One case of the menu conditioned by other - c++

Question

I have a c++ program with a menu of 9 cases. I need that the case 7 is only executed if the case 3 has been used before case 7. How can I do it?
Thanks.
I don't know how to use a case in an "if".

Answer 1

Here's an example of using an if inside a case statement:
case 7:
{
if (previous_selection == 3)
{
Do_Operation();
}
}
break;
This one of many possible solutions, there are others.

Answer 2

You have to flag somehow that case 3 has been executed. For example, with a bool variable, setting it as part of the case 3 block of instructions, and checking it as part of the case 7 block of instructions.
[Demo]
#include <iostream> // cout
int main() {
bool case_3_already_used{ false };
for (auto index : { 7, 6, 5, 3, 2, 1, 8, 7, 9 }) {
switch (index) {
case 0: { std::cout << "0\n"; break; }
case 1: { std::cout << "1\n"; break; }
case 2: { std::cout << "2\n"; break; }
case 3: {
std::cout << "3\n";
case_3_already_used = true;
break;
}
case 4: { std::cout << "4\n"; break; }
case 5: { std::cout << "5\n"; break; }
case 6: { std::cout << "6\n"; break; }
case 7: {
if (case_3_already_used) {
std::cout << "7\n";
} else {
std::cout << "Cannot execute case 7 since case 3 has not been executed yet.\n";
}
break;
}
case 8: { std::cout << "8\n"; break; }
case 9: { std::cout << "9\n"; break; }
default: break;
}
}
}
// Outputs:
//
// Cannot execute case 7 since case 3 has not been executed yet.
// 6
// 5
// 3
// 2
// 1
// 8
// 7
// 9
Now, were the switch logic in a separate function, you would need to keep the state of that boolean variable between different calls. A way to do that would be to mark the flag as static.
[Demo]