Thread: Printing integers backwards

You can get each of the digits by dividing the number by ten. Take the remainder and that's the last digit of the number. The quotient is the rest of the digits. So make the quotient your 'new number', then loop back until your quotient is zero. For example:

num is initially 1234
Here's what happens:

digit = num % 10 //num == 1234, digit == 4
myarray[1] = digit
num = num / 10 //num now equals 123

digit = num % 10 //num == 123, digit == 3
myarray[2] = digit
num = num / 10 //num is now 12

digit = num % 10 //num == 12, digit == 2
myarray[3] = digit
num = num / 10 //num is now 1

digit = num % 10 //num == 1, digit == 1
myarray[4] = digit
num = num / 10 //num is now 0

//num is 0, so we're done. myarray now equals 4321.

char *_itoa( int value, char *string, int radix );

Code:

#include <stdlib.h>
#include <stdio.h>

#define NUM_DIGITS

char ch[NUM_DIGITS + 1]; 
int i = 10;

atoi(i,ch,10);

printf("%d as a string %s\n",i,ch);

reverse ch, and print it again.

The algorithm Tman gave, was what I meant. To convert the array to an integer, you could almost do the same. But instead of dividing, you should do multiplying with 10, so like this:

array [0] * 10^0 +
array [1] * 10^1 +
array [2] * 10^2 +
... +
array [n] * 10^n

Assume array has the following values:

array [0] = 1;
array [1] = 2;
array [2] = 3;
array [3] = 4;
array [4] = 5;

Then the calculation would be done as:

integer = 1 * 1 + 2 * 10 + 3 * 100 + 4 * 1000 + 5 * 10000 = 12345

[edit]
In programming this would be done as a loop, it could look like:

Code:

integer := 0;
for i from 0 to N do
    integer = integer + array [i] * 10^i;

[/edit]

Why make it so complicated with arrays when you can easily reverse an integer into another integer:

Code:

static unsigned reverse ( unsigned val )
{
  unsigned ret = 0;
  do
    ret = ( val % 10 ) + ret * 10;
  while ( ( val /= 10 ) != 0 );
  return ret;
}

By the way, if this is homework then you'd best understand how it works if you plan on turning it in. Teachers have a way of asking you how you came up with something if it looks like it was..."borrowed".

-Prelude

You could do it another way that none of you've shown...

Code:

sprintf( buf, "%d", mynumber );
for( x = strlen(buf-1); x > -1; x-- )
    printf("%c",buf[x]);

After all, the task is just to print the number reversed...

Quzah.