Palindrome using recurison - c

Question

The following code needs to check for palindromes and write the user input backwards using recursion, and then find the Greatest Common Denominator using the same method. The code section that finds the gcd and tells whether or not it is a palindrome works, but it crashes every time it gets the the part where it reverses the code. Why is it crashing every time.
#include <stdio.h>
#include <stdlib.h>
/*Delare the prototypes*/
char palindromes(char[], int);
char backwards(char[], int, int);
int gcdfunc(int, int);
int findgcd(int, int);
int main ()
{
char userinput[10];/*Declares the character array for the input*/
int index= 0;/*Index for the counting loop*/
int counter= 0;/*Counts number of elements entered in the array*/
int printindex= 0;/*Index to print the values on the screen*/
int gcd= 0;/*Sets a value for the GCD*/
int palcheck = 0;
int value1= 0;/*User value 1*/
int value2= 0;/*User value 2*/
int flipindex=0;/*Sets an index for the gcd function*/
printf("Please enter a series of nine or less characters to test for a palindrome.\n");
scanf(" %9s%n", &userinput, &counter);
printf("\n");
palcheck = palindromes(userinput, counter-1);
if(palcheck == 0)
{
printf("Your input was not a palindrome \n");
}/*End of if statement*/
else
{
printf("Your input was a palindrome \n");
}/*End of else statement*/
backwards(userinput, counter-1, flipindex);
printf("Your input backwards is: ");
for(printindex; printindex <= counter; printindex++)
{
printf("%c", userinput[printindex]);
}/*End of printing backwards loop*/
printf("\n");
printf("\nEnter two numbers: ");
scanf("%d %d",&value1,&value2);
gcd=gcdfunc(value1, value2);
printf("The GCD of %d and %d is: %d",value1,value2,gcd);
system("pause");
}/*End of main function*/
char palindromes(char userinput[], int counter)
{
int palindex= 0;/*Declares the index to check for a palindrome*/
int palendinx= counter;
int modulus = counter%2;
if(modulus = 0)
{
if(userinput[palindex]==userinput[palendinx])
{
palindex++;
palendinx--;
if(palindex==(counter/2) && userinput[palindex]==userinput[palendinx])
return 1;
palindromes(userinput, counter);
}
else
{
return 0;
}
}
else
{
if(userinput[palindex]==userinput[palendinx])
{
palindex++;
palendinx--;
if(palindex==(counter/2) && userinput[palindex]==userinput[palendinx])
return 1;
palindromes(userinput, counter);
}
else
{
return 0;
}
}
}/*End of palidrome function*/
char backwards(char userinput[], int counter, int flipindex)
{
while(flipindex<(counter/2))
{
char temp;/*Sets a temporary value to swap the two values*/
temp = userinput[flipindex];
userinput[flipindex] = userinput[counter-flipindex];
userinput[counter-flipindex] = temp;
backwards(userinput, counter, flipindex++);
}
}/*End of reverse function*/
int gcdfunc(int value1, int value2)
{
int gcd;
gcd=findgcd(value1,value2);
return gcd;
}
int findgcd(int value1,int value2)
{
while(value1!=value2)
{
if(value1>value2)
return findgcd(value1-value2,value2);
else
return findgcd(value1,value2-value1);
}
return value1;
}

Answer 1

in your function there is no pass for the variables and it stays in the same cycle over and over and in recursion the same function does not share the same variables it create new one and if you modify one it does not affect the others variables.
char palindromes(char userinput[], int counter)
{
int palindex= 0;/*Declares the index to check for a palindrome*/
int palendinx= counter;
int modulus = counter%2;
if(modulus = 0)
{
if(userinput[palindex]==userinput[palendinx])
{
palindex++;
palendinx--;
if(palindex==(counter/2) && userinput[palindex]==userinput[palendinx])
return 1;
palindromes(userinput, counter);
}
else
{
return 0;
}
}
else
{
if(userinput[palindex]==userinput[palendinx])
{
palindex++;
palendinx--;
if(palindex==(counter/2) && userinput[palindex]==userinput[palendinx])
return 1;
palindromes(userinput, counter);
}
else
{
return 0;
}
}
}/*End of palidrome function*/
So here is the code i did
int palindromes(char userinput[], int counter,int index , int palendex)
{
if(userinput[palendex] == userinput[index])
{
if(counter%2 == 0 && ( index - palendex) == 1)
return 0;
if( index == palendex )
return 0;
else
return palindromes(userinput, counter , index-1 ,palendex+1 );
}
else
return 1;
return 1;
}
In your backward function is that it never leaves the cycle while and do a lot of instances that the memory available runs out. so I changed it to an if statement and it looks like this (and a few changes)
int backwards(char userinput[], int counter, int flipindex)
{
if(flipindex<(counter/2))
{
char temp;/*Sets a temporary value to swap the two values*/
temp = userinput[flipindex];
userinput[flipindex] = userinput[counter-flipindex-1];
userinput[counter-flipindex-1] = temp;
backwards(userinput, counter, flipindex+1);
}
return 1;
}

Answer 2

It looks like you go past the end of the buffer with this code:
for(printindex; printindex <= counter; printindex++)
{
printf("%c", userinput[printindex]);
}/*End of printing backwards loop*/
because printindex == counter and userinput[counter] is out of bounds. Try this for-statement instead:
for(printindex; printindex < counter; printindex++)

Answer 3

if(palindex==(counter/2) && userinput[palindex]==userinput[palendinx])
return 1;
palindromes(userinput, counter);
It call recursively condition in the if statement is not satisfied. but It will stack overflow repeated calls indefinitely because there is no change in the situation.