answer.code: In function ‘main’:
answer.code:94:14: warning: implicit declaration of function ‘is_index_in_array’ [-Wimplicit-function-declaration]
         if (!is_index_in_array(ret_, ret_size, i) && !is_index_in_array(ret_, ret_size, j)) {
              ^~~~~~~~~~~~~~~~~
answer.code:39:5: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d", &n);
     ^~~~~~~~~~~~~~~
answer.code:42:9: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
         scanf("%d", &arr[i]);
         ^~~~~~~~~~~~~~~~~~~~
/usr/bin/ld: /tmp/ccMkkmpj.o: in function `main':
answer.code:(.text.startup+0x23c): undefined reference to `is_index_in_array'
/usr/bin/ld: answer.code:(.text.startup+0x251): undefined reference to `is_index_in_array'
collect2: error: ld returned 1 exit status