CS 101

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CS 101

• Lecture 5

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Functions

• In math, we write a function as f(x) x2 . If
  we want to write this same function in C, we
  use the following syntax
• double f(double x)return xxThe first
  double says that f returns a double, which means
  that when the call the function, i.e., when the
  expression f(a) occurs, e.g., on the right side
  of an assignment statementdouble zf(a) where a
  is a double variable. We can also call f by
  writing coutnest calls of f as double zf(f(a)) The return
  value of f is the double value that f(a)
  represents. We cannot write f(a) 3, any more
  than we could write xy 3 The second double,
  in double x, is a declaration of the variable x.
  x is called a local variable of f, and also
  occurs inside the body of f which is return
  xx. If a is a double variable which is
  declared in main, when we call f by writing
  coutof f and the value of a is copied to the local
  variable x, then x is squared in xx, and finally
  f(a) holds a copy of xx. The variable x ceases
  to exist when control returns from f to main
  again.F is said to have one argument.Let us look
  at a program which calls f.

3
Using a function in a program

• includedouble f(double)//functio
  n declaration or prototypemain()double
  a3.4cout11.56 to screendouble f(double x)return
  xx//function definitionNotice that it is not
  necessary to write x inside the function
  declaration. It is not wrong to do so, but we may
  write double x or double y, it makes no
  difference. We have made a point of
  distinguishing a from x, since a is called an
  actual parameter and x is called a formal or
  local parameter. A function must be declared
  before it is called. Here are some other ways
  that f can be declared defined and called.

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Alternate possibilities for declaring, defining
and calling f.

• includedouble f(double
  x)return xx/counts as declaration and
  definition/main()Double a3.4coutlAlsoincludemain()double
  a3.4double f(double)//declaration of f must
  come before f(a)coutf(double x)return xx//definition of ff
  cannot be defined inside main or any other
  function, but it can be declared.

5
Functions with no arguments

• The main difference between math functions and
  C functions is that functions in C can DO
  things like printing out to the screen or to a
  file and math functions cant. Here is an example
  of a function f with no arguments and no
  return.void f()coutarguments and no return.\nYou may have
  noticed that main has no arguments either, since
  we write main(). The return type of main is int,
  though we dont have to write that for the g
  compiler. For other compilers, such as Borland or
  Visual C, we have to write int main(). One of
  the most common errors is calling f as f rather
  than the correct way which is f() Notice that we
  cant write coutanything, nor can we write int zf()

6
Another program with function calls

• includeint sum(int x,int
  y)return xyint prod(int x,int y)return
  xymain()coutintegers.\nint a,bcinabcoutof product is

7
Quiz 4 and solution

• Write a program which will use a function
  named sqs which will take three int arguments,
  return an int which is the sum of the squares of
  its three arguments. The values of the int
  variables which are to be passed to sqs are to be
  entered from the keyboard. Make sure you define
  sqs appropriately. includeint
  sqs(int x, int y, int z)return
  xxyyzzmain()coutintegers.\nint a,b,ccinabccoutsum of the squares of the integers you just
  entered is main to use sqs is to print out its return value
  to the screen.