Function that reverse one part(half) of integer - c++

Question

I want to write a function to reverse one of two parts of number :
Input is: num = 1234567; part = 2
and output is: 1234765
So here is part that can be only 1 or 2
Now I know how to get part 1
int firstPartOfInt(int num) {
int ret = num;
digits = 1, halfDig = 10;
while (num > 9) {
ret = ret / 10;
digits++;
}
halfDigits = digits / 2;
for (int i = 1; i < halfDigits; i++) {
halfDigits *= 10;
}
ret = num;
while (num > halfDigits) {
ret = ret / 10;
}
return ret;
}
But I don't know how to get part 2 and reverse the number. If you post code here please do not use vector<> and other C++ feature not compatible with C

Answer 1

One way is to calculate the total number of digits in the number and then calculate a new number extracting digits from the original number in a certain order, complexity O(number-of-digits):
#include <stdio.h>
#include <stdlib.h>
unsigned reverse_decimal_half(unsigned n, unsigned half) {
unsigned char digits[sizeof(n) * 3];
unsigned digits10 = 0;
do digits[digits10++] = n % 10;
while(n /= 10);
unsigned result = 0;
switch(half) {
case 1:
for(unsigned digit = digits10 / 2; digit < digits10; ++digit)
result = result * 10 + digits[digit];
for(unsigned digit = digits10 / 2; digit--;)
result = result * 10 + digits[digit];
break;
case 2:
for(unsigned digit = digits10; digit-- > digits10 / 2;)
result = result * 10 + digits[digit];
for(unsigned digit = 0; digit < digits10 / 2; ++digit)
result = result * 10 + digits[digit];
break;
default:
abort();
}
return result;
}
int main() {
printf("%u %u %u\n", 0, 1, reverse_decimal_half(0, 1));
printf("%u %u %u\n", 12345678, 1, reverse_decimal_half(12345678, 1));
printf("%u %u %u\n", 12345678, 2, reverse_decimal_half(12345678, 2));
printf("%u %u %u\n", 123456789, 1, reverse_decimal_half(123456789, 1));
printf("%u %u %u\n", 123456789, 2, reverse_decimal_half(123456789, 2));
}
Outputs:
0 1 0
12345678 1 43215678
12345678 2 12348765
123456789 1 543216789
123456789 2 123459876

Answer 2

if understand this question well you need to reverse half of the decimal number. If the number has odd number of digits I assume that the first part is longer (for example 12345 - the first part is 123 the second 45). Because reverse is artihmetic the reverse the part 1 of 52001234 is 521234.
https://godbolt.org/z/frXvCM
(some numbers when reversed may wrap around - it is not checked)
int getndigits(unsigned number)
{
int ndigits = 0;
while(number)
{
ndigits++;
number /= 10;
}
return ndigits;
}
unsigned reverse(unsigned val, int ndigits)
{
unsigned left = 1, right = 1, result = 0;
while(--ndigits) left *= 10;
while(left)
{
result += (val / left) * right;
right *= 10;
val = val % left;
left /= 10;
}
return result;
}
unsigned reversehalf(unsigned val, int part)
{
int ndigits = getndigits(val);
unsigned parts[2], digits[2], left = 1;
if(ndigits < 3 || (ndigits == 3 && part == 2))
{
return val;
}
digits[0] = digits[1] = ndigits / 2;
if(digits[0] + digits[1] < ndigits) digits[0]++;
for(int dig = 0; dig < digits[1]; dig++) left *= 10;
parts[0] = val / left;
parts[1] = val % left;
parts[part - 1] = reverse(parts[part - 1], digits[part - 1]);
val = parts[0] * left + parts[1];
return val;
}
int main()
{
for(int number = 0; number < 40; number++)
{
unsigned num = rand();
printf("%u \tpart:%d\trev:%u\n", num,(number & 1) + 1,reversehalf(num, (number & 1) + 1));
}
}

Answer 3

My five cents.:)
#include <iostream>
int reverse_part_of_integer( int value, bool first_part = false )
{
const int Base = 10;
size_t n = 0;
int tmp = value;
do
{
++n;
} while ( tmp /= Base );
if ( first_part && n - n / 2 > 1 || !first_part && n / 2 > 1 )
{
n = n / 2;
int divider = 1;
while ( n-- ) divider *= Base;
int first_half = value / divider;
int second_half = value % divider;
int tmp = first_part ? first_half : second_half;
value = 0;
do
{
value = Base * value + tmp % Base;
} while ( tmp /= Base );
value = first_part ? value * divider + second_half
: first_half * divider +value;
}
return value;
}
int main()
{
int value = -123456789;
std::cout << "initial value: "
<< value << '\n';
std::cout << "First part reversed: "
<< reverse_part_of_integer( value, true ) << '\n';
std::cout << "Second part reversed: "
<< reverse_part_of_integer( value ) << '\n';
}
The program output is
initial value: -123456789
First part reversed: -543216789
Second part reversed: -123459876

Answer 4

Just for fun, a solution that counts only half the number of digits before reversing:
constexpr int base{10};
constexpr int partial_reverse(int number, int part)
{
// Split the number finding its "halfway"
int multiplier = base;
int abs_number = number < 0 ? -number : number;
int parts[2] = {0, abs_number};
while (parts[1] >= multiplier)
{
multiplier *= base;
parts[1] /= base;
}
multiplier /= base;
parts[0] = abs_number % multiplier;
// Now reverse only one of the two parts
int tmp = parts[part];
parts[part] = 0;
while (tmp)
{
parts[part] = parts[part] * base + tmp % base;
tmp /= base;
}
// Then rebuild the number
int reversed = parts[0] + multiplier * parts[1];
return number < 0 ? -reversed : reversed;
}
int main()
{
static_assert(partial_reverse(123, 0) == 123);
static_assert(partial_reverse(-123, 1) == -213);
static_assert(partial_reverse(1000, 0) == 1000);
static_assert(partial_reverse(1009, 1) == 109);
static_assert(partial_reverse(123456, 0) == 123654);
static_assert(partial_reverse(1234567, 0) == 1234765);
static_assert(partial_reverse(-1234567, 1) == -4321567);
}