Lab 8 - char*

We strongly encourage teamwork in this lab.

This lab will run as follows:

1. Review char * strings
2. Implement a variant of strlen
3. Compare your implementation with that of another student
4. Find a way to make strlen shorter/simpler, and note what you did in a comment
5. Implement a variant of strtok
6. Compare your implementation with that of another student
7. Find a way to make strtok shorter/simpler, and note what you did in a comment
8. Show a TA how you made each function smaller

String Overview

Layout

In C, strings are pointers to arrays of characters, or char *s. They do not include length information; instead, they use the special value 0 to note the end of the string. Thus, if memory contains

Then if char *x contains 0x20, it looks like the string "welcome" and if it contains 0x28, it looks like the string "to char". Note that there is no magic to starting at the beginning of something; if x contains 0x22 then it looks like the strong "lcome" and if it is 0x27 it looks like the empty string "".

Const

String literals are generally placed by the compiler in read-only memory, and have the type const char * not just char *. Thus,

const char *c = "hello";
c[1] = 'u'; /* error: cannot modify read-only memory */
c = c + 2;  /* OK: c can change but, the memory it points to cannot */

If you want to get a mutable copy of a const string, you can use strdup. From man strdup we find

#include <string.h>
char *strdup(const char *s);

The  strdup()  function  returns  a pointer to a new
string which is a duplicate of the string s.  Memory
for the new string is obtained with malloc, and can be
freed with free.

This means you’d use is as

const char *s1 = "hello"; /* initialized in read-only memory */
char *s2 = strdup(s1);    /* make a copy in mutable memory   */
s2[1] = 'u';              /* change one letter of the copy   */
puts(s2);                 /* display the altered copy        */
free(s2);                 /* don't litter: discard the copy  */

Note that the free when you are done is somewhat important, but won’t break your code if you leave it off in this lab. We’ll discuss more about free in class later this week.

Display

The simplest way to display a string in C is using the function puts. An extract from the manual page man puts says

#include <stdio.h>
int puts(const char *s);

puts() writes the string s and a trailing newline to stdout.
puts() returns a nonnegative number on success, or EOF on error.

Thus

const char *s = "welcome\0to char";
puts(s);
puts(s+2);
puts(s+7);
puts(s+8);

will display

welcome
lcome
to char

Writing string code

Let’s start with some basic string functions. These are in the standard C library, but are worth implementing by hand to better understand them.

String length

The strlen function is described in man strlen as

size_t strlen(const char *s);

The strlen() function calculates the length of the string s, excluding the terminating null byte (‘\0’).

Write an implementation of this function, naming it mystrlen instead of strlen.

const char *s = "even elephants exfoliate";
size_t slen = mystrlen(s);
puts(s + (slen/2));

ts exfoliate

After you’ve written an implementation, compare with another student. Work together to make a simpler version (you’ll need to explain what you did to a TA later, so remember your changes).

There exist very short solutions to this; I know of one where the entire function body has just 3 statements and 31 non-white characters…

Simplified strtok

Two library functions, strsep and strtok, both implement a string-splitting behavior. You will implement a simplified version

char *simple_split(char *s, char delim);

Given a string and a delimiter character, do the following:

• if s is NULL or s[0] is \0, return NULL
• find the first delim character in s
  • if there is none, return NULL
  • otherwise, replace it with \0 and return a pointer to the character after it

char *s = strdup("can all aardvarks quaff?");
char *bit = simple_split(s, 'a');
puts(s);
puts(bit);
free(s);

c
n all aardvarks quaff

char *trash, *bit, *s;
trash = bit = s = strdup("can all aardvarks quaff?");
do {
    s = bit;
    bit = simple_split(s, 'a');
    puts(s);
} while(bit);
free(trash);

c
n 
ll 

rdv
rks qu
ff?

After you’ve written an implementation, compare with another student. Work together to make a simpler version (you’ll need to explain what you did to a TA later, so remember your changes).

There exist very short solutions to this; I know of one where the entire function body has just 5 statements and 70 non-white characters…

Check off

Show a TA your implementations and describe how you made them smaller (or why you beleive they are as small as they can be).