How to properly convert std::string to an integer vector

Question

My high level goal is to convert any string (can include non-ascii characters) into a vector of integers by converting each character to integer.

I already have a python code snippet for this purpose:

bytes = list(text.encode())

Now I want to have a C++ equivalent. I tried something like

int main() {
  char const* bytes = inputText.c_str();
  long bytesLen = strlen(bytes);
  auto vec = std::vector<long>(bytes, bytes + bytesLen);
  for (auto number : vec) {
      cout << number << endl;
  }
  return 0;
}

For an input string like "testΔ", the python code outputs [116, 101, 115, 116, 206, 148].

However C++ code outputs [116, 101, 115, 116, -50, -108].

How should I change the C++ code to make them consistent?

"I know how to do this in another language" is not a reason to use that language's tag. — Karl Knechtel
– Karl Knechtel, Commented Nov 10, 2020 at 19:58
If you are using unicode characters you cannot use char* you'll likely want to use wide characters and unicode literals stackoverflow.com/questions/6796157/… for example Python's str.encode uses utf-8 by default, so here's an explanation of utf-8 support in C++ stackoverflow.com/questions/50403342/… — Cory Kramer
– Cory Kramer, Commented Nov 10, 2020 at 19:59
If you're sticking in ASCII space, an unsigned datatype in the vector should help a lot. — user4581301
– user4581301, Commented Nov 10, 2020 at 20:01
True enough. I was watching TV the other night and apparently the aliens built the pyramids. Maybe they are responsible for triangles, too. — user4581301
– user4581301, Commented Nov 10, 2020 at 20:54

Karl Knechtel · Accepted Answer · 2020-11-10 20:02:31Z

2

However C++ code outputs [116, 101, 115, 116, -50, -108].

In C++, the char type is separate from both signed char and unsigned char, and it is unspecified whether or not it should be signed.

You thus explicitly want an unsigned char*, but the .c_str method gives you char *, so you need to cast. You will need reinterpret_cast or a C-style cast; static_cast will not work.

answered Nov 10, 2020 at 20:02

Karl Knechtel

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2 Comments

Karl Knechtel Over a year ago

To be clear: static_cast will work on individual elements, but not on the pointer.

ireneisgood Over a year ago

thanks Karl. I modified my code and confirmed it's working. Really appreciate your help (as well as the other comments below)

jhkouy78reu9wx · Accepted Answer · 2020-11-10 20:14:13Z

1

You can iterate over std::string contents just fine, no need to convert it to std::vector. Try this:

int main()
{
    std::string str = "abc";
    for (auto c : str)
    {
        std::cout << static_cast<unsigned int>(c) << std::endl;
    }
}

static_cast here is needed just because standard operator<< outputs char as it is, not as a number. Otherwise, you can work with it just like with any other integral type. We cast it to unsigned int to ensure that output is strictly positive, for signedness of char is implementation-defined.

edited Nov 10, 2020 at 20:14

answered Nov 10, 2020 at 20:07

jhkouy78reu9wx

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2 Comments

ireneisgood Over a year ago

thanks for the suggestion! My use case actually needs the vecotr<long> explicitly. I added the cout just to help explain my problem

jhkouy78reu9wx Over a year ago

@ireneisgood you can construct it like this: std::vector<unsigned long> result(str.begin(), str.end());

eerorika · Accepted Answer · 2020-11-10 20:01:56Z

0

How should I change the C++ code to make them consistent?

The difference appears to be that Python uses unsigned char values while char is signed in your C++ implementation. One solution: Reinterpret the string as array of unsigned char.

answered Nov 10, 2020 at 20:01

eerorika

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