Brainfuck interpreter in C++ produces incorrect result

Question

I'm writing a Brainfuck interpreter in C++ and I'm having trouble getting this to work correctly. I'm new to C++, but I know JavaScript, which is why I'm writing this interpreter using my JavaScript code (which works perfectly) for reference.

#include <iostream>
#include <vector>

using namespace std;
void printVec(vector<int> arr) {
  for (int i = 0; i < arr.size(); i ++) {
    cout << arr[i] << ", " << endl;
  }
}
vector<int> brain(string code, string input = "") {
  vector<int> tape(1);
  vector<string> stack(0);
  int cur = 0;
  int mem = 0;
  for (int i = 0; i < code.length(); i ++) {
    switch (code[i]) {
      case '+':
        tape[cur]++;
        tape[cur] %= 256;
        break;
      case '-':
        tape[cur]--;
        if (tape[cur] < 0) tape[cur] = 256 - tape[cur] * -1;
        break;
      case '>':
        cur++;
        try {
          tape.at(cur);
        } catch (out_of_range& oor) {
          tape.push_back(0);
        }
        break;
      case '<':
        cur--;
        if (cur < 0) {
          cout << "Index too low!" << endl;
          throw 1;
        }
      case '[':
        mem = i;
        break;
      case ']':
        if (tape[cur] > 0) i = mem;
        break;
    }
  }
  return tape;
}
int main() {
  printVec(brain("+++[>++<-]"));
  return 0;
}

This produces 0, 2 as the output, when it should be 0, 6. I think the problem is with the way I handle Brainfuck loops (cases '[' and ']') but I can't figure out the exact issue. In JavaScript, the code for these cases looks almost exactly the same, but it works fine.

Any help will be appreciated, including any tips on how to write other parts of this code better.

look at case '<', and see if its missing something. also you shouldn't use exceptions for flow control. — Mor A., Nov 09 '17 at 06:29
I suggest you learn C++ first, ignoring your javascript knowledge. — juanchopanza, Nov 09 '17 at 06:33
@juanchopanza do you mean taking a class? This is the way I have learned languages in the past (trial and error). — Eli Richardson, Nov 09 '17 at 07:50
Thank you. After adding the break statement the problem is solved. @M.Aroosi — Eli Richardson, Nov 09 '17 at 07:52

score 1 · Answer 1 · answered Dec 19 '20 at 16:53

Your case '<': is missing a break and thus will fallthrough and perform the function of case '[' as well. Add the missing break; and see if that is your only issue.

  case '<':
    cur--;
    if (cur < 0) {
      cout << "Index too low!" << endl;
      throw 1;
    }
    break;        // Add this
  case '[':
    mem = i;
    break;

Brainfuck interpreter in C++ produces incorrect result

1 Answers1