recursion execution in reverse string - arrays

Question

#include <stdio.h>
#define MAX 100
char *reverse(char[]);
int main()
{
char str[MAX], *rev;
enter code here //printf("Enter a String: ");
scanf("%s", str);
rev = reverse(str);
printf("The reversed string is : %s\n", rev);
return 0;
}
char* reverse(char str[])
{
static int i= 0;
static char rev[MAX];
if (*str)
{
reverse(str+1);
rev[i++]= *str;
}
return rev;
}
how is the execution taking place in the function reverse......pls explain each step of the execution with example
thank you

Answer 1

The main point of the function reverse is recursion.
Recursive function as you know, are functions that call themselves from within their own code. Generally, each call got a smaller "problem" than the previous one.
Let's look at the function. We have two static variables that are static so as dxiv said in the comment, they will "keep" their value once they are initialized the first time. But in certain cases, "static" variables are also a problem to deal with.
Now we have an if that check whether we have still some character inside the string or not. That means that if we reach the end of the string, the piece of the code inside the if will not be executed.
Let's look at this example:
I added a printf to check the string content each time and as you see, the function goes to the last element of the string (if we think of it as an array) and once it reaches the end, it starts to do the next instruction.
So the function will call itself, increasing str time by time:
reverse(str + 1);
Once we reach the end of str, the program passes with the following instruction:
assign a char to the reverse array that in this case will start from zero since the counter variable i is still at 0.
The returning value is considered just at the end and not during the recursion "phase".
At this point the game is done, recursion function calls that are on the stack are going to be removed one by one, and each of them now can add the letter at the reverse array. Doing this i will increase and we are considering each time the letter before in the string.
In this way, in the end, we will have the reversed string.
Some tips:
Use fgets(..., ..., ...) instead of scanf so that you can take also sentences instead of just a word let's say. But there are still some other advantages, that I let you discover.
Generally, that returning value should be used also in the recursion process
Personally, I prefer to don't have static variables in a recursive function (unless I really need it). I prefer to have (if it is possible) the variables that I need as parameters.
I'm going to attach the code with fgets(...) right here below:
#include <stdio.h>
#include <string.h>
#define MAX 100
char *reverse(char[]);
int main() {
char str[MAX], *rev;
printf("Enter a String: ");
fgets(str, sizeof(str), stdin);
str[strcspn(str, "\n")] = '\0';
rev = reverse(str);
printf("The reversed string is: %s\n", rev);
return 0;
}
char *reverse(char str[]) {
static int i = 0;
static char rev[MAX];
if (*str) {
reverse(str + 1);
rev[i++] = *str;
printf("\n%s", str);
}
return rev;
}
I really hope that this can help you :D
Best regards,
Denny