C++ Extract Number from the Middle of a String

C++ Extract number from the middle of a string

updated for C++11

(important note for compiler regex support: for gcc. you need version 4.9 or later. i tested this on g++ version 4.9[1], and 9.2. cppreference.com has in browser compiler that i used.)

Thanks to user @2b-t who found a bug in the c++11 code!

Here is the C++11 code:

#include <iostream>
#include <string>
#include <regex>

using std::cout;
using std::endl;

int main() {
    std::string input = "Example_45-3";
    std::string output = std::regex_replace(
        input,
        std::regex("[^0-9]*([0-9]+).*"),
        std::string("$1")
        );
    cout << input << endl;
    cout << output << endl;
}

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boost solution that only requires C++98

Minimal implementation example that works on many strings (not just strings of the form "text_45-text":

#include <iostream>
#include <string>
using namespace std;
#include <boost/regex.hpp>

int main() {
    string input = "Example_45-3";
    string output = boost::regex_replace(
        input,
        boost::regex("[^0-9]*([0-9]+).*"),
        string("\\1")
        );
    cout << input << endl;
    cout << output << endl;
}

console output:

Example_45-3
45

Other example strings that this would work on:

• "asdfasdf 45 sdfsdf"
• "X = 45, sdfsdf"

For this example I used g++ on Linux with #include <boost/regex.hpp> and -lboost_regex. You could also use C++11x regex.

Feel free to edit my solution if you have a better regex.

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Commentary:

If there aren't performance constraints, using Regex is ideal for this sort of thing because you aren't reinventing the wheel (by writing a bunch of string parsing code which takes time to write/test-fully).

Additionally if/when your strings become more complex or have more varied patterns regex easily accommodates the complexity. (The question's example pattern is easy enough. But often times a more complex pattern would take 10-100+ lines of code when a one line regex would do the same.)

---

[1]

[1]
Apparently full support for C++11 <regex> was implemented and released for g++ version 4.9.x and on Jun 26, 2015. Hat tip to SO questions #1 and #2 for figuring out the compiler version needing to be 4.9.x.

How to extract numbers from string in c?

You can do it with strtol, like this:

char *str = "ab234cid*(s349*(20kd", *p = str;
while (*p) { // While there are more characters to process...
    if ( isdigit(*p) || ( (*p=='-'||*p=='+') && isdigit(*(p+1)) )) {
        // Found a number
        long val = strtol(p, &p, 10); // Read number
        printf("%ld\n", val); // and print it.
    } else {
        // Otherwise, move on to the next character.
        p++;
    }
}

Link to ideone.

Find and extract a number from a string

go through the string and use Char.IsDigit

string a = "str123";
string b = string.Empty;
int val;

for (int i=0; i< a.Length; i++)
{
    if (Char.IsDigit(a[i]))
        b += a[i];
}

if (b.Length>0)
    val = int.Parse(b);

How to convert a part of string into integer?

Another way: You could replace non-digits with spaces so as to use stringstring extraction of the remaining integers directly:

std::string x = "15a16a17";
for (char &c : x)
{
    if (!isdigit(c))
        c = ' ';
}

int v;
std::stringstream ss(x);
while (ss >> v)
    std::cout << v << "\n";

Output:

15
16
17

how to extract a number from string in c++ [duplicate]

You can use atoi and isdigit:

// Example string
std::string str = "this is my id 4321.";

// For atoi, the input string has to start with a digit, so lets search for the first digit
size_t i = 0;
for ( ; i < str.length(); i++ ){ if ( isdigit(str[i]) ) break; }

// remove the first chars, which aren't digits
str = str.substr(i, str.length() - i );

// convert the remaining text to an integer
int id = atoi(str.c_str());

// print the result
std::cout << id << std::endl;

Get a part of an integer in C

Getting a single digit as an int can be done mathematically:

int num = 897;
int dig1 = (num /   1 ) % 10;
int dig2 = (num /  10 ) % 10;
int dig3 = (num / 100 ) % 10;

Division by one on the first line is only for illustration of the concept: you divide by n-th power of ten, starting with 10⁰ = 1.

C++ - extract numbers from a string

A job for stringstreams, see it live: http://ideone.com/e8GjMg

#include <sstream>
#include <iostream>

int main()
{
    std::istringstream iss(" abcd 1234    -6242          1212");

    std::string s;
    int a, b, c;

    iss >> s >> a >> b >> c;

    std::cout << s << " " << a << " " << b << " " << c << std::endl;
}

Prints

abcd 1234 -6242 1212

C-Language: How to get number from string via sscanf?

A combination of digit filtering and sscanf() should work.

int GetNumber(const char *str) {
  while (!(*str >= '0' && *str <= '9') && (*str != '-') && (*str != '+')) str++;
  int number;
  if (sscanf(str, "%d", &number) == 1) {
    return number;
  }
  // No int found
  return -1; 
}

Additional work needed for numbers that overflow.

A slower, but pedantic method follows

int GetNumber2(const char *str) {
  while (*str) {
    int number;
    if (sscanf(str, "%d", &number) == 1) {
      return number;
    }
    str++;
  } 
  // No int found
  return -1; 
}

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