Introduction to Functions

• Functions are also sometimes called subroutines, or procedures.
• Functions are really just small programs
• Functions allow us to break the problem into smaller parts.
• This allows us to
  • Focus our problem solving on a smaller problem.
  • Focus our programming on a smaller problem.
  • Focus our debugging on a smaller amount of code.
  • Work in groups.
  • reuse code.
• We have seen functions all semester
  • regular functions: pow, sqrt, ...
  • class member functions: setw, size, ...
• There is little difference between these, but we will start with regular functions.
• Transferring information to/from functions is done
  • via parameters
  • setw(4) in this case, 4 is a parameter
  • or can be a return value
  • rand() returns an int
• Different functions take different parameters depending on what is to be doe
  • celif just takes the value you want to find the ceiling of
  • pow takes both the base and the exponent.
  • find takes the thing to search for and possibly a starting location.
• When we design a function, we need to know the information we will be
  • Passing into the function
  • returning from the function
  • and possibly sending to the function to be modified.
• The number, type and variability of parameters to a function depend on what the function is doing and what information needs to be transferred.
  • A function which takes a string and converts it to upper case would only require a string.
  • A function which takes a planet name and returns if it is a valid planet or not would take a string input and produce a boolean output
  • A function which plays a guessing game as the guesser would pass no information
• Information is passed into a function in the form of parameters
  • A parameter is a variable declared in a function heading.
  • Parameters must have a type.
  • Parameters can be marked as variable, which means the value can be changed in the function.
• When a function is called, the programmer passes arguments to the function
  • There is one argument per parameter
  • parameters and arguments are matched by order
    • first parameter matches to first argument
    • second parameter matches to second argument
  • They types of parameters and arguments must match (within coercion)
  • The variability must match. (More on this later)
• Parameters (and arguments) vary by function, there is no set requirement other than "What the function needs, produces".
• This makes creating functions somewhat challenging
  • The mechanism is different from other syntax we have seen
  • It is not static (like a for loop), but depends on the parameters needed.
  • It is a system which allows the user to build what they need.