C++ Integer [?]

Question

In Java, strings have a charAt() function.

In C++, that function is simply stringname[INDEX]

However, what if I wanted to use a particular number at a certain index of an integer?

E.g.

int value = 9123;

Let's say I wanted to work with the index 0, which is just the 9.

Is there a way to use index at's with integers?

Answer 1

int value = 9123;
std::stringstream tmp;
tmp << value;
char digit = (tmp.str())[0];

Answer 2

No, there is no standard function to extract decimal digits from an integer.

In C++11, there is a function to convert to a string:

std::string string = std::to_string(value);

If you can't use C++11, then you could use a string stream:

std::ostringstream stream;
stream << value;
std::string string = stream.str();

or old-school C formatting:

char buffer[32];  // Make sure it's large enough
snprintf(buffer, sizeof buffer, "%d", value);
std::string string = buffer;

or if you just want one digit, you could extract it arithmetically:

int digits = 0;
for (int temp = value; temp != 0; temp /= 10) {
    ++digits;
}

// This could be replaced by "value /= std::pow(10, digits-index-1)"
// if you don't mind using floating-point arithmetic.
for (int i = digits-index-1; i > 0; --i) {
    value /= 10;
}
int digit = value % 10;

Handling negative numbers in a sensible way is left as an exercise for the reader.

Answer 3

You can use the following formula (pseudo-code) :

currDigit = (absolute(value) / 10^index) modulo 10; // (where ^ is power-of)

Answer 4

Just to make things complete, you can also use boost::lexical_cast, for more info check out the documentation here.

Basically its just a nice wrapper around the code which can be found at Andreas Brinck answear.

Answer 5

Another solution, which does use 0 for the lestmost digit. digits is used to break down value into individual digits in written order. (i.e. "9347" becomes 9,3,4,7). We then discard the first index values. I.e. to get the 3nd digit, we discard the first two and take the new front.

if (value==0 && index ==0) return 0; // Special case.
if (value <0) { ... } // Unclear what to do with this.
std::list<char> digits;
while (value) {
  digits.push_front(value % 10);
  value /= 10;
}
for(; index > 0 && !digits.empty(); index--) {
  digits.pop_front();
}
if (!digits.empty()) {
  return digits.front();
} else
{
  throw std::invalid_argument("Index too large");
}

Answer 6

An integer is not a string and therefor you can not do that. What you need is indeed to convert an integer to string. You can use itoa or have a look here.

Answer 7

Try sprintf to write the integer out to a string:

http://www.cplusplus.com/reference/clibrary/cstdio/sprintf/

Then you can index into the char array that you've just printed into.

Answer 8

Longer version respect to Andreas Brink.

The C++ library is designed so that between "sequences" and "values" there is a "mediator" named "stream", that actually act as a translator from the value to their respecting sequence.

"sequences" is an abstract concept whose concrete implementation are "strings" and "files". "stream" is another abstract concept whose correspondent concrete implementation are "stringstream" and "fstream", that are implemented in term of helper classes "stringbuf" and "filebuf" (both derived form the abstract "streambuf") and from a helper object of "locale" class, containing some "facets".

The cited answer code, works this way:

1. The tmp object of class stringstream is default-constructed: this will construct also internally a stingbuf and a string, plus a locale referencing the facets of the system global locale (the default one remaps the "classic" or "C" locale)
2. The operator<< between stream and int function is called: there is one of them, for all the basic types
3. The "int version" gets the num_put facet from the locale, and a "buffer iterator" from the buffer, and calls the put function passing the format flags of the given stream.
4. the "put function" actually converts the number into the character sequence thus filling the buffer
5. When the buffer is full, or when a particular character is inserted or when the str function is called, the buffer content is "sent" (copyed, in this case) to the string, and the string content returned.

This very convoluted process looks complex at first but:

• Can be completely hidden (resulting in two lines of code)
• Cam be extended to virtually anything but...
• It is often kept as a (sort of ) misery in its details in the most of C++ courses and tutorials

Answer 9

I've implemented a variant of giorashc s solution, with all the suggested fixes and issues resolved: Its a bit long but it should be fast if everything is inlined: Most of the code is tests which I've left in for completeness.

#include <iostream>
#include <math.h>

char get_kth_digit( int v, int index)
{
  assert(v>0);
  int mask = pow(10,index);
  return '0'+(v % (mask*10))/mask;
}

int count_digits( int v )
{
  assert(v>0);

  int c=0;
  while(v>0)
  {
    ++c;
    v/=10;
  }

  return c;
}

char get_int_index(int v, int index)
{
  if( v==0 ) return '0';
  if( v <  0 )
  {
    if(index==0) { return '-'; }
    return get_int_index(-v,index-1);
  }

  // get_kth_digit counts the wrong way, so we need to reverse the count
  int digits = count_digits(v);
  return get_kth_digit( v, digits-index-1);
}



template<typename X, typename Y>
void compare(const X & v1, const Y & v2, const char * v1t, const char * v2t, uint32_t line, const char * fname )
{
  if(v1!=v2)
  {
    std::cerr<<fname<<":"<<line<<": Equality test failed "<< v1t  << "("<<v1<<") <> " << v2t <<" ("<<v2<<")"<<std::endl;
  }
}

#define test_eq(X,Y) compare(X,Y,#X,#Y,__LINE__,__FILE__)

int main()
{
  test_eq( 1, count_digits(1) );
  test_eq( 1, count_digits(9) );
  test_eq( 2, count_digits(10) );
  test_eq( 2, count_digits(99) );
  test_eq( 3, count_digits(100) );
  test_eq( 3, count_digits(999) );

  test_eq( '1', get_kth_digit(123,2) );
  test_eq( '2', get_kth_digit(123,1) );
  test_eq( '3', get_kth_digit(123,0) );

  test_eq( '0', get_kth_digit(10,0) );
  test_eq( '1', get_kth_digit(10,1) );

  test_eq( '1', get_int_index(123,0) );
  test_eq( '2', get_int_index(123,1) );
  test_eq( '3', get_int_index(123,2) );

  test_eq( '-', get_int_index(-123,0) );
  test_eq( '1', get_int_index(-123,1) );
  test_eq( '2', get_int_index(-123,2) );
  test_eq( '3', get_int_index(-123,3) );

}

Answer 10

I would convert it to a string, then index it -- CPP also has the:

str.at(i) 

function similar to Java's.

Another simpler loop in C++11 would be a range based loop --

int i = 0
for(auto s : int_or_str){
    if(i == idx)
        cout << s;
    else
        i++  
}

I guess this isn't easier than the standard for loop -- thought auto may be helpful, not really. I know this is answered, but I prefer simple and familiar answers.

Zach