Unable to get Reverse of 5-digit number as output

Hi,


This is the first time that I'm posting on this forum. I just singed up today. I had done a course on Computer Application Development about 7-8 years ago. I just wanted to brush up on my coding skills. Thus, I started off with C.


As part of getting myself oriented I am doing the basic coding exercises. At this stage I am not supposed to use 'if-else' decision statements or loops.


The Exercise is to "Write a programme to reverse a 5-digit number input by the user." I have typed the following code in Turbo C. However, I'm not getting the expected result...


I know some people might say that Turbo C is dinosaur like. However, I'd like to know why I'm not getting the correct result, even though there are no errors found during compiling. Anyway, if I'm left with no choice, then, I might as well used a better IDE or something... Following is my code. Please help...

Code:

---

/* Program to reverse a 5-digit number */


#include<stdio.h>
#include<conio.h>


int main ()
{
  int num, n1,n2,n3,n4,n5,rev;
 
  clrscr();
  printf("\nPlease enter a 5-digit number: ");
  scanf("%d",&num);
 
  n1 = num % 10;
  num = num / 10;
 
  n2 = num % 10;
  num = num / 10;


  n3 = num % 10;
  num = num / 10;


  n4 = num % 10;
  num = num / 10;


  n5  = num;


  rev = n1*10000 + n2 * 1000 + n3 * 100 + n4 * 10 + n5;


  printf("\nReverse of the number is %d",rev);
  getch();
 
  return 0;
}

---

I wonder what exactly is the problem... I wonder what it is that is invisible to my eyes....


Thanks in advance.

If it doesn't reverse a number, what does it do? (That is to say, what behavior of the program makes you believe there is an error?)

It prints: "Reverse of the number is -11215" :frown:
I fail to understand why the values of n1, n2, n3, n4 and n5 do not remain the same.

I tried inserting printfs after each modulus operation, i.e.

(Let's assume that the 5-digit number input by user is 12345)

n1 = num %10;

printf("Value of n1 = %d",n1);

It prints: "Value of n1 is 5"

Now, this is what I want n1 to store. However, the value of n1 somehow seems to get modified at the final arithmetic operation stage, I suppose, i.e.

rev = n1 * 10000

at this above step....

Quote:

---

Originally Posted by Anish Kuriakose

It prints: "Reverse of the number is -11215" :frown:

---

What do you mean? It prints that no matter what you enter? What input did you give?

I'm sorry for creating a confusion. I assumed the input as 12345. Well, the problem is that no matter what input I give (I mean a valid 5 digit number) it's giving me a negative value... (e.g. -11215 or -11561 or something of that sort.) So, that simply means my code is wrong..., right? :frown:

The logic looks correct, and I am unable to replicate your problem when I tested with:

Code:

---

#include<stdio.h>

 
int main(void)
{
    int num, n1, n2, n3, n4, n5, rev;

    printf("\nPlease enter a 5-digit number: ");
    if (scanf("%d", &num) == 1)
    {
        n1 = num % 10;
        num = num / 10;

        n2 = num % 10;
        num = num / 10;

        n3 = num % 10;
        num = num / 10;

        n4 = num % 10;
        num = num / 10;

        n5  = num;

        rev = n1 * 10000 + n2 * 1000 + n3 * 100 + n4 * 10 + n5;

        printf("\nReverse of the number is %d\n", rev);
    }

    return 0;
}

---

Since you're getting a negative number this suggests to me that Turbo C is using 16-bit ints and you're getting a signed overflow; i.e. the range of a signed 16-bit integer is −32,768 to 32,767 (assuming 2s complement representation). Therefore if you enter any number > 32767 things are not likely to be what you expect.

Try this on your compiler:

Code:

---

#include<stdio.h>
 
int main ()
{
  int num, n1,n2,n3,n4,n5,rev;

  num =65535;

  printf("Number is %d\n",num);
 
  n1 = num % 10;
  num = num / 10;
 
  n2 = num % 10;
  num = num / 10;
 
 
  n3 = num % 10;
  num = num / 10;
 
 
  n4 = num % 10;
  num = num / 10;
 
 
  n5  = num;
 
 
  rev = n1*10000 + n2 * 1000 + n3 * 100 + n4 * 10 + n5;
 
 
  printf("\nReverse of the number is %d",rev);
 
  return 0;
}

---

If Turbo C is indeed using 16-bit ints then you'll get something along the lines of

Code:

---

Number is -1

Reverse of the number is -10000

---

I have no idea if Turbo C has limits.h but if it does (it should) check to see what the values of INT_MIN and INT_MAX are.

For example:

Code:

---

#include <stdio.h>
#include <limits.h>
 
int main(void)
{
    printf("Range is %d to %d\n", INT_MIN, INT_MAX);
    return 0;
}

---

For some reason, your compiler is using a 16-bit integer. 54321 is too big to properly fit and wraps around to a negative number. Look up "two's complement representation" for more info.

So, unless you're using a very old computer, you really do have to update that compiler!

Quote:

---

Originally Posted by laserlight

The logic looks correct, and I am unable to replicate your problem when I tested with:

---

It can be replicated using short rather than int ;)

In other words it does come down to TurboC being a dinosaur in this case.

Thank you so much Hodor. You and all the other users have pointed me to the right direction. You're right. I'm using the 16-bit compiler and that's the root cause of the problem. BTW, Turbo C does have limits.h and it does display the limits. Thank you all. I feel so glad that I'm now part of this great forum. So much of intellect. I should have joined long ago.

Lol ;)

Then replace 'int' declarations with 'long' to force it to use 32-bit integers. Use %ld in print format specifier.