I can only think of ways to do credit.h using functions from string.h and math.h, but they're not covered in the week's lecture. Are we allowed to use them?
You are allowed to use any c resources that you wish. This applies more and more in the later psets.
However, in the early exercises and lectures, everything you need will usually be presented. You might want to consider whether the resource you want to use is a good idea, is it a better solution, or should you focus on implementing the pset with what has been presented in order to master a technique.
Having said that, also remember that there is never just one right solution. Some may be "more right" or more efficient than others. ;-)
If this answers your question, please click on the check mark to accept. Let's keep up on forum maintenance. ;-)