Why I am not getting the number from string

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i am trying to take a number as string input and convert it to the number but its showing a little error cann't understand what to do..suppose the input was 123456789
and my code showing output as 123456785 change in the last digit..help please..

What I have tried:

C++

#include <iostream>
#include <cmath>
using namespace std;

int main()
{

    string s;
    while(cin>>s)
    {
        long long number=0;
        int len=s.size();
        for(int i=len-1,j=0;i>=0;i--)
        {
            int digit=s[i]-'0';
            number+=digit*pow(10,j);
        j++;
        }
        cout<<number<<endl;
      
    }

    //cout << "Hello world!" << endl;
    return 0;
}

Posted 19-Apr-16 21:46pm

Member 12270086

Updated 19-Apr-16 22:34pm

Patrice T

v4

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Comments

Richard MacCutchan 20-Apr-16 3:07am

That should not even compile since pow does not accept or return ints.

Jochen Arndt 20-Apr-16 3:13am

The VS math.h header contains a template:

_Ty _Pow_int(_Ty _X, int _Y)

which is used by some inline functions including:

inline double __cdecl pow(int _X, int _Y) {return (_Pow_int(_X, _Y)); }

Richard MacCutchan 20-Apr-16 3:34am

The sample above does not include that header, so std::pow is implied.

Jochen Arndt 20-Apr-16 3:40am

There may have been cmath or math.h behind the empty #include (it has been added now by ppolymorphe).

Even with std::pow there are versions with exp as integer (since C++11) and a promotion for other types which may have been already included with the used development environment.

Richard MacCutchan 20-Apr-16 5:17am

Not according to https://msdn.microsoft.com/en-us/library/dt5dakze.aspx.

But this is all academic anyway, as none of us actually knows what the OP did in his/her real test.

Patrice T 20-Apr-16 3:16am

Is it actual code ?
For me, this should not even compile.

Jochen Arndt 20-Apr-16 3:21am

The <cmath> file part seems to have vanished during copy & paste (interpreted as tag).

Patrice T 20-Apr-16 3:38am

Where did you declare the j variable ?

Jochen Arndt 20-Apr-16 3:44am

Inside the loop where i is also declared:

for(int i=len-1,j=0;i>=0;i--)

Carlo and I have compiled the original source with success after adding cmath.

2 solutions

Solution 1

You code works fine on my Lubuntu box (g++ 4.7.3).
You may also write it this way

C++

 #include <iostream>
 #include <cmath>
using namespace std;

int main()
{
  string s;
  while ( cin >> s)
  {
    long long number = 0;
    for (const auto & c : s)
    {
      number *= 10;
      number += c -'0';
    }
    cout << number << endl;
  }
}

Posted 19-Apr-16 21:59pm

CPallini

v2

Comments

Member 12270086 20-Apr-16 3:02am

what does for loop codition meaning

CPallini 20-Apr-16 3:09am

It is the range-based for (similar to other languages foreach construct), see
http://en.cppreference.com/w/cpp/language/range-for

Sergey Alexandrovich Kryukov 20-Apr-16 3:39am

5ed.
—SA

CPallini 20-Apr-16 3:48am

Thank you.

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Jochen Arndt · Accepted Answer · 2016-04-19T22:09:00

I just tested it with GCC (g++) on Ubuntu and it is showing the correct result.

Your wrong output indicates that there is probably a rounding error which may be introduced by using the pow() function. Depending on your compiler and the math.h / cmath header file it may use floating point or a template.

[EDIT]
When using VS there is a _Pow_int template used with inline functions in the math.h header file:

inline double __cdecl pow(int _X, int _Y)
        {return (_Pow_int(_X, _Y)); }

So the power is caluclated from integers and the returned result is converted to double. This result is then converted to long long. These two conversions may lead to rounding errors.
[/EDIT]

But you can avoid using pow() by rewriting your function:

long long number=0;
int len = s.size();
for (int i = 0; i < len; i++)
{
    int digit=s[i] - '0';
    number *= 10;
    number += digit;
}
cout<<number<<endl;

Why I am not getting the number from string

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Why I am not getting the number from string

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