Limiting to characters on a string

**Erik Ingvoldsen** · 02-26-2015

I'm trying to set up a function in which you type in one character, but the character you type in is limited to the string input. Which is to say you can only put in characters that exist in the given string, but I'm not sure how to go about it. This is what I have so far, but it's not compiling.

Code:

char UserInputHandler::GetSingleCharacter(string limit) {
  std::string str;
  char letter;
  int i;
  bool loopend = false;
  while (loopend == false) {
    std::getline(std::cin, str);
    i = (int)str.length();
    if (std::string::npos == str.find_first_not_of(limit)) {
      if (i > 1) {
        std::cout << invalid_char_input() << std::endl;
      } else {
        letter = str[0];
        std::cout << "Character Entered: " << letter << endl;
        loopend = true;
      }
    } else {
        std::cout << char_not_allowed() << std::endl;
    }
  }
  return letter;
}

**jimblumberg** · 02-26-2015

This is what I have so far, but it's not compiling.

Why is it not compiling? Do you get some kind of warning or error messages? If so it would be extremely helpful if you posted them, all of them exactly as they appear in your development environment. It would also be helpful if you posted the smallest possible complete program that illustrates the problem, so we can compile and see the errors for ourselves if we so desire.

Jim

**Alpo** · 02-26-2015

It appears to work for me. the only possible thing I can see that might not compile is that you forgot the std:: prefix to the string parameter.

The only logical confusion I have is if this if condition is correct? It seems superfluous since you are using the index [] operator to retrieve that character. (Not saying it's wrong, it might just be redundant).

Code:

if (i > 1) {std::cout << invalid_char_input() << std::endl;
}

I tested it with the following code, it worked correctly AFAIK:

Code:

#include <string>
#include <iostream>

using namespace std;

class UserInputHandler
{
public:
    char GetSingleCharacter(std::string limit);
    std::string invalid_char_input() 
    {
        return std::string("You must enter a single character.");
    }
    std::string char_not_allowed() 
    {
        return std::string("That character is forbidden.");
    }
};

char UserInputHandler::GetSingleCharacter(std::string limit)
{
    std::string str;
    char letter;
    int i;
    bool loopend = false;
    while (loopend == false) {
        std::getline(std::cin, str);
        i = (int)str.length();
        if (std::string::npos == str.find_first_not_of(limit)) {
            if (i > 1) {
                std::cout << invalid_char_input() << std::endl;
            } else {
                letter = str[0];
                std::cout << "Character Entered: " << letter << endl;
                loopend = true;
            }
        } else {
            std::cout << char_not_allowed() << std::endl;
        }
    }
    return letter;
}

int main()
{
    UserInputHandler uih;
    uih.GetSingleCharacter("abcd");
}

**Erik Ingvoldsen** · 02-27-2015

@jimblumberg

Sorry about that. It had been working before I tried to make it based on the input string, so I figured it would be a very easy fix.

@Alpo

It worked, thanks! I just had to make it a std::string. The only other thing I need help with now is setting it so the string is empty by default--which is to say there's no limits at that point.

**whiteflags** · 02-27-2015

The only other thing I need help with now is setting it so the string is empty by default--which is to say there's no limits at that point.

Assuming that find_first_not_of doesn't just break when you do that, because I'm not actually sure what happens if the argument is an empty string. You could use default args if there is no other problem.

I would kinda rewrite it. I hate guessing about what the STL does:

Code:

char X::GetChar() {
  std::string s;
  while(true) {
    geline(std::cin, s);
    if (s.length() == 1)
      return s[0];
  }
}

char X::GetChar(std::string limit) {
  char c;
  do
    c = GetChar();
  while (std::string::npos == limit.find(c));
  return c;
}

Not saying it's better though. There shouldn't be much of a problem as long as these functions are written in terms of one another to avoid unnecessary code duplication.

**Elysia** · 02-27-2015

>>i = (int)str.length();
I suggest you don't do this. Declare "i" to be the same type str.length() returns (i.e. size_t) or use C++11's auto. Many bugs have come from not matching type and signedness, so just be consistent and match it.
Also declare "i" near first use. As it's not needed before this line and you initialize there, you should declare it there

size_t i = str.length();

**Erik Ingvoldsen** · 02-27-2015

Changed i. How does one set an empty string to start with? I've done it with ints and boolean before, but never an empty string.

**Elysia** · 02-27-2015

A string is empty by simply default-constructing it, i.e.
std::string s;

You can check whether a string is empty by checking against an empty string (e.g. s == "") or using the member function empty().

**whiteflags** · 02-27-2015

Originally Posted by Erik Ingvoldsen

Changed i. How does one set an empty string to start with? I've done it with ints and boolean before, but never an empty string.

