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Lecture Notes: 04 More C

··2 mins

Write average program #

  • Read # of ints from argv[1]
  • Allocate stack array with dynamic array.
  • Read that # of ints in loop
  • Function to calculate average (pointer, size).

Programming in C #

  • Let’s write “prime”, which prints the Nth prime number.
  • Top-down design.
    • Allocate an array for N primes.
      • Show stack and heap allocation
    • Pre-load first 3 primes.
    • A number is-prime if it’s not divisible by any prime up to sqrt(x).
  • Makefile first.
    • gcc command
    • clean rule
    • tabs
  • Specifics:
    • Local variables are “on the stack”.
    • Can allocate arrays “on the stack”.
    • Standard library functions and headers:
      • stdio.h: printf
      • stdlib.h: atol
      • math.h: ceil, sqrt – show manpage, talk about libm
    • Casts. For numbers, these convert.
    • prime

Arrays and Pointers #

  • Arrays in C are just like arrays in ASM. They’re a contigous sequence of memory starting at some address.
  • Array variables store the address of the start of the array.
  • C knows how big the type stored in array is.
  • For arrays in ASM, we had to manually calculate the byte offsets.
  • An array of words in C would be xs[0], xs[1], etc.
  • Pointers are variables that store memory addresses with an associated type so “int*” (int pointer) is the address of an int.
  • C lets you do arthmetic with pointers.
    • If you add one to an int pointer, it steps to the next int-sized chunk of memory. So if p is a pointer to a 4 byte int, then (p + 1) is the address increased by 4 bytes.
    • Show sizeof: char, short, int, long, float, double, char*, long*
  • Array varibles and pointers are the same thing with different notation.
  • If pp is a pointer and aa is an array, both of the same type pointing to the same address, then pp[3] and *(pp + 3) are the same thing.
  • Strangely, pp[ii] and ii[pp] are the same thing too.
    • Why?