Finding palindromes in C

Question

I am new to C language and would like to understand the code. The standard input stream is given a string consisting of words with a maximum length of 100 characters. Words consist of Latin characters and are separated by one space. Write to standard output a string containing only palindromes. A palindrome is a word that reads the same in both directions. For example, for input: dad peep aaas. To the exit: dad peep

#include <stdio.h>
#include <ctype.h>
#include <string.h>

int main(void) {
    int i = 0;
    char word[100];
    int ch, bk, fr;

    while (EOF != (ch = getchar())) {
        if (isspace(ch) != 0) {
            if (i > 0) {
                fr = 0;
                bk = i - 1;
                while (word[fr] == word[bk]) {
                    ++fr;
                    --bk;
                }
                if (fr < bk)//not palindromes
                    i = 0;
                else {
                    while (i > 0)
                        printf("%c", word[--i]);
                    printf("%c", ch);
                }
            }
            if (ch == '\n')
                break;
        }
        else {
            word[i++] = tolower(ch);//check i < sizeof(word)
        }
    }
}

It is not clear to me how the algorithm selects the words we need. I would like to know this step by step due to my extreme inexperience.

Answer 1

The code is bad and has undefined behavior.

For example let's assume that the input consists from one character 'A' and a space "A ". The word "A" is a palindrome.

But in this while loop

while (word[fr] == word[bk]) {
    ++fr;
    --bk;
}

where initially fr is equal to 0 and bk is equal to 0 due to these assignments (I think fr means front and bk means back)

if (i > 0) {
    fr = 0;
    bk = i - 1;
    //...

there will be access to memory beyond the array word. That is in the first iteration of the while loop the condition evaluates to the logical true.

word[fr] == word[bk]

that is equivalent to

word[0] == word[0]

So the variable bk will be decremented. And in the next iteration of the loop the condition will look like

word[1] == word[-1]
           ^^^^^^^^

My advice: do not search a bad code in the internet and do not spend your time to understand it.

As for you question then within this while loop

while (EOF != (ch = getchar())) {

characters are being written in the character array word starting from the position i equal to 0 until a white space character is encountered.

    if (isspace(ch) != 0) {
        //...
    }
    else {
        word[i++] = tolower(ch);//check i < sizeof(word)
    }

If a white space is encountered

    if (isspace(ch) != 0) {

then the variable i points to the position in the array word after the last written character and the program checks whether the sequence of characters stored in word of the length equal to i is a palindrome.