C Arrays, Strings

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Short Quiz 2: High 10, low 0, median 7, mean 6.5
• Reminder: Programming assignment #1 is due tonight, and Homework #2 is due Wednesday. Questions?
  
• Today: Continue with cat, then look at C arrays and strings.

Continue with cat

• Last Wednesday we wrote a simple cat program.
• I've since finished the error handling.
• Let's revise it to include line numbers, ala cat -n.

C Arrays

• Assuming we know the number of elements, we can easily create and manipulate arrays of items.

  int i[5];
  
  i[0] = i[1] = 1;
  i[2] = 2;
  i[i[2] + 1] = i[2] + 2;
  i[i[2] + i[2]] = i[3] + i[2];
  

• Unfortunately, we cannot set all values of an array at once, nor assign one array to another. Instead, we must iterate through each item.

  int i, a[10];
  
  // clear or copy all values
  for ( i = 0; i < 10; i++ ) {
     a[i] = 0;  or   a[i] = x[i];
  }
  

Array Initialization

• We can, however, initialize the elements of an array when declared.
• All of these are equivalent:
  
  

  unsigned short int a[10] = {1, 2, 3, 4, 5, 6, 0, 0, 0, 0};
  unsigned short int a[10] = {1, 2, 3, 4, 5, 6};
  unsigned short int a[] = {1, 2, 3, 4, 5, 6, 0, 0, 0, 0};
  unsigned short int a[] = {[0]=1, [1]=2, [2]=3, [3]=4, [4]=5, 
                             [5]=6, [9]=0};
  

  
  
• The number of elements can be inferred from the initializers, and missing initializers are always set to 0.

Multidimensional Arrays

• You can have arrays of any type, including arrays of arrays.

  int i, j;
  float a[3][10] = { {14.2, 1, 0, 9.4},
                     {0, 144, 7, 2.5},
                     {2, 3, 109, 42.0} };
  

• Could print out contents:

  for (j = 0; j < 10; j = j + 1)
     printf("\t%d:", j);
  printf("\n");
  
  for (i = 0; i < 3; i = i + 1) {
     printf("%d:", i);
     for (j = 0; j < 10; j = j + 1)
  	printf("\t%f", a[i][j]);
     printf("\n");
  }
  

• Note that we could have gone in the opposite order (2nd subscript first).

Character Arrays

• An array of characters works the same. We can create and initialize them with the same syntax.

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

• There are many characters that are not letters, numbers, or keyboard symbols. Such characters are represented by an escape sequence using the backslash.
• Common characters include:

    \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.