CS156 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)

printf("%d %d %d %d\n", 'C', '%', '2', '\n');

67 37 50 10

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';
printf("%s\n", state);

Texas

    ┌────┬────┬────┬────┬────┬────┐
    │ 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 make the mistake 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";
printf("%s %s %s\n", sam1, sam2, sam3);

sam sam sam

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

char name[4];
strcpy(name, "sam");  // 's', 'a', 'm', '\0'
printf("My name is \"%s\"\n", name);

My name is "sam"

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 z1[5] = "Zulu";
char z2[] = "Zulu";
char z3[5] = { 'Z', 'u', 'l', 'u', '\0' };
char z4[] = { 'Z', 'u', 'l', 'u', '\0' };
/* This one has extra space for growth */
char z5[10] = { 'Z', 'u', 'l', 'u', '\0' };
printf("%s%s%s%s%s\n", z1, z2, z3, z4, z5);

ZuluZuluZuluZuluZulu

           ┌────┬────┬────┬────┬────┐
    z1…z4: │ Z  │ u  │ l  │ u  │ \0 │
           └────┴────┴────┴────┴────┘
             0    1    2    3    4

        ┌────┬────┬────┬────┬────┬────┬────┬────┬────┬────┐
    z5: │ 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";
printf("%zu\n", strlen(state));
printf("%zu\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 %zu is how you print a length.
- %ld, %lu, and %zd are common, but wrong.

A String Mystery

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

char s1[] = "Russell Brand", s2[] = "Russell Brand";
if (strcmp(s1, s2) == 0)
    printf("The strings are equal.\n");
else
    printf("The strings are not equal.\n");

The strings are equal.

A common misunderstanding

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

char s1[] = "Russell Brand", s2[] = "Russell Brand";
if (strcmp(s1, s2))  // Bad code!  Bad!
    printf("They're the same.\n");   // Liar!
else
    printf("The strings are not equal.\n");

The strings are not equal.

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?

char a[] = "omega", b[] = "alpha";
printf("%d\n", strcmp(a,b));

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.

CS156: Intro to C, Part I

Spring 2018

Strings

CS156 Strings

Characters in C

Strings in C

Don’t forget the null character

Assigning Strings to Variables

Assigning Strings to Variables

The C String Library

strlen and the length of strings

A String Mystery

strcat: Concatenating Strings

strcmp: Comparing Strings

A common misunderstanding

strcmp: Comparing Strings

String Use Tips