Why is the variable value changing automatically?

Question

This is my code

    int front=-1, rear=-1, CQUEUE[MAX];
    int isFull()
    {
      if((rear=MAX-1 && front==0) || front==rear+1)
        return 1;
      else
        return 0;
    }
    void enQueue()
    {
      printf("\nValue of rear=%d front=%d",rear,front);
      char ch;
      if(!isFull())
      {
        printf("\nValue of rear=%d front=%d",rear,front);
      ......

This is the output on calling the function enQueue()

Value of rear=-1 front=-1
Value of rear=0 front=-1

The value of rear is getting increased by 1 although I'm not increasing it manually.

Answer 1

Look at your first if statement.

if (rear=MAX-1  ...)
// maybe better if you type:
if (rear==MAX-1 ...)

Answer 2

You are using the assignment operator = where you should be using the eq comparison operator == in isFull

int isFull()
{
  if((rear==MAX-1 && front==0) || front==rear+1)
    return 1;
  else
    return 0;
}