string reverse using recursion - c

Question

can someone please tell me what is the problem in my code?
it is showing some strange output.
#include<stdio.h>
#include<string.h>
void reverse(char string[])
{
char b[200];
int t;
t = strlen(string);
if (t==1)
printf("%c",string[0]);
else
{
printf("%c",string[t-1]);
for (int i=0;i<t-1;i++)
b[i]=string[i];
reverse(b);
}
}
int main()
{
char a[200];
scanf("%s",&a);
reverse(a);
return 0;
}

Answer 1

If you had tried to use a debugger, you would see that t is going mad on second iteration. This is because after you have copied string into b you forgot to insert \0 symbol at the end (position with index t-1). This causes t become literally anything on the next iteration because of strlen() needs a null-terminating string and it results in an undefined behaviour as mentioned in docs:
The behavior is undefined if str is not a pointer to a null-terminated
byte string
So a quick fix is as folows:
...
for (int i=0;i<t-1;i++)
{
b[i]=string[i];
}
b[t-1] = '\0';
reverse(b);
...
And as already mentioned in comments by #LPs : change scanf("%s",&a); to scanf("%199s",a); (199 because we need to leave a space for '\0' at the end, thanks to #RoadRunner for noticing that)
Note: take a look at strncpy_s (if you use C11) and use it instead of that for loop:
printf("%c",string[t-1]);
strncpy_s(b, 200, string, t-1); // 200 because char b[200]
reverse(b);
or strncpy:
printf("%c",string[t-1]);
strncpy(b, string, t-1);
b[t-1] = '\0';
reverse(b);
Still another approach is not to copy:
else
{
string[t-1] = '\0'; // you don't need array 'b' at all
reverse(string);
}
And the simpliest way is just to use a loop:
for (int i = strlen(string) - 1; i >= 0; --i)
{
printf("%c", string[i]);
}

Answer 2

You mix up two approaches, i.e. recursive and loop, in one function. Further, for me it remains unclear whether you want to change the input string to a reverse order, or if you just want to print it out in reverse order.
Anyway, see a solution that provides loop bases reverse printing and recursion based reverse printing; hope it helps in understanding the steps the code such goes through:
#include <stdio.h>
#include <string.h>
void reverse_loop(char string[]) {
long i = strlen(string) - 1;
while (i >= 0)
printf("%c",string[i--]);
}
void reverse_recursive (char string[]) {
if (string[0] != '\0') {
reverse_recursive(&string[1]);
printf("%c", string[0]);
}
}
int main()
{
char a[200] = { "some test string" };
reverse_recursive(a);
return 0;
}

Answer 3

The easiest way to reverse a string using recursion is like this:
// C++ program to reverse a string using recursion
# include <stdio.h>
# include<string>
using namespace std;
string in="";
/* Function to print reverse of the passed string */
void reverse(char *str)
{
if (*str)
{
reverse(str+1);
in=in+*str; /* If you want to save the reversed string */
printf("%c", *str); /*If you just want to print the reversed string */
}
}
/* Driver program to test above function */
int main()
{
char a[] = "Reversing a string using recursion";
reverse(a);
printf("\n%s\n",in.c_str());
return 0;
}
/* output : noisrucer gnisu gnirts a gnisreveR */
Explanation:
Recursive function (reverse) takes string pointer (str) as input and calls itself with next location to passed pointer (str+1). Recursion continues this way, when pointer reaches ‘\0’, all functions accumulated in stack print char at passed location (str) and return one by one.
Time Complexity: O(n)