如何从char数组中提取数字字符串成员

Question

i want to extract number string values of a char array. Actually I want to extract numbers embeded in file names for some file management. For example if there is a file name as file21 then i want the decimal number 21 from this file name.

How can i extract these values?

I tried the following but it results in an unexpected value. I think it is as a result of the implicit typecasting from the char to int while doing the arthimetic operation.

char * fname;
cout<<"enter file name";
cin>>fname;

int filenum=fname[4]%10+fname[5];
cout<<"file number is"<<filenum;

NOTE: The filenamse are strictly in the format fileXX , XX being numbers between 01 and 99

Answer 1

You're getting undefined behavior because you're never allocating memory for the char* you read into:

char * fname = new char[16];  //should be enough for your filename format

or better yet

char fname[16];

Also, what do you expect:

fname[4]%10+fname[5];

to do? Magically concatenate the numbers?

First, you convert the first char to an int , multiply it by 10 , convert the second char to an int and add to the first one. A simple google search for char to int would get you there.

Answer 2

You need to subtract '0' to get the decimal value of a digit character:

int filenum=(fname[4]-'0')*10+(fname[5]-'0');

Better yet, you should use atoi :

int filenum = atoi(fname+4);

Answer 3

How can i extract these values?

There are an infinite number of ways. One way is to use std::istringstream :

#include <string>
#include <sstream>
#include <iostream>

int main () {
  std::string fname;
  std::cout << "Enter file name: ";
  std::getline(std::cin, fname);

  int filenum;
  std::istringstream stream(fname.substr(4,2));

  if(stream >> filenum)
    std::cout << "file number is " << filenum << "\n";
  else
    std::cout << "Merde\n";
}

Answer 4

Here is the simplest code:

#include <iostream>
#include <sstream>
#include <string>
using namespace std;
int main()
{
  int filenum;
  string fname;
  cout<<"enter file name";
  cin>>fname;

  string str2 = fname.substr(4,2);
  istringstream(str2) >> filenum;
  cout<<"file number is"<<filenum;
  return 0;
}

Answer 5

If your input is that much defined, the simplest solution is scanf:

int main()
{
    int theNumber = 0;
    scanf("file%d.txt", &theNumber);
    printf("your number is %d", theNumber);
}

Check it out in action, reading from char* instead of stdio: http://codepad.org/JFqS70yI

scanf (and sscanf) also throws in checking for proper input format: returns number of fields read successfully. In this case if return value is any other than 1, the input was wrong.