Why does an error occur when I modulus the value in array?

Question

may i know what exactly have gone wrong in this code? cause the output of the even number is not what i expected if i input the value in the comment

//int number[10]={0, 2, 5, 8, -2, 0, 6, 4, 3, 1};
int number[10], divided_number[10], total_odd_numbers, total_even_numbers, a;
for(int i=0;i<=9;i++){
    a=i+1;
    cout<<"Number "<<a<<": ";
    cin>>number[i];
    
}
for(int i =0; i<=9; i++){
    cout<<*(number+i)<<"  ";
}
cout<<endl;
for(int i=0;i<=9; i++){
    divided_number[i]=number[i]%2;
    if(divided_number[i]!=0){
        total_odd_numbers +=1;
    }
    else if (divided_number[i]==0){
        total_even_numbers++;
    }
}
for(int i=0;i<=9; i++){
    cout<<*(divided_number+i)<<"  ";
}
cout<<"\n Total odd number: "<<total_odd_numbers<<endl;
cout<<"\n Total even number: "<<total_even_numbers<<endl;

This is a small amount of code that you could debug yourself. When you say it's not what you expect, _what_ output do you expect and what output are you actually getting? Note that you also have a negative number and you're taking the modulo. By the rules of that operator, the result can be negative. Something to keep in mind. — paddy, Nov 19 '21 at 04:50

score 1 · Accepted Answer · edited Nov 19 '21 at 07:45

The problem is that total_odd_numbers and total_even_numbers are not initialized before being accessed. This is called undefined behavior. In this case, the program will take whichever trash data is already in the memory assigned to those variables and just use it as-is (and so, of course, the output may or may not be correct, depends on what's in that memory at the time of assignment).

It should be noted that :

Using an unitialized value is, by itself, not undefined behavior, but the value is simply indeterminate. Accessing this then is UB if the value happens to be a trap representation for the type. Unsigned types rarely have trap representations, so you would be relatively safe on that side.

Modified program:

#include <iostream>
using namespace std;

int main()
{
    int number[10]={0, 2, 5, 8, -2, 0, 6, 4, 3, 1};
    int divided_number[10], total_odd_numbers = 0, total_even_numbers = 0, a;

    for(int i =0; i<=9; i++){
        cout<<*(number+i)<<"  ";
    }
    cout<<endl;
    for(int i=0;i<=9; i++){
        divided_number[i]=number[i]%2;
        if(divided_number[i]!=0){
            total_odd_numbers +=1;
        }
        else if (divided_number[i]==0){
            total_even_numbers++;
        }
    }
    for(int i=0;i<=9; i++){
        cout<<*(divided_number+i)<<"  ";
    }
    cout<<"\n Total odd number: "<<total_odd_numbers<<endl;
    cout<<"\n Total even number: "<<total_even_numbers<<endl;
}

Output:

0  2  5  8  -2  0  6  4  3  1
0  0  1  0  0  0  0  0  1  1
 Total odd number: 3

 Total even number: 7

And, of course, the code could be cleaned up much further:

int number[10]={0, 2, 5, 8, -2, 0, 6, 4, 3, 1};
int odds = 0, evens = 0;

for(int i=0;i<=9; i++)
{
    int left = number[i] % 2; cout << left << " ";
    if (left != 0) {odds++;} else {evens++;}
}

cout<<"\nTotal odd number: "<<odds<<'\n'<<"Total even number: "<<evens;

Why does an error occur when I modulus the value in array?

1 Answers1