In C++, How should I write a function that takes in a number and outputs it in reverse order?

Question

Sorry, I just have programmers block today. What way do you recommend doing it yourself? pseudocode welcomed, actual code preferred. Thank you.

Note: By reverse order I mean like this,

input:
1234567

output
7654321

Answer 1

Both above posters suggest excellent methods (one numeric, one via string). Just for kicks, here's a recursive solution:



void reverse(int input) {

if (input < 10) { // if there's only one digit, we can just print this number and we're done

printf("%d", input);

return;

}



// Otherwise, we should print out the least significant digit first, which is input % 10

printf("%d", input % 10);



// Then, we should print out the rest of the number (i.e. input / 10) in reverse order.

// A recursive call can do this, because that is exactly what this function does

reverse(input / 10);



}

Answer 2

Peel the digits off the right of the input one by one and shift them onto the right of the output one by one, like this:



int rev(int in)

{

int out=0;

while(in>0)

{

out=(out*10)+(in%10);

in/=10;

};

return out;

}

Answer 3

I would do these steps:

- Convert the number into a string using something like sprintf()

- Use a string function like strrev() to reverse the string

- Then output that string using printf(), or if you want first convert the string back to a number using something like atoi()