Function calling other functions of the program
int main()
{
    int a[5];
    input(a);
    display(a);
    sort(a);
    display(a);
    getch();
    return 0;
}

OUTPUT:

Enter 5 numbers : 34
53
97
108
347
34      53      97      108     347
34      53      77      97      108

the value '347' is automatically getting replaced by '77'. When I input values >70, compiler replaces one of the values with 77 and displays it after sorting. And if I input values <70 then the program works fine. Can someone explain this please? And how to fix this?

Answer 1

This code block

        if(*(p+i)>*(p+i+1))
        {
            temp=*(p+i);
            *(p+i)=*(p+i+1);
            *(p+i+1)=temp;
        }

results in undefined behavior when i is equal to 4 because the expression *(p+i+1) accesses memory beyond the array.

Pay attention to that it is a bad design when functions depend on the magic number 4.

For example the function sort could be declared like

void sort( int *p, size_t n );

where the parameter n specifies the number of elements in the pointed array.