Get a part of an integer in C

Question

I came from python and it was easy to get the middle digit of an integer, for example, from 897 I want the 9:

>>> num = 897
>>> num2 = int(str(num)[1])
>>> num2
9

But how i can do this on C? It's so difficult to convert into string...

`snprintf` to get the integer into a string, and `strlen` to help you calculate the "middle". Or if you know the number of digits, divide by `10` in a loop a couple of times, followed by a modulo by `10`. — Some programmer dude, Sep 07 '19 at 10:08
@Gianla The formula you showed does not extract the middle digit for an arbitrary number.:) — Vlad from Moscow, Sep 07 '19 at 10:08
Possible duplicate of [Converting int to string in C](https://stackoverflow.com/questions/5242524/converting-int-to-string-in-c) — Amaresh S M, Sep 07 '19 at 10:09
@Gianla By the way what is the middle digit of a number that contains two digits? — Vlad from Moscow, Sep 07 '19 at 10:10

score 3 · Accepted Answer · answered Sep 07 '19 at 10:09

Getting a single digit as an int can be done mathematically:

int num = 897;
int dig1 = (num /   1 ) % 10;
int dig2 = (num /  10 ) % 10;
int dig3 = (num / 100 ) % 10;

Division by one on the first line is only for illustration of the concept: you divide by n-th power of ten, starting with 10⁰ = 1.

score 1 · Answer 2 · answered Sep 07 '19 at 10:09

You can try the modulus operator (%).

In your case

num = 897

lastDigit = num%10; // Which returns 7

lastDigit/= 10;

secondLastDigit = lastDigit%10; //Which returns 9

Do this using a while loop for as long as you want. EG.

while(num>0)
{
    digit = num%10;
    num/=10;
}

Steve Summit · Answer 3 · 2019-09-07T13:17:02.013

The closest approximation in C to the given Python fragment, and its str operator, is sprintf/snprintf:

int num = 897;
char numstr[30];
sprintf(numstr, "%d", num);
char num2 = numstr[1];
int num2n = num2 - '0';
printf("%d\n", num2n);

To guard against buffer overflow, it's safer to get in the habit of using snprintf rather than sprintf:

snprintf(numstr, sizeof(numstr), "%d", num);

Instead of sprintf and %d, some systems provide an itoa() function (the opposite of atoi) for converting integers to strings, but it's not standard. For more information about converting numbers to strings, see Converting int to string in C.

This is "harder" in C than it is in Python for a couple of reasons. Constructing strings from numbers typically involves calls to snprintf, which is both more flexible and more of a nuisance than just calling str(). You typically have to worry about how big the destination array (here numstr) needs to be; this difficulty stems from C's lack of a first-class string type. And C has no convenience method for converting back and forth between digit characters and their values, so C programmers get used to adding and subtracting the constant '0' (or, if they like to make more work for themselves, 48).

(And the other part of the difficulty stems, I guess, from practicality. C was originally designed for writing operating systems and text editors and system utilities and things. How often do you need the middle digit of a number, anyway?)

Get a part of an integer in C

3 Answers3