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CS156 Strings: Strings

Characters in C

• Modern computers use the ASCII character to represent characters as numbers. It does, so you don’t have to.
• All characters in the ASCII set are represented by numbers between 0 and 127.
  • 'A'==65
  • '&'==38
  • '5'==53
  • Notably, '2' is not the same as 2. Really!
• You can also regard them as octal or hexadecimal numbers (we’ll cover these later)

Strings in C

• In C, strings are just arrays of characters ending with the special character '\0'
• '\0' is the null (not NULL) character

char state[6];
state[0] = 'T'; state[3] = 'a';
state[1] = 'e'; state[4] = 's';
state[2] = 'x'; state[5] = '\0';

    ┌────┬────┬────┬────┬────┬────┐
    │ T  │ e  │ x  │ a  │ s  │ \0 │
    └────┴────┴────┴────┴────┴────┘
      0    1    2    3    4    5

• String literals, or string constants, are represented as a sequence of characters in double quotes: "hello".

Don’t forget the null character

• You have to specify the '\0' to end the string.
• Don’t fall into the trap of “If I don’t put a null character in there, it’ll just go to the end of the array.”
• No, it’ll go until it finds a '\0'.
• It’ll find one eventually, or die trying.

    char name[7];
    name[0] = 'F'; name[1] = 'o'; name[2] = 'o';
    printf("%s\n", name);

    ┌───┬───┬───┬───┬───┬───┬───┐
    │ F │ o │ o │ ¥ │ ¶ │ € │ ☃ │
    └───┴───┴───┴───┴───┴───┴───┘
      0   1   2   3   4   5   6

Assigning Strings to Variables

• Strings can be assigned to character arrays in several ways:

    char sam1[4] = {'s','a','m','\0'};
    char sam2[4] = "sam";
    char sam3[] = "sam";

• strcpy: The C string library provides a function for copying strings:

    char name[4];
    strcpy(name, "sam");  // 's', 'a', 'm', '\0'

• It is your job to ensure there is enough space allocated, including the null character at the end!

Assigning Strings to Variables

• Multiple ways to define a character array as a string:

    // These all do the same thing:
    char name1[5] = "Zulu";
    char name2[] = "Zulu";
    char name3[5] = { 'Z', 'u', 'l', 'u', '\0' };
    char name4[] = { 'Z', 'u', 'l', 'u', '\0' };

    ┌────┬────┬────┬────┬────┐
    │ Z  │ u  │ l  │ u  │ \0 │
    └────┴────┴────┴────┴────┘
      0    1    2    3    4

    /* This one has extra space for growth */
    char name5[10] = { 'Z', 'u', 'l', 'u', '\0' };

    ┌────┬────┬────┬────┬────┬────┬────┬────┬────┬────┐
    │ Z  │ u  │ l  │ u  │ \0 │ ?? │ ?? │ ?? │ ?? │ ?? │
    └────┴────┴────┴────┴────┴────┴────┴────┴────┴────┘
      0    1    2    3    4    5    6    7    8    9    

The C String Library

• The C string library: #include <string.h>
• The string library provides a number of useful functions for handling strings.
  • strlen — get the length of a string
  • strcpy — copy one string into another
  • strcat — add one string onto the end of another
  • strcmp — compare two strings for equality

strlen and the length of strings

• The length of a string is not equal to the length of the character array that contains it.
  
  

  char state[50] = "Texas";
  strcpy(state,"Texas");
  printf("%zd\n", strlen(state));
  printf("%zd\n", sizeof(state));
  

  5
  50
  
• The length of a string is the number of characters before the '\0' (doesn’t include the '\0'). It is returned by strlen.
• The character array holding a string must be larger than the string (by one), but can be much larger. The size of the array is returned by sizeof.
• Note that %zd is how you print a length.

A String Mystery

char alpha[] = "123";
char beta[16] = "1234567890abcdef";
char gamma[] = "12345678";
printf("%zd %zd %zd\n", sizeof(alpha), sizeof(beta), sizeof(gamma));
printf("%zd %zd %zd\n", strlen(alpha), strlen(beta), strlen(gamma));

4 16 9
3 19 8

• How can strlen(beta) possibly be more than sizeof(beta)?
• Why that length, particularly?

strcat: Concatenating Strings

• strcat takes a character array as its first argument and either a character array or string literal as its second argument.

    char name[12];
    strcpy(name, "Sue");       // name = "Sue", length=3
    strcat(name, " Storm");    // name = "Sue Storm", length=9
    strcat(name, " Richards"); // Runtime failure: too long
    strcat("Ben", "Grimm");    // Compile-time failure

• A bad idea:

    char buf[10];
    strcat(buf, "xyz");        // Adding on to the end of … what?

strcmp: Comparing Strings

• strcmp returns 0 if two strings are equal and otherwise returns a non-zero answer

    if (strcmp(string1, string2) == 0)
        printf("The strings are equal.\n");

• It’s strcmp, not strequal. The result of 0 for equal strings can be confusing:

    if (strcmp(string1, string2))	 // Bad code!  Bad!
        printf("They're the same.\n");   // Liar!

strcmp: Comparing Strings

int strcmp(const char s1[], const char s2[]);

Return value indicates the lexicographical relation of string1 to string2:

Relationship of s1 to s2	Value
s1 less than s2	some number <0
s1 identical to s2	0
s1 greater than s2	some number >0

Vague, isn’t it?

String Use Tips

• Always make sure the character array is long enough to hold the string and the '\0'.
• Use strcpy to assign strings, not =.
• Use strcmp to compare, not ==, <, >, etc.
• Remember the meaning of strcmp results.
• Don’t confuse strcpy and strcat
• Don’t try to cat or copy a string into a string literal.