Subtracting from an array to leave a specific number

Question

I'm looking for a bit of help and to confirm if what i'm trying to do is possible at all.

I have an array (for this test ) of 3 numbers one of them I want to use as the number to subtract by.

My starting number is 100 and I would like to leave 40 at the end as this links in to the bigger program.

I know how to loop through the array to find a matching number and have used this as a starting point, however I'm getting totally lost trying to expand this to do the above.

Any help would be greatly appreciated!

My code so far.

int main() {
    int arr[3] = {12,13,60};  //Array of 3 numbers
    int j = 100; // j set to 100 for the test
    int t = 0; //to be used 
    int result;

    for (int i= 0; i <= sizeof(arr); i++) {   //loops through the array 
        if (j == arr[i] - j == 40) {   //if j = array number - j == 40
           possible = true; 
           t = arr[i] - j; //sets t to the correct array number
        }
   }

    if (possible == 1) {
        cout << "This is possible to do" << endl;
        result = (j - t) << endl; //calculates the sum which leaves 40
    }
    else {
        cout << "This is not possible" << endl;
    }

return 0;
}

Like I said above I know this is not a working code however I'm totally lost as to where to turn next.

Answer 1

I'm not 100% clear on what you are trying to do but this looks like it is wrong:

    if (j == arr[i] - j == 40) {   //if j = array number - j == 40
       possible = true; 
       t = arr[i] - j; //sets t to the correct array number
    }

The if statement here means "if the current array value minus 100 is equal to 40 and 100"... which is impossible.

I think what you were looking for was:

if( j - arr[i] == 40 )

Which means "if 100 minus the current array value is 40"

Answer 2

I think the correct code for what you are trying to do is,

int main() {
    int arr[3] = {12,13,60};  //Array of 3 numbers
    int j = 100; // j set to 100 for the test
    int t = 0; //to be used 
    int result;

    for (int i= 0; i <= sizeof(arr); i++) {   //loops through the array 
        if (j - arr[i] == 40) {   //if j - array number == 40
           possible = true; 
           t = arr[i]; //sets t to the correct array number
        }
   }

    if (possible == true) {
        cout << "This is possible to do" << endl;
        cout << result = (j - t) << endl; //calculates the sum which leaves 40
    }
    else {
        cout << "This is not possible" << endl;
    }

return 0;
}

Answer 3

The

   if (j == arr[i] - j == 40) {   //if j = array number - j == 40

is surely bad. It evaluates as this:

  if ((j == arr[i] - j) == 40)

Which means:

  if (true == 40)

And that will be always false.

Answer 4

By looking at the comment

//if j = array number - j == 40

i think what you are trying to do is

if ((j=arr[i]-j) ==  40){
   possible=true;
   t=arr[i]-j;
}

If you actually want to assign value to j and then compare it.