Recursive function to calculate successor of a numeric string - c

Question

I want to write a recursive function which calculates Succ(‘2468’) = '2469'. ‘2468’ is a numeric string.
The exercice give me some predefined function such as last(ch), which returns the last caracter of a string, start(ch), returns ch without the last caracter, addEnd(ch, c), adds c at the end of ch and ask me to return a string as final result (i.e., suc("123")="124")
I tried this code but it works only for a string with 2 characters. If the length of my string is > 2, it doesn't work:
int successor (char*ch)
{
if (strlen (ch)==1)
return (int(*ch))+1);
else
return ((int(*ch))*10+successor(ch+1));}

Answer 1

There seems to be no need for multiplication or use of powers. Given the extra predefined functions you indicated were provided, I think the intention here was for a recursive way to express long addition with carry. I don't know C but here's an illustration in JavaScript that has very close syntax. I hope this helps.
function successor(s){
if (s.length == 1){
if (s == '9'){
return '10';
} else {
// Return the successor character,
// not sure how this looks in C.
return String.fromCharCode(
s.charCodeAt(0) + 1);
}
}
let rightmost = successor(last(s));
// No carry so just return
// the string with the last
// character incremented
if (rightmost != '10'){
return addEnd(start(s), rightmost);
// We have a carry so
// continue the recursion
} else {
return addEnd(successor(start(s)), '0');
}
}
function last(s){
return s.substr(-1);
}
function start(s){
return s.substr(0, s.length - 1);
}
function addEnd(s, c){
return s + c;
}
console.log(successor('2999'));

Answer 2

A key problem is this logic:
(int(*ch))*10+successor(ch+1)
the multiplication by 10 is insufficient for larger numbers. We need to multiply by a power of 10 and we already calculated that power but didn't hang onto it:
strlen (ch)
or more specifically:
strlen(ch) - 1
A complete solution:
#include <math.h>
#include <stdio.h>
#include <string.h>
#define digit(c) (c - '0')
int successor(char *string)
{
size_t power = strlen(string) - 1;
if (power == 0)
{
return digit(*string) + 1;
}
return digit(*string) * pow(10, power) + successor(string + 1);
}
int main() {
printf("%d\n", successor("2999"));
return 0;
}
OUTPUT
> ./a.out
3000
>
TODO
What happens if successor() is passed an empty string:
printf("%d\n", successor(""));
How can you modify the code to fix this? First decide what the function should return in this situation. What happens if successor() is passed a string that represents a number too large to be contained in an int:
printf("%d\n", successor("8589934592"));
How can you modify the code to fix this? Again, first decide what the function should return in this situation.