Find palindromes in sentence - c

Question

I am trying to write a piece of C code that takes a sentence and returns all the palindromes in that sentence, each in a new line. For example, the sentence "I like to race a civic racecar" would return:
civic
racecar
I've tried to use some debugging software (lldb, as I'm a mac user), but found it a bit confusing. The code below is what I have written. It's returning a segmentation fault, and I'm having trouble identifying it within my program.
int is_palin(char c[], int length)
{
int front = 0;
int back = length - 1; /* account for length starting at 0 */
if (length % 2 == 0){ /* check for even palindromes */
int middle = (length /2) -1 ;
while (front< middle + 1){
if (c[front] != c[back]){
return 0;}
front = front + 1;
back = back -1;
}
}
else { /* check for odd palindromes */
int middle = ((back - 2) / 2 ) + 1;
while (front != middle){
if (c[front] != c[back]){
return 0;}
front = front + 1;
back = back -1;}
}
return 1;
}
int is_delimiting_char(char ch)
{
if(ch == ' ') //White space
return 1;
else if(ch == ',') //Comma
return 1;
else if(ch == '.') //Period
return 1;
else if(ch == '!') //Exclamation
return 1;
else if(ch == '?') //Question mark
return 1;
else if(ch == '_') //Underscore
return 1;
else if(ch == '-') //Hyphen
return 1;
else if(ch == '(') //Opening parentheses
return 1;
else if(ch == ')') //Closing parentheses
return 1;
else if(ch == '\n') //Newline (the input ends with it)
return 1;
else
return 0;
}
/////////////////////////////////////////////
//---------------------------------------------------------------------------
// MAIN function
//---------------------------------------------------------------------------
int main (int argc, char** argv) {
char input_sentence[100];
int i=0;
char current_char;
int delimiting_char;
char word[20];
int word_length;
int have_palindrome = 0;
/////////////////////////////////////////////
/////////////////////////////////////////////
/* Infinite loop
* Asks for input sentence and prints the palindromes in it
* Terminated by user (e.g. CTRL+C)
*/
while(1) {
i=0;
print_char('\n');
print_string("input: ");
/* Read the input sentence.
* It is just a sequence of character terminated by a new line (\n) character.
*/
do {
current_char=read_char();
input_sentence[i]=current_char;
i++;
} while (current_char != '\n');
/////////////////////////////////////////////
print_string("output:\n");
int char_index = 0;
for(int k=0; k<i; k++) {
palin = 1;
current_char = input_sentence[k];
delimiting_char = is_delimiting_char(current_char);
if(delimiting_char) {
if (char_index > 0) { //Avoids printing a blank line in case of consecutive delimiting characters.
word[char_index++] = '\n'; //Puts an newline character so the next word in printed in a new line.
word_length = word_length + 1;
if (is_palin(word, word_length) && word_length > 1){
have_palindrome = 1;
for(int j=0; j<char_index; j++) {
print_char(word[j]);
}
word_length = 0;
char_index = 0;
}
} }
else {
word[char_index++] = current_char;
word_length = word_length + 1;
}
}
if (have_palindrome == 0){
print_string("Sorry! No palindromes found!"); }
}
return 0;
}
Also wondering if anyone has good videos or sites for learnign how to use lldb, when one has never used anything of the sort before. Thanks!

Answer 1

There are several things wrong here:
word_length is uninitialised at first use, so statements like word_length = word_length + 1 lead to undefined behaviour. In fact, you have two different variables, char_index and word_length, that should always have the same value. Instead of going through the hassle to keep them in sync, use just one variable.
You reset both char_index and word_length to zero only if a palindrome was found. You should reset if after every word, of course.
The line palin = 1; is probably a leftover from older code. You should also reset have_palindrome after each line. In general, you should take more care when defining variables.
By adding a newline to your word you make printing a bit easier, but you will never find a palindrome, because the newline at the end is taken into account when checking for the palindrome.
Your code for reading with read_char, which is probably an alias to getchar, needs to check for the end of input.
You don't need to distinguish between even and odd sized palindromes. Just make the condition that front < back and be done with it. The middle character of an odd sized palindrome doesn't matter. (That's not an error, your code is just needlessly complicated.)