Strings and Pointers

Last time: C Using UNIX, part 2

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• Short Quiz #3 today
• HW#2: Low 5, High 10, Mean 9.5.
• SQ#2: Low 0, High 10, Mean 6.9.
• I've posted Programming assignment #3. Due Wednesday.
• The dollar sign ($) is not valid in C variable names. However, as some of you noted, it is permitted by some C compilers. Why is this? It turns out that some compilers permit it so that C can call functions written in other languages. In your homework, you received credit whether you said $ was valid or not.
• Review example of passing arrays

Recall Character Arrays

• We can create and initialize arrays of characters:

  char hello[] = {'h', 'e', 'l', 'l', 'o', '!'};
  

• Common characters needing escapes:

    \n    a ``newline'' character
    \b    a backspace
    \r    a carriage return (without a line feed)
    \'    a single quote (e.g., in a character constant)
    \"    a double quote (e.g., in a string constant)
    \\    a single backslash
  

• Character constants always use single quotes.

C Strings

• A string constant is zero or more characters enclosed in double quotes.

  printf("Hello world.\n");
  

• Strings in C are always terminated internally by a null character ('\0').

  char word[] = { "Hello!" };
  char word[7] = "Hello!";
  char word[] = "Hello!";
  char word[] = { 'H', 'e', 'l', 'l', 'o', '!', '\0' };
  
  printf("She said %s\n", word);
  

• Let's write code to concatenate two strings. Here is a skeleton that we can fill out.

Using strings in C

• There are many useful functions to help us use strings in C.
• For example, since C does not let us assign entire arrays, we use the strcpy(3c) function to copy one string to another:

  #include <string.h>
  
  char string1[] = "Hello, world!";
  char string2[20];
  
  strcpy(string2, string1);
  

Using strings in C

• Another lets us compare arrays.

  char string3[] = "this is";
  char string4[] = "a test";
  
  if(strcmp(string3, string4) == 0)
     printf("strings are equal\n");
  else	
     printf("strings are different\n");
  

  This code fragment will print "strings are different". Notice that strcmp does not return a boolean result.
• And of course the string concatenate function (which we implemented previously) is available as strcat(3c).

Using strings in C

• Often you'll need to know how long a string is
• E.g., to see if a copy of it will fit into a destination buffer
• You can call strlen(3c), which returns the length of the string (i.e., the number of characters in it), not including the terminating null:

  char string7[] = "abc";
  int len = strlen(string7);
  printf("%d\n", len);
  

  Which might be implemented as

  int mystrlen(char str[]) {
    int i;
    for(i = 0; str[i]; i++)   ;
    return i;
  }
  

• We can print strings using printf() with the %s format (e.g., printf("%s\n", string5);).

Pointers Revisited

• With pointers, we can manipulate addresses or contents referenced by the addresses.
• We can first declare a pointer variable

int *ip;

Which tells the compiler that variable ip is of type (int *) or equivalently that *ip is of type int.
• Pointers contain addresses (Note that ip is currently undefined). We can set ip to the address of another variable easily, as in

  int i = 5;
  ip = &i;
  

  At this point, the contents of ip and the address of i are the same -- they both refer to the same memory location, which contains the number 5.

Pointers

• We can read the value referenced by a pointer by prefixing the pointer name with an asterix, as in

  printf("%d\n", *ip);
  

• We can also write to the address contained in a pointer in the same way:

  *ip = 4;
  

  So now the value of i will also be 4.
• We can also examine the variable ip itself, by casting it to a type that is usable in printf(), as in:

  printf("%uld\n", (unsigned long) ip);
  

  Note that here we will see the 32-bit value of ip (that is, the memory location), and not the value to which ip points.

Pointer Declarations

• As with any other variable types, you can initialize the value of a pointer variable when you declare it, as in

int *ip = &i;

but you cannot initialize the value of the memory location to which it points, as something like

int *ip = 5;

will only tell the compiler to use address 5 as the initial value for ip (and the contents of address 5 are undefined, and probably off-limits to your program anyway).

Pointer Declarations

• While the compiler thinks these are equivalent,

  int *j;
  int* j;
  

  the latter leads to possible problems later, such as writing

  int* i,j;
  

  when you wanted two pointers.