I am trying to convert octal number to decimal number using a string as an input but the answer is c-CodePudding

This is the given code. I am accessing each element of the string and trying to multiply it with powers of 8 but, the result is coming out to be wrong.

#include<iostream>
#include<algorithm>
#include<math.h>
using namespace std;

int od(string s)
{
    reverse(s.begin(),s.end());
    
    int ans=0;
    
    for(int i=0;i<s.length();i  )
    {
        ans=ans s[i]*pow(8,i);
    }
    
    return ans;
}

int main()
{
    string s;
    cin>>s;
    
    cout<<od(s);
    
    return 0;
}

CodePudding user response：

The problem lies in the line:

 ans=ans s[i]*pow(8,i);

Here s[i] is a character. When numerical operations are performed with it, its ASCII value is taken. So here's an easy fix:

 ans=ans ((s[i]-'0'))*pow(8,i);

This will give you the correct answers for obvious reasons.

CodePudding user response：

This code is the issue

ans=ans s[i]*pow(8,i);

string characters (chars) are ASCII, so "0" in ASCII is actually 48. ASCII mappings are here https://en.cppreference.com/w/cpp/language/ascii

I suppose you could check each character and map it to the decimal value, or you could use something like std::stoi

std::stoi(myoctalstring, nullptr, 8);

https://en.cppreference.com/w/cpp/string/basic_string/stol

CodePudding user response：

There are two issues: the string's elements are digits, not numbers ('0' is not the same thing as 0), and pow is a floating-point operation, which you should not use in conjunction with integers.

You can get the number that corresponds to a digit by subtracting '0' from it, and you can keep track of the current power of eight as you loop.

int od(string s)
{
    reverse(s.begin(), s.end());
    int ans = 0;
    int power = 1;
    for(int i = 0; i < s.length(); i  )
    {
        ans = ans   (s[i] - '0') * power;
        power *= 8;
    }   
    return ans;
}

but it's actually simpler to not reverse the string and multiply the temporary result on each step instead:

int od(string s)
{
    int ans = 0;
    for(int i = 0; i < s.length(); i  )
    {
        ans = ans * 8   s[i] - '0';
    }   
    return ans;
}

CodePudding user response：

You need to use atoi or _ttoi, something like atoi(s[i]).