In the context of a default argument you will have to do something like this:

Code:

char X::GetChar(std::string limit = "");

This allows you to define a function that takes an argument, limit, that will be an empty string or something else. In place of a literal empty string ("") you can write std::string(). In fact any type T with a default constructor, it can be invoked in this context with T().

**Erik Ingvoldsen** · 02-27-2015

Alright, well it's default on empty, but now I have a problem with how it treats empty strings. I'm trying to set it so that if the string is empty, it won't check if the character input is inside the string. And if the string is not empty, it will check. The problem is, it ignores the string completely now and will let the user input anything.

Code:

char UserInputHandler::GetSingleCharacter(std::string limit = "") {
  std::string str;
  char letter;
  bool loopend = false;
  while (loopend == false) {
    std::getline(std::cin, str);
    size_t i = str.length();
    if (limit == "") {
      if (i > 1) {
        std::cout << invalid_char_input() << std::endl;
      } else {
        letter = str[0];
        std::cout << "Character Entered: " << letter << endl;
        loopend = true;
      }
    } else if (std::string::npos == str.find_first_not_of(limit)) {
      if (i > 1) {
        std::cout << invalid_char_input() << std::endl;
      } else {
        letter = str[0];
        std::cout << "Character Entered: " << letter << endl;
        loopend = true;
      }
    } else {
        std::cout << char_not_allowed() << std::endl;
    }
  }
  return letter;
}

In the main method I have this:

Code:

UserInputHandler handler;
handler.GetSingleCharacter("abcd");

So it should only be letting me put in a, b, c, or d, but it's allowing me to put in e, f, g, etc. too.

**whiteflags** · 02-28-2015

Originally Posted by Erik Ingvoldsen

So it should only be letting me put in a, b, c, or d, but it's allowing me to put in e, f, g, etc. too.

That makes logical sense if you think about it.

Code:

 str.find_first_not_of("abcd");

Find first not of returns the first thing in the string that isn't these things. If the first character is 'e', etc. it's going to return that.

What you need to do is try to find the character in a list of acceptable characters. I've already done this in example code earlier in the thread.

**Erik Ingvoldsen** · 02-28-2015

Sorry about that. I just wanted to try and do as much as I could with my own coding so I didn't just copy/paste something without understanding it.

Code:

char X::GetChar() {
  std::string s;
  while(true) {
    geline(std::cin, s);
    if (s.length() == 1)
      return s[0];
  }
}

Is this part a function within a function?

**Alpo** · 02-28-2015

Originally Posted by Erik Ingvoldsen

Sorry about that. I just wanted to try and do as much as I could with my own coding so I didn't just copy/paste something without understanding it.

Code:

char X::GetChar() {
  std::string s;
  while(true) {
    geline(std::cin, s);
    if (s.length() == 1)
      return s[0];
  }
}

Is this part a function within a function?

It is just a normal function with a loop that collects input into the string, it returns the string on the condition the length is 1.

The definition of the function looks like this:

Code:

return_type class_type::function_name (parameters...)
{
    body
}

The function_name should be one of the functions (methods) declared in the class_name class, the :: operator specifies the scope the function_name belongs to.

**Erik Ingvoldsen** · 02-28-2015

Hmmm well the problem is, this seems to be two functions

Code:

char X::GetChar() {
  std::string s;
  while(true) {
    geline(std::cin, s);
    if (s.length() == 1)
      return s[0];
  }
}
 
char X::GetChar(std::string limit) {
  char c;
  do
    c = GetChar();
  while (std::string::npos == limit.find(c));
  return c;
}

And I need to have everything in one function. Is there a possible way to format it so that an empty string will allow everything?

**whiteflags** · 02-28-2015

I'm sorry that the two functions is causing such a problem. We can forget about it though.

This function looks okay:

Code:

char UserInputHandler::GetSingleCharacter(std::string limit = "") {
  std::string str;
  char letter;
  bool loopend = false;
  while (loopend == false) {
    std::getline(std::cin, str);
    size_t i = str.length();
    if (limit == "") {
      if (i > 1) {
        std::cout << invalid_char_input() << std::endl;
      } else {
        letter = str[0];
        std::cout << "Character Entered: " << letter << endl;
        loopend = true;
      }
    } else if (std::string::npos == str.find_first_not_of(limit)) {
      if (i > 1) {
        std::cout << invalid_char_input() << std::endl;
      } else {
        letter = str[0];
        std::cout << "Character Entered: " << letter << endl;
        loopend = true;
      }
    } else {
        std::cout << char_not_allowed() << std::endl;
    }
  }
  return letter;
}

Like I said before, you need to replace find first not of with find, though.

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