C++ Program to Implement atol Function

This C++ Program which implements atol function which converts a C++ string to an integer value. The function atol( ) skips any trailing blanks and alphabets and if the string iterator has reached the end of the string, a negative value is returned to indicate that no digit is present in the string else the iterator goes through the remaining string until a non-digit character is encountered.

Here is source code of the C++ program which implements atol function which converts a C++ string to an integer value. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
/*
```

 * C++ Program to Implement atol Function

```
 */
```
```
#include <iostream>
```
```
#include <cctype>
```
```
#include <string>
```
```
 
```

/* Function converts string to integer */

```
int atol(std::string s)
```
```
{
```
```
    int num = 0;
```
```
    std::string::const_iterator i;
```
```
 
```

    for (i = s.begin(); i != s.end(); i++)

```
    {
```

        if (*i == ' ' || *i == '\t' || isalpha(*i))

```
            continue;
```
```
        else
```
```
            break;
```
```
    }
```
```
    if (i == s.end())
```
```
        return -1;
```

    for (std::string::const_iterator j = i; j != s.end(); j++)

```
    {
```
```
        if (isdigit(*j))
```

            num = num * 10 + (*j - '0');

```
        else
```
```
            break;
```
```
    }
```
```
    return num;
```
```
}
```
```
 
```
```
int main()
```
```
{
```
```
    std::string s;
```
```
    int num;
```
```
 
```

    std::cout << "Enter a numerical string : ";

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    std::cin >> s;
```
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    num = atol(s);
```
```
    if (atol(s) >= 0)
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        std::cout << "The Numerical Value is  : " << num << std::endl;

```
    else
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        std::cout << "No numerical digit found " << std::endl;

```
}
```

$ g++ main.cpp
$ ./a.out
Enter a numerical string :      956
The Numerical Value is  : 956
$ ./a.out
Enter a numerical string : nonumber
No numerical digit found

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