BigInteger check in C++ from a string

Question

You might consider this question a follow-up of:

BigInteger check in C from a string

Although, this time, I used C++.

I am on Linux Mint 19.1 with the compiler version:

g++-8 (Ubuntu 8.2.0-1ubuntu2~18.04) 8.2.0

I tried to push myself hard in C, but it obviously requires one of:

• more skill / experience

• lower-level approach

or both.

That is why I decided to transition to C++, already bought one C++ online course on Udemy, but back to coding, the current version looks straightforward to me, I see no more pointers, etc., which sounds ideal to me:

---

#include <iostream>
//#include <string> // I don't get why it compiles without this header

bool string_contains_integer(std::string str)
/*
    This function iterates through an array of chars,
    and checks each character to be a digit;
    optionally including a starting "+/-" sign.

    Returns true if the string contains a number (string of digits);
    Returns false if the string contains another character(s).

    Starting "+/-" gets ignored, as we accept all integer numbers.
*/
{
    // if the string is empty, return right away
    if ( str.empty() ) return false;

    // I'd like to avoid repeated length() calls
    unsigned long long string_length = str.length();

    // find out if there is a "+/-" sign
    bool sign_present = ( (str[0] == '-') || (str[0] == '+') );

    // check if there is the sign only
    if ( sign_present && (string_length == 1) ) return false;

    // iterate through all characters in the string
    for (unsigned long long i = 0; i < string_length; i++)
    {
        // skip the digit check for the sign at the beginning
        if ( (i == 0) && sign_present ) continue;

        // this is actually the core part checking on digits
        if ( ! std::isdigit( (unsigned char) str[i] ) ) return false;
    }

    // If we got here, then all the characters are digits,
    // possibly starting with a sign.
    return true;
}

int main(void)
{
    if ( string_contains_integer("-123456789123456789123456789123456789123456789123456789123456789") )
    {
        std::cout << "PASS: Input is a number.\n";
        return 0;
    }
    else
    {
        std::cerr << "FAIL: Input is not a number!\n";
        return 1;
    }
}

---

This program I compile as follows:

g++-8 -std=c++17 -Wall -Wextra -Werror -Wpedantic -pedantic-errors -o bigInteger bigInteger.cpp

Answer 1

You do need to include <string>. Your platform seems to bring it in as a side-effect of other includes, but you can't portably rely on that.

If there's no need to modify the contents of the string, prefer to pass by reference, to reduce copying:

bool string_contains_integer(const std::string& str)
//                           ^^^^^            ^

Instead of looping with indexes, learn to use iterators. If you really must use indexes, use the correct type (std::string::size_type, not unsigned long long).

We don't need to write our own loop by hand, as there's a standard algorithm that will do that for us; we just need to supply the correct predicate function:

#include <algorithm>
#include <iostream>
#include <string>

bool string_contains_integer(const std::string& str)
/*
    This function iterates through an array of chars,
    and checks each character to be a digit;
    optionally including a starting "+/-" sign.

    Returns true if the string contains a number (string of digits);
    Returns false if the string contains any other character(s).

    Starting "+/-" gets ignored, as we accept all integer numbers.
*/
{
    auto from = str.begin();
    auto to = str.end();

    if (from == to) {
        return false;
    }

    if (*from == '+' || *from == '-') {
        ++from;
    }

    return from != to
        && std::all_of(from, to,
                       [](unsigned char c){ return std::isdigit(c); });
}

Answer 2

In this self-mini-review, let it be clear, I just want to point out several things I do differently now. These were untouched by Toby Speight, so could be beneficial to someone. All of these are opinion-based!

Styling points

• I add spaces around parentheses now, like in this case:

  bool string_contains_integer ( const std::string & str )
  

• I write clearer function's function comments:

  // True : If the string contains an integer number (possibly starting with a sign).
  // False: If the string contains some other character(s).
  

• I no longer use redundant parentheses, like in this case:

  if ( i == 0 && sign_present ) continue;
  

• I put the function's function comment on top of the function:

  // True : If the string contains an integer number (possibly starting with a sign).
  // False: If the string contains some other character(s).
  bool string_contains_integer ( const std::string & str )
  

  because I found out there is a helper in Visual Studio Code - if If I hover the mouse over the function call anywhere in the code:

  

---

Compilation points

I use -Wc++11-compat flag to ensure compatibility with old compilers.

Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.

Even since I use -Wall already, it could be used explicitly in case you for any reason opt to remove -Wall. The compatibility flag could be useful.

---

Code points

I no longer call str.empty() as this was basically a redundant call of str.length():

std::size_t str_length = str.length();
if ( str_length == 0 ) return false;

---

Editor points

Further, I completely switched to Visual Studio Code, where it looks just great for a reader: