How to use pointers into arrays? - c++

Question

So currently I am still confused in using pointers and reference and I do not know if what I am doing is right. My program is a calculator that stores everything the user inputs. There is an option that the user will be asked if he/she want to view the history and if he/she wants it the program will show all the data he/she inputs.And I need to use pointers and reference in my program but I am still confused on how to use pointers and reference into array
Here is my initialization:
int main() {
int size = 1, fNum[size], sNum[size];
char oprtn[size], answer;
;
float result[size];
int *ptrf = &fNum[size];
int *ptrs = &sNum[size];
char *ptro = &oprtn[size];
float *ptrRes = &result[size];
while (true) {
cout << "=====CALCULATOR=====\n\n";
cout << "ENTER TWO NUMBERS:" << endl;
while (!(cin >> *ptrf >> *ptrs)) {
system("cls");
cout << "INVALID INPUT. PLEASE ENTER TWO NUMBERS:\n";
cin.clear();
cin.ignore(2);
}
cout << endl;
do {
cout << "Choose Operation to be Used: \n"
<< " + --- Addition \n"
<< " - --- Subtraction \n"
<< " * --- Multiplication \n"
<< " / --- Division \n"
<< " % --- Remainder \n";
cout << "Answer: ";
cin >> answer;
cout << endl;
switch (answer) {
case '+':
cout << "ADDITION\n";
break;
case '-':
cout << "SUBTRACTION\n";
break;
case '*':
cout << "MULTIPLICATION\n";
break;
case '/':
cout << "DIVISION\n";
break;
case '%':
cout << "REMAINDER\n";
break;
default:
answer = false;
system("cls");
cout << "PLEASE ENTER A VALID ANSWER. CHOOSE BELOW.\n\n";
cout << "FIRST NUMBER: " << *ptrf << endl;
cout << "SECOND NUMBER: " << *ptrs;
cout << endl << endl;
continue;
}
} while (!answer);
cout << "DO YOU WANT TO TRY AGAIN? (Y / N): ";
cin >> answer;
switch (answer) {
case 'Y':
case 'y':
system("cls");
continue;
default:
cout << "VIEW HISTORY? (Y / N): ";
cin >> answer;
switch (answer) {
case 'Y':
case 'y':
cout << "HISTORY\n\n";
break;
default:
return 0;
}
}
}
}

Answer 1

This is not valid C++ code:
int size = 1, fNum[size], sNum[size]; // wrong: C++ forbids Variable Length Arrays (1)
char oprtn[size], answer; // ditto...
;
float result[size]; // ditto...
int *ptrf = &fNum[size]; // wrong: this syntax makes ptrf points one past end of array (2)
int *ptrs = &sNum[size]; // ditto...
char *ptro = &oprtn[size]; // ditto...
(1): VLA are a C language concept. Some compilers (gcc and CLang) allow it as an extension but it is useless in C++ because of the containers from the standard library
(2): the idiomatic way to initialize a pointer to the beginning of an array is just int *ptrf = Num; When used as a rvalue (in short at the right side of an = sign), an array decays to a pointer to its first element. So it reads (int *) ptr = &(fNum[0]);: ptr is a pointer to int and its initial value is the address of the first element of the array fNum

Answer 2

fNum[size]
is an array of one integer.
int *ptrf = &fNum[size];
Is a pointer to one past the last element of this array, therefore it's out of bounds access, this is undefined behaviour.
Since it only has one element, you declaring it as an array is pointless.
A pointer to the beginning of an array would be:
int *ptrf = fNum;
Or
int *ptrf = &fNum[0];
You can then cycle trough the array incrementing the pointer ptrf++.
To assing a pointer to a variable:
int x;
int *ptr = &x;
So the variable declarations and assignments:
int size = 1, fNum[size], sNum[size];
char oprtn[size], answer;
float result[size];
int *ptrf = &fNum[size];
int *ptrs = &sNum[size];
char *ptro = &oprtn[size];
float *ptrRes = &result[size];
Are the same as:
int fNum, sNum;
char oprtn, answer;;
float result;
int *ptrf = &fNum;
int *ptrs = &sNum;
char *ptro = &oprtn;
float *ptrRes = &result;
That said, C++ has mutch nicer data containers you can use like std::vector or std::array.
One last note, variable length arrays(fNum[size]) are forbidden in C++.