Palindrome Program - c

Question

I've created a program that takes an inputted string and outputs the string backwards. I've created a function that counts the number of characters in a string. With that information, I've created another function that puts the first character of the original array at the end of the new array. Thus, when the loop in the function has finished, the new array contains the letters of the original array but backwards. I'm running into some difficulty however. I'm still wary of character arrays and I'm not sure if I'm using them correctly. Could someone help me out?
#include <stdio.h>
#define MAXLINE 1000
char backward(char str[], int count)
int count(char str[])
int main() {
char orig[MAXLINE];
int c;
scanf("%s", orig);
c = count(orig);
printf("%s", backward(orig, c));
return 0;
}
char backward(char str[], int count) {
char back[MAXLINE];
int i;
for(i = 0; i <= count; ++i) {
back[count] = str[i];
--count;
return back;
}
int count(char str[]) {
int i;
for (i = 0; str[i] != '\0'; ++i)
;
return i;
}
Edit: Input: Let's say "minivan". Expected output would be "navinim". There's no actual output yet. I'm getting this error:
palindrome.c:8: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘int’

Answer 1

The error you're getting is because your prototypes are wrong:
#include <stdio.h>
#define MAXLINE 1000
char backward(char str[], int count) /* missing ; */
int count(char str[]) /* missing ; */
int main() {
You should also consider replacing count() with strlen(3) from the <string.h> header file. The standard library is full of utilities that have been debugged and known to work well -- it is worth using them. (Yes, there's educational value in re-implementing them. But in that case, be prepared to test with a variety of corner cases.)

Answer 2

Your code probably won't compile because you missed a closing brace in backward().
char backward(char str[], int count) {
char back[MAXLINE];
int i;
for(i = 0; i <= count; ++i) {
back[count] = str[i];
--count;
/** You should add a } character here! **/
return back;
}
MAJOR EDIT 1:
I tried to compile your code and there were lots of things wrong with it. Here is working code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAXLINE 1000
char * backward(char * str);
int main() {
char orig[MAXLINE];
int c;
scanf("%s", orig);
printf("%s\n", backward(orig));
return 0;
}
char * backward(char * str) {
int count = strlen(str);
char * back = malloc(MAXLINE);
int i;
for(i = 0; i < count; i++)
{
back[count-i-1] = str[i];
}
back[count] = 0; // Add the null termination character.
return back;
}
When I run this code and enter "abcde", it outputs "edcba". Let me know if you have any specific questions about how this works. I got rid of your count() function because it is a duplicate of strlen(), and I moved the call to strlen() inside the backward() to make the interface to the backward() function simpler.
Another thing that made your code fail is that you were decrementing count inside the for loop, which would cause your for loop to terminate earlier than you wanted it to.

Answer 3

This line has an off by one mistake:
back[count] = str[i];
count is not the last character of the string, it's the '\0' character.
Also you are returning a stack variable - this is not going to work.
You should reverse the string in place:
for(i = 0; i <= count; ++i) {
--count;
char tmp = str[i];
str[i] = str[count];
str[count] = tmp;
}
You can then return str.

Answer 4

You program have lot of errors. Firstly you have missed out semicolons(;) while declaring the prototypes for backward and count. Then you haven't closed the for loop in backward function with ending braces(}).
But the biggest three problems are
1. You have defined backward function as a char data type but you are returning a pointer to the array of char data type
2. You are accessing back[count] instead of back[count-1]. Suppose the string is minivan. The count will hold 7. the last character is at 6th position not 7th because in array subscripts starts with 0 not 1. And back[count] will point to null character i.e '\0'
3. You are incrementing i and decrementing count and checking i<=count. This will process your string only till mid of it.
Although the right program has been posted by David Greyson, but since he have removed the count function and introduced the strlen and malloc function, I am posting your program again by just correcting your algorithm
#include <stdio.h>
#define MAXLINE 1000
char *backward(char str[], int count);
int count(char str[]);
int main() {
char orig[MAXLINE];
int c;
scanf("%s", orig);
c = count(orig);
printf("%s", backward(orig, c));
return 0;
}
char *backward(char str[], int count) {
char back[MAXLINE];
int i;
for(i = 0; i < count; ++i) {
back[count-i-1] = str[i];
}
back[i] = '\0';
return back;
}
int count(char str[]) {
int i;
for (i = 0; str[i] != '\0'; ++i)
;
return i;
}

Answer 5

You can create a character pointer then add length of string to this pointer, it should likely char *reverse = (char *)name, then add reverse = reverse + strlen(name) -1.
Then you can use while loop through through the start of string. The whole code looks,
void main(void)
{
char *name = "Kernel";
char *reverse = (char *)name;
reverse = reverse + strlen(name) - 1;
while(*reverse)
printf("%c", *reverse);
}

Answer 6

#include<Stdio.h>
#include<string.h>
int main()
{
size_t i,j;
char arr[10]={'\0'};//orignal array
char arr2[10]={'\0'};//it's reverse to be
gets(arr);//prompt
strncpy(arr2,arr,10);//copy
strrev(arr2);
if(strncmp(arr,arr2)==0)
printf("plaindrom");
return 0;
}