CODEWITHSUNDEEP
c++arraysrandom
Alex Santisteban
Alex Santisteban

Reputation: 25

Where is this array value coming from?

So I'm practicing coding before classes begin again and I wanted to create a random array of 10 integers and wanted to find which two numbers, when added together, equal greater than 100.

When I run the code below

int arr[9];

for(int i = 0; i<=9; i++){
    arr[i]=(rand()%100)+1;
    cout<<arr[i]<<endl;
}

cout<<endl;

for(int k = 0; k<=9; k++){
    for (int j = k+1; j<=9; j++){
        if((arr[k]+arr[j])>100){
            cout<<arr[k]<<" + "<<arr[j]<<" "<<"over 100"<<endl;
        }
        else{
            cout<<arr[k]<<" + "<<arr[j]<<" "<<"under 100"<<endl;
        }
    }
}


return 0;

I notice that the value of arr[9] is equal to one number but the program has another number for it. For example: the array that's created is [42 68 35 1 70 25 79 59 63 65]

but my output shows a 9 as arr[9]

42 + 68 over 100

42 + 35 under 100

42 + 1 under 100

42 + 70 over 100

42 + 25 under 100

42 + 79 over 100

42 + 59 over 100

42 + 63 over 100

42 + 9 under 100

(I'll just include the first loop to not make this any longer)

Where did my 65 go??

Where did the 9 come from??

Did I write something wrong??

Thanks

Upvotes: 2

Views: 53

Answers (1)

Waqar
Waqar

Reputation: 9366

I notice that the value of arr[9] is equal to one number but the program has another number for it.

Counting in computers starts from 0 and ends at one number less than the total size. For int arr[9]

  • the ninth index is arr[8]
  • the 1st index is arr[0]
  • the 2nd index is arr[1]

When you do arr[9], you are crossing the boundary (accessing an element that's not in the range) which leads to undefined behaviour.

As a result, your loop should stop at i < 9:

for(int i = 0; i < 9; i++) // i < 9 now vs i <=9

The same changes need o be made to the loops below the first one.

Upvotes: 1

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