C Programming Introduction Part II

1
C Programming IntroductionPart II

• Geb Thomas

2
Learning Objectives

• Learn how to use C functions
• Learn how to use C arrays
• Understand what a pointer is and how to use it
• Understand how to open a file and write to it.

3
C Functions

• Generally, you pass variables to a function, it
  does some work, and returns a variable.
• double Sqrt(double val) a function that takes a
  value and returns its square root.
• double pow(double v1, double v2) a function
  that raises v1 to the power v2.
• Sometimes you dont need or want a return value
  (such as the function to close a file).
• Sometimes you dont need to pass anything to the
  function or get anything back (such as a function
  to clear the monitor of any data).

4
Declaring C Functions

• All functions should be declared at the top of
  the program (before they are used).
• int count(void)
• int openFile(char fileName)
• The declaration tells the compiler how the
  function should be used -- what it should take
  and what it should return.

5
Defining Functions

• The function is defined in the body of the
  c-file, outside of any other functions.
• The definition explains what the function does
  with the information passed in and how it
  determines the information to return.
• int factorial(int inVal) / start of function
  definition /
• int cntr, returnVal 1 /declaration of
  local variables /
• for (cntr 2 cntr lt inVal cntr) / note
  use of inVal /
• returnVal returnValcntr
• return returnVal / function will return this
  value /
• / end of function definition /

6
Using Functions

• To use a function, just pass variables or
  arguments in the correct order.
• You can use the return value to set another
  variable.

int main(int argc, void argv) int a 4,
b b factorial(a) b factorial(6)
7
C Arrays

• C uses square brackets for defining and using
  arrays.
• double F6 declares a six-dimensional vector.
• The six values in f are ordered starting from 0.
• F0 3 /set 1st element to 3. /
• F3 14.1234 / set 4th element to 14 /
• F6 234.341 / error, this would be the 7th
  element in a six-dimensional vector /
• You can pass element values to functions
• F4 sqrt (F3)

8
Pointers

• C allows the programmer to access specific
  locations in memory with pointers.
• Pointers user two operators
• the address of operator
• the indirection or dereferencing operator

9
How Pointers Work

• Instead of referring to a variable directly,
  pointers provide the location where the variable
  exists in memory.
• This is useful when you want to change a variable
  in memory within a function.
• Normally a function copies its arguments and
  changes to the passed arguments to not affect the
  main program.
• If you pass a pointer, then when you change the
  values at the address pointed to, the changes DO
  affect the main program.

10
How to Use Them

• Declare a pointer to a float
• float floatPtr
• Declare a pointer to a double
• double dblPtr
• Set the pointer to a variable.
• floatPtr myFloatVariable
• Set the value in the pointer
• floatPtr 6
• Use the value in a pointer
• printf(Pointer value is f\n, floatPtr)

11
File Access

• FILE fptr / pointer to a file structure /
• fptr fopen(filename, r)
• fprintf(Print this f, baby\n, val)
• fscanf(f, val) / reads a value /
• fclose(fptr)

12
Learning Objectives

• Learn how to use C functions
• Learn how to use C arrays
• Understand what a pointer is and how to use it
• Understand how to open a file and write to it.