The most efficient way to reverse a number - c++

Question

I am looking for an efficient algorithm to reverse a number, e.g.
Input: 3456789
Output: 9876543
In C++ there are plenty of options with shifting and bit masks but what would be the most efficient way ?
My platform: x86_64
Numbers range: XXX - XXXXXXXXXX (3 - 9 digits)
EDIT
Last digit of my input will never be a zero so there is no leading zeros problem.

Answer 1

Something like this will work:
#include <iostream>
int main()
{
long in = 3456789;
long out = 0;
while(in)
{
out *= 10;
out += in % 10;
in /= 10;
}
std::cout << out << std::endl;
return 0;
}

Answer 2

#include <stdio.h>
unsigned int reverse(unsigned int val)
{
unsigned int retval = 0;
while( val > 0)
{
retval = 10*retval + val%10;
val /= 10;
}
printf("returning - %d", retval);
return retval;
}
int main()
{
reverse(123);
}

Answer 3

You may convert the number to string and then reverse the string with STL algorithms. Code below should work:
long number = 123456789;
stringstream ss;
ss << number;
string numberToStr = ss.str();
std::reverse(numberToStr.begin(), numberToStr.end());
cout << atol(numberToStr.c_str());
You may need to include those relevant header files. I am not sure whether it is the most efficient way, but STL algorithms are generally very efficient.

Answer 4

static public int getReverseInt(int value) {
int resultNumber = 0;
for (int i = value; i != 0;) {
int d = i / 10;
resultNumber = (resultNumber - d) * 10 + i;
i = d;
}
return resultNumber;
}
I think this will be the fastest possible method without using asm. Note that d*10 + i is equivalent to i%10 but much faster since modulo is around 10 times slower than multiplication.
I tested it and it is about 25 % faster than other answers.

Answer 5

int ans=0;
int rev(int n)
{
ans=(ans+(n%10))*10; // using recursive function to reverse a number;
if(n>9)
rev(n/10);
}
int main()
{
int m=rev(456123); // m=32
return 0;
}

Answer 6

//Recursive method to find the reverse of a number
#include <bits/stdc++.h>
using namespace std;
int reversDigits(int num)
{
static int rev_num = 0;
static int base_pos = 1;
if(num > 0)
{
reversDigits(num/10);
rev_num += (num%10)*base_pos;
base_pos *= 10;
}
return rev_num;
}
int main()
{
int num = 4562;
cout << "Reverse " << reversDigits(num);
} ``

Answer 7

// recursive method to reverse number. lang = java
static void reverseNumber(int number){
// number == 0 is the base case
if(number !=0 ){
//recursive case
System.out.print(number %10);
reverseNumber(number /10);
}
}

Answer 8

This solution is not as efficient but it does solve the problem and can be useful.
It returns long long for any signed integer(int, long, long long, etc) and unsigned long long for any unsigned integer (unsigned int, unsigned long, unsigned long long, etc).
The char type depends of compiler implementation can be signed or unsigned.
#include <iostream>
#include <string>
#include <algorithm>
template <bool B>
struct SignedNumber
{
};
template <>
struct SignedNumber<true>
{
typedef long long type;
};
template <>
struct SignedNumber<false>
{
typedef unsigned long long type;
};
template <typename TNumber = int,
typename TResult = typename SignedNumber<std::is_signed<TNumber>::value>::type,
typename = typename std::void_t<std::enable_if_t<std::numeric_limits<TNumber>::is_integer>>>
TResult ReverseNumber(TNumber value)
{
bool isSigned = std::is_signed_v<TNumber>;
int sign = 1;
if (value < 0)
{
value *= -1;
sign = -1;
}
std::string str = std::to_string(value);
std::reverse(str.begin(), str.end());
return isSigned ? std::stoll(str) * sign : std::stoull(str) * sign;
}
int main()
{
std::cout << ReverseNumber(true) << std::endl; //bool -> unsigned long long
std::cout << ReverseNumber(false) << std::endl; //bool -> unsigned long long
std::cout << ReverseNumber('#') << std::endl; //char -> long long or unsigned long long
std::cout << ReverseNumber(46) << std::endl; //int -> long long
std::cout << ReverseNumber(-46) << std::endl; //int -> long long
std::cout << ReverseNumber(46U) << std::endl; //unsigned int -> unsigned long long
std::cout << ReverseNumber(46L) << std::endl; //long -> long long
std::cout << ReverseNumber(-46LL) << std::endl; //long long -> long long
std::cout << ReverseNumber(46UL) << std::endl; //unsigned long -> unsigned long long
std::cout << ReverseNumber(4600ULL) << std::endl; //unsigned long long -> unsigned long long
}
Output
1
0
64
64
-64
64
64
-64
64
64
Test this code
https://repl.it/#JomaCorpFX/IntegerToStr#main.cpp

Answer 9

If it is 32-bit unsigned integer (987,654,321 being max input) and if you have 4GB free memory(by efficiency, did you mean memory too?),
result=table[value]; // index 12345 has 54321, index 123 has 321
should be fast enough. Assuming memory is accessed at 100 ns time or 200 cycles and integer is 7 digits on average, other solutions have these:
7 multiplications,
7 adds,
7 modulo,
7 divisions,
7 loop iterations with 7 comparisons
if these make more than 100 nanoseconds / 200 cycles, then table would be faster. For example, 1 integer division can be as high as 40 cycles, so I guess this can be fast enough. If inputs are repeated, then data will coming from cache will have even less latency.
But if there are millions of reversing operations in parallel, then computing by CPU is absolutely the better choice (probably 30x-100x speedup using vectorized compute loop + multithreading) than accessing table. It has multiple pipelines per core and multiple cores. You can even choose CUDA/OpenCL with a GPU for extra throughput and this reversing solution from other answers look like perfectly embarrassingly parallelizable since 1 input computes independently of other inputs.

Answer 10

This is the easiest one:
#include<iostream>
using namespace std;
int main()
{
int number, reversed=0;
cout<<"Input a number to Reverse: ";
cin>>number;
while(number!=0)
{
reversed= reversed*10;
reversed=reversed+number%10;
number=number/10;
}
cout<<"Reversed number is: "<<reversed<<endl;
}