Comments for COMP2710 Spring 2018

COMP2710 README.md

This a collection of tips and feedbacks for COMP2710 Software Construction Spring 2018.

1. Grading script for hw1 gist.
2. Grading script for hw2 gist.

hw1.md

Some comments about HW1.

1. MAKE SURE YOU PROGRAM COMPILES ON LINUX. For example, system("pause") is Windows specific.
2. implicit conversion. Please be aware of implicit conversion in C++. For example

   int a = 0, b = 1, c = 2;
   a = b / c;         // a is 0
   a = 2.0 * (b / c); // a is 0
   a = 2.0 * b / c;   // a is 1

3. File encoding. Make sure that your cpp files are in ASCII encoding. If you copy and paste code snippets from website (which is fine), chances are you might be copying some non-printable Unicode characters as well.
4. Over-engineering is bad. I could see that some students are really trying to modularize the code. Instead of a 20-line code, we see a file of about 100 lines. I'm not saying there is anything wrong about the added functionality and encapsulation (if any). However more thoughts are definitely needed over simplicity, functionality, code structure, and etc. For this simple problem whatever you do probably does not matter much. In a large code base, however, these trade-offs should be considered with caution.

hw2.md

Some feedbacks for HW2.

1. MAKE SURE YOUR SOURCE FILE COMPILES.
2. There is NO dollar sign before the rate.
3. Be aware of the edge cases where the monthly payment is lower than the interest.

General tips on C++.

1. bool and int values can be implicitly converted to each other.

   1. bool to int: false is 0, and true is 1.
   2. int to bool: 0 is false, non-zero is true.

   So at_least_two_alive can be implemented as

   bool at_least_two_alive(bool a, bool b, bool c)
   {
     return a + b + c >= 2;
   }

2. When testing a boolean value, do not use if (a == true) or if (true == a), simply use if (a) instead.

3. When returning a boolean value from a function, instead of

   if (a) return true;
   else return false;

   Use

   return a;

4. Configure your editor to use tabs or spaces consistently. DO NOT MIX TABS AND SPACES.

hw3.md

Feedback for HW3.

1. MAKE SURE YOUR SOURCE FILE COMPILES.
2. The probability may be off by 1-2%, no points off as long as they are close enough.

hw4.cpp

#include <fstream>
#include <iostream>
#include <vector>
using namespace std;

vector<int> merge(const vector<int>& v0, const vector<int>& v1)
{
  vector<int> ret(v0.size() + v1.size(), 0);
  int i, j, k;
  for (i = 0, j = 0, k = 0; i < v0.size() && j < v1.size(); ++k)
    if (v0[i] < v1[j])
      ret[k] = v0[i++];
    else
      ret[k] = v1[j++];
  for (; i < v0.size(); ++i, ++k) ret[k] = v0[i];
  for (; j < v1.size(); ++j, ++k) ret[k] = v1[j];
  return ret;
}

int main()
{
  string fname;
  ifstream fin;
  int val;

  cin >> fname;
  fin.open(fname);
  vector<int> arr1;
  while (fin >> val) arr1.push_back(val);
  fin.close();

  cin >> fname;
  fin.open(fname);
  vector<int> arr2;
  while (fin >> val) arr2.push_back(val);
  fin.close();

  vector<int> res = merge(arr1, arr2);
  cin >> fname;
  ofstream fout(fname);
  for (int n : res) fout << n << endl;
  fout.close();

  return 0;
}

hw4.org

HW4 Feedback

Since we are using C++, please upgrade your code from C to C++. And C++11 has been around for quite a while, since 2011!! Even C++14 was approved back in 2014. GCC 6.1 supports C++14 with GNU extensions by default. And GCC 6.1 was released on April 27, 2016. Please upgrade your code from C++03 to C++11 as well.

1. Avoid .h headers. C++ headers are like <iostream>, <algorithm>, <vector>, etc. You probably will never need something like <something.h>.
2. Prefer vector over raw array.
   • vector is a dynamic-sized array, which automatically grows as needed, you don’t need to worry about the maximum size.
   • Accessing individual elements is the same as the raw array.
   • Many convenient functions, e.g., size(), empty(), etc.
3. Prefer string over raw char array. std::basic_fstream::open() takes string as input since C++11, so you do not need the c_str() conversion if you use C++11.
4. Mixing cin and getline is MESSY. A simple solution is cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); after you do cin, which ignores everything before a newline and the newline itself.
5. Avoid eof() to test the end of input. Just Google “eof c++ not working”.
   • eof() does not return true until after a failed reading.
   • You may run into other problems when reading a stream, e.g., incompatible types.

   Just test the stream itself, e.g., while (cin >> a >> b).

hw5.org

HW5 Feedback

1. Define structure.

   struct Node {
     int val;
     Node *next;
   
     Node(int v = 0) : val(v), next(nullptr) {}
   };
       

2. Segment fault usually means you are visiting an address that does not belong to you program.
3. Memory leak problem.

   struct Node {
     int val;
     Node *next;
   
     Node(int v = 0) : val(v), next(nullptr) {}
   };
   
   int main()
   {
     Node *node = new Node(13);
     // do something with node
     delete node;
     return 0;
   }
       

   Use shared_ptr instead.

midterm-q5.cpp

#include <iostream>
using namespace std;

class PrimeNumber
{
 public:
  PrimeNumber(int n = 2) : n_(n) {}

  friend ostream &operator<<(ostream &os, const PrimeNumber &pn);

  PrimeNumber &operator++();   // prefix
  PrimeNumber operator++(int); // postfix

  PrimeNumber &operator--();   // prefix
  PrimeNumber operator--(int); // postfix

 private:
  bool isprime(int n);
  int next_prime(int n);
  int prev_prime(int n);

 private:
  int n_;
};

ostream &operator<<(ostream &os, const PrimeNumber &pn)
{
  os << pn.n_;
  return os;
}

PrimeNumber &PrimeNumber::operator++()
{
  n_ = next_prime(n_);
  return *this;
}

PrimeNumber PrimeNumber::operator++(int)
{
  PrimeNumber p = *this;
  ++*this;
  return p;
}

PrimeNumber &PrimeNumber::operator--()
{
  n_ = prev_prime(n_);
  return *this;
}

PrimeNumber PrimeNumber::operator--(int)
{
  PrimeNumber p = *this;
  --*this;
  return p;
}

int PrimeNumber::next_prime(int n)
{
  if (2 == n) return 3;
  for (n += 2; !isprime(n); n += 2)
    ;
  return n;
}

int PrimeNumber::prev_prime(int n)
{
  if (3 == n) return 2;
  for (n -= 2; n > 1 && !isprime(n); n -= 2)
    ;
  return n > 1 ? n : 2;
}

bool PrimeNumber::isprime(int n)
{
  for (int i = 2; i * i <= n; ++i)
    if (n % i == 0) return false;
  return true;
}

int main()
{
  PrimeNumber pn;
  cout << pn << endl;

  for (int i = 0; i < 10; ++i)
    cout << pn++ << endl;

  for (int i = 0; i < 10; ++i)
    cout << pn-- << endl;

  for (int i = 0; i < 10; ++i)
    cout << ++pn << endl;

  for (int i = 0; i < 10; ++i)
    cout << pn-- << endl;

  cout << pn << endl;
  return 0;
}

Project 1 Feedback.org

Project 1 Feedback

1. MAKE SURE YOUR CODE COMPILES WITH GNU G++.
2. Write your own code, or please at least fix the bugs in the code from unknown sources.
3. Please create the text file if not exist.
4. If you split your implementation into a couple of files, please provide me with a simple Makefile.