Checking whether string is palindrome or not using Functions in C - c

Question

I want to check whether a string entered by the user is palindrome or not. Without using functions I have done this problem but when I use functions for this problem I am always getting the same output:
The Entered String Is Not Palindrome!
Even if the string entered by the user is palindrome I get the same output. Here is my code:
#include <stdio.h>
#include <string.h>
void reverse(char [],int);
int main() {
char a[100];int len, i = 0;
printf("Please enter the string to check: \n");
fgets(a,100,stdin);
//Loop to replace the '\n' inserted at the end of string due to fgets() with '\0'
for (int i = 0; i < strlen(a); ++i)
{
if (a[i] == '\n')
{
a[i] = '\0';
}
}
len = strlen(a);
reverse(a,len);
return 0;
}
void reverse(char b[100], int n) {
char c[100];int k=0;
//Read characters from b[] from the end and store them into c[]
for (int i = n-1; i >= 0; i--)
{
c[k] +=b[i];
k++;
}
c[k] = '\0';
//Check if the reversed string c[] and the actual string b[] are equal
if(strcmp(c,b) == 0)
printf("The Entered String Is Palindrome!\n");
else
printf("The Entered String Is Not Palindrome!\n");
}
The code in the reverse() function is the same as the one I used to solve the same problem without functions(and that program worked perfectly fine). But still it isn't giving the correct output. What am I doing wrong here?
EDIT: Okay, so I have removed the c[k] += b[i] according to an users suggestion and now it works perfectly fine. But I still don't get it. I mean I used the same line in the same program, the only difference being I didn't use the functions concept there and it worked perfectly fine. Here is the code:
#include <stdio.h>
#include <string.h>
int main() {
char a[100], b[100];
int k=0;
printf("Please enter the string: \n");
fgets(a,100,stdin);
//To replace the '\n' at the end of the string inserted by fgets()
for (int i = 0; i < strlen(a); ++i)
{
if(a[i] == '\n')
a[i] = '\0';
}
for (int i = strlen(a)-1; i >=0 ; i--)
{
b[k] += a[i];
k++;
}
if (strcmp(a,b) == 0)
{
printf("The entered string is palindrome!\n");
}
else
printf("The entered string is not palindrome! \n");
return 0;
}
Is there some concept behind it that I am unaware of? If so then please enlighten me.

Answer 1

You invoked undefined behavior by using values of an uninitialized variable having automatic storage duration, which is indeterminate.
Instead of adding in c[k] +=b[i];, just assign value from b as c[k] = b[i];.