How does this pseudo code is false (does not output 43)

Question

I am studying Software System Development A-Level with Cea. The code is displayed below. The mark scheme says that the pseudo code is false, can someone please explain why that is?

Pseudo Code:

The output from the following pseudo code is 43.
(method) – void incrementNum( int numParam ){
 numParam++
}
(main method) – void main(){
int numArg = 42
incrementNum( numArg )
output numArg // writes 43
}

Answer 1

Have a look into function evaluation strategies, in particular Call by Value. The function incrementNum receives a copy of numArg called numParam, which is increased. The original variable is untouched. Depending on the actual programming language, you have multiple solutions:

• rewrite incrementNum such that it returns its argument (see cybermonkey's solution)
• use a different evaluation strategy. e.g. in C++ you can call by reference (note the int& vs int in the signature of increase):

#include <iostream>

void increase(int& v) {
  v++;
}

int main() {
  int i = 42;
  increase(i);
  std::cout << i << std::endl;

  return 0;
}

outputs 43.