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Lesson Learned:
1. When you define a local variable inside the loop {...}, it means that this variable will be initialized every time the loop executes.
2.

isdigit() and isalpha() can be used to check if the item falls in the range we expected.
3. When you wrote the loop using while, remember to advance the variable so that the loop can end:
while(*p!='\0')
{
    p++; //Remember to add this statement
}