弦回文功能

Question

got a quick question.

So I've tried to design a palindrome function in C in one of my "beginner" programs.

For those unaware what palindrome is, basically it is a set of characters (usually a word, but could also be number - although in this case specifically word) that is spelled the same way backwards and forwards.

Example of a palindrome - wow, lol, aaafaaa, ...

So you got the point. So I began with my function

int palindrome(char input[]){

So my presumption was, that ideally I'd want to run through the string with an index and compare it letter by letter.

int palindrome(char input[]){
 int start = 0, length = 0, end;
 /* Until we reach end of the word */
 while (input[start++] != '\0'){
   length++;

   for(start = 0, end = length - 1; start = length / 2; end--){
   /*If they do not match, return 0 */
     if (input[start] != input[end]){
        return 0;
        break;
     }
   }
 }
return 1;
}

That's how my palindrome function looks like. Now I only want to check user input from standard stdin.

So my main function looks like this

int main(){
char uInput[30];

/* Welcome user */
printf("Hello, please enter some text \n);
scanf("%29s", uInput);

if palindrome(uInput){
printf("The word: %s is a palindrome \n", uInput);
}

else {
printf("The word: %s is not a palindrome \n", uInput);
}

return 0;
}

So pretty straightforward code there, unfortunately, my results are

"The word (word) is not a palindrome"

regardless of whether it is a palindrome or not.

So there probably is something wrong with my function altogether. Also I am aware that this could be done through other libraries such as string.h and others, but I personally would prefer to do it this way as a form of exercise rather than using predefined functions.

So yeah, I've got a strong suspicion I'm not using my returns correctly in the function, but I'm not exactly sure what is the actual error with them.

Answer 1

Your idea is correct, there are just a few typos and omittings.

Your for loop is wrong, it should be:

for(start = 0, end = length - 1; start != length / 2; start++, end--)

instead of:

for(start = 0, end = length - 1; start = length / 2; end--)

And your while loop includes the whole for loop as well which is nonsense. It should be just:

while (input[start++] != '\0')
  length++;

and the } before return 1 should be removed.

And int start = 0 is not necessary as you initialize start at the beginning of the for loop anyway; int start is enough. But this is not really an error.

Answer 2

There are multiple errors in the palindrome function

We can do with a single loop and not a loop inside a loop. Also note the terminating condition start != (length / 2) of for loop and increment of both start and end .

There are also some compilation errors fixed. The complete code is below.

#include <stdio.h>
int palindrome(char input[]){
 int start = 0, length = 0, end;

 /* Until we reach end of the word */
 while (input[length] != '\0')
   length++;

 for(start = 0, end = length - 1; start != (length / 2); start++, end--){
 /*If they do not match, return 0 */
     if (input[start] != input[end]){
        return 0;
     }
 }

 return 1;
}

int main(){
char uInput[30];

/* Welcome user */
printf("Hello, please enter some text \n");
scanf("%29s", uInput);

if (palindrome(uInput)){
printf("The word: %s is a palindrome \n", uInput);
}

else {
printf("The word: %s is not a palindrome \n", uInput);
}
return 0;
}