Had no idea this could get solved like this, thanks for the trick. Took me time to wrap my head around it and hopefully i get to use it in the future with problems that needs intricate solving!

thanks, I didn't know you could start the counting at an arbitrary number in enumerate.

Smart usage of formatting

Based on my understanding, it is a difference of stack variable vs heap variable.

char res[len + 1] would make a local variable on the stack.

There are two problems with this. 1) it is a local variable, so it can't be easily seen or used outside of the function. 2) it is on the stack, and the compiler needs to know ahead of time exactly how much space to put on the stack.

If you don't know ahead of time how much space you need, the variable must be allocated on the heap. char *res = malloc(len + 1); creates a space on the heap.

Since we don't know ahead of time how long the input DNA sequence is going to be, the program will only work if you allocate it on the heap.

Also, the test suite that "uses" our code expects that we allocated on the heap, so it will try to free().

Yup. Forgot that % operator formatting detail myself that it accepts tuples.

What does the % v specifically the % ...? Thanks !

Learned string formatting! Thanks :)

he need change the value of ret

the return must be a pointer to a (free-able) dynamically allocated string

@meonu, because x can become very large given large num