C Functions, Structures

Last time: Using UNIX, part 2

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Collect HW2
• myncat.c (with small improvements) has been posted online
• Questions about Monday's filter program from lab?
• Program #2 has been posted.
• Quiz Friday -- on C basics

Functions: What and Why

• What is a function?

 


• Why do we want to use functions?

Functions: What and Why

• What is a function?
• a piece of code, written by you or by someone else, which performs some useful, compartmentalizable task
• Why might we want to use functions?
• to re-use someone else's code
• to make your code re-usable by yourself or others
• the rest of the program might not care how the task is implemented
• to break up code into smaller, simpler, modules that are easy to understand, develop, and maintain
• as a programming paradigm (as in the LISP, Scheme, ML, and Miranda languages)

Using Functions

• A function is called by using its name followed by parentheses. Arguments, if any, are within the parenthesis, separated by commas.

  printf("Hello, world!\n");
  printf("%d\n", i);
  sqrt(144.);
  getchar();
  

• Arguments can be arbitrary expressions.

  printf("sum = %d\n", a + b + c);
  

• Many fuctions return values, which can be used as arguments to other functions.

  c = sqrt(a * a + b * b);
  x = r * cos(theta);
  i = f1(f2(j));
  

Defining Functions

• Here is a simple function:

  void printMessage(void) {
    printf("Programming is fun.\n");
  }
  

• Among other things, the first line (the function prototype declaration) tells us
  • The type of value it returns
  • Its name
  • The arguments that it takes
• One with a parameter and return value:

  int multbytwo(int x) {
    return x * 2;
  }
  

• Could declare function separately from function definition (often needed).

Parameters in C Functions

• Parameters to C functions are always call-by-value.
  • So formal parameters are copies of actual arguments.
• Arrays are passed by using the name of the array, which effectively passes the address of the array.
  • Changes to array elements will affect original array
• Passing the address (using &) of a variable is how we permit functions to modify the contents of variables.
• Must use the *var convention to access or modify contents.

  void multbytwo(int *x) {
    *x = *x * 2;
  }
  
  void main(void) {
    int a = 4;
    multbytwo(&a);
    mutlbytwo(&a);
    printf("a is %d\n", a);
  }
  

Structures

• Sometimes it is useful to collect a set of variables together, and reference them as a unit.
• In C, this is accomplished with a struct(ure).

  struct date {
    int month;        // one member
    int day;
    int year;
  };                  // this is a new type
  
  struct date today;  // this is a variable of the new type
  

• Access members using the period operator

  today.day = 2;
  today.year = 2005;
  today.month = 2;
  

• Can be passed as a parameter to a function.
  • Why is this not always desirable?
• Can have arrays of structs, and structs containing arrays or other structs.

C libraries use structs

• Consider acquiring the current time.
• The time(2) call returns the number of seconds since the UNIX epoch (Jan 1, 1970).
• The localtime(3c) function converts that to a struct:

  struct tm {
    int  tm_sec;    /* seconds after the minute - [0, 61] */
                    /* for leap seconds */
    int  tm_min;    /* minutes after the hour - [0, 59] */
    int  tm_hour;   /* hour since midnight - [0, 23] */
    int  tm_mday;   /* day of the month - [1, 31] */
    int  tm_mon;    /* months since January - [0, 11] */
    int  tm_year;   /* years since 1900 */
    int  tm_wday;   /* days since Sunday - [0, 6] */
    int  tm_yday;   /* days since January 1 - [0, 365] */
    int  tm_isdst;  /* flag for daylight savings time */
  };
  

Recursive Functions

• A recursive function is one that calls itself (perhaps indirectly).

  void printndots(int n) {
    if (n<1)
      return;
    printf(".");
    printndots(n-1);
  }
  

• Consider rolling two six-sided dice until a desired sum comes up. Let's implement this both iteratively and recursively.

Looking Ahead

• Friday's quiz will be on C Basics.
• Don't forget about project #2.
• Friday we will talk about strings and pointers.