Laboratory Exercise on Characters and Strings

Goals

This laboratory exercise examines characters, details of string storage, and the operations of string library functions within the C programming language.

Preparation before Class

• King: Section 7.3, Chapter 13, pages 134-141, 277-299, OR
• Kernighan & Ritchie: 1.9, 7.8.1, 7.8.2, B.3
• Sample program: char-example.c

Work Started in Class

Character Arrays, Strings, char * and Storage

Program string-intro.c shows several variations related to the declaration of character arrays, strings, and char * variables.

One run of this program produced the following output:

first 3 characters in each array
   first: Col
  second: Wor
   third: Com
  fourth: Wor
   fifth: Hel
Variable addresses and array base addresses
   first address: 359157264,   array base address: 359157264
  second address: 359157248,   array base address: 359157248
   third address: 359157232,   array base address: 359157232
  fourth address: 359157224,   array base address: 359157248
   fifth address: 359157216,   array base address: 4196464
variables printed as strings
   first: Cold\ufffd
  second: World
   third: Computer ScienceWorld
  fourth: World
   fifth: Hello

Understanding this program and output can provide substantial insights to how C works with arrays, characters, strings, and pointers.

1. Copy string-intro.c to your account, compile and run it, and examine the output.

   1. Each run of this program likely places variables in different memory locations. Absolute addresses may change, but do the relative addresses change? That is, to what extent does the memory schematic in the above table need to change for different runs of the program.

   2. Immediately after the declaration of all arrays, but before any printing, insert the line:

      first[3] = second[3] = third[3] = 0;
      

      This line inserts a null character at index 3 for each of the three strings.

      Recompile and rerun the program, describe what (if any) differences result in the output printed, and explain why this output is obtained.

   3. Immediately after the declaration of all arrays, but before any printing, insert the line:

      fifth[3] = 0;
      

      What happens when you try to compile and run this program? Why do you think this result occurs?

Declaring Strings

2. Here are a number of different string declarations.

   char *baboon;
   char *chimpanzee = "animal";
   char dolphin[];
   char emu[] = "animal";  
   char fox[4] = "animal";
   char giraffe[8] = "animal";
   char elephant[10];
   elephant = "animal";
   

   1. Which are valid and which are invalid?
   2. How do the valid declarations differ?
   3. What happens if you switch fox and giraffe? How do you think this can be explained? Think about the bounds of arrays, and the layout of characters in main memory.

Initialized and Uninitialized Strings

• The sizeof function tells you how much memory has been allocated to a something.
• The strlen function tells you the length of a string, which is the number of characters before you hit the null character('\0').

3. Start a new program for several experiments with strings.

   1. Copy the following declaration and code into a main procedure, making sure that you include the library string.h:

      char computerscience[16] = "isawesome";
      char isawesome[16] = "computerscience";
      
      printf ("strlen (computerscience): %d\n", strlen (computerscience) );
      printf ("strlen (isawesome): %d\n", strlen (isawesome) );
      
      printf ("computerscience: %s\n", computerscience );
      printf ("isawesome: %s\n", isawesome );
      

      What output is obtained? Briefly explain why these results are printed.

   2. What would you expect to get if you had written:

      char computerscience[16];
      

      instead of:

      char computerscience[16] = "isawesome";
      

      Run the program to check whether your expectations match what is printed.

   3. Restore the initialization in Step 3a and then add this line of code:

      printf ("Concatenate the strings: %s", strcat (isawesome, computerscience));
      

      • What is the result? Is this what you expected? Change the bounds of array isawesome to 32 and see what happens? What happened now?
      • What did the string operator strcat() do? Explain conceptually what happens in the array and where the null character(s) is/are?

Characters as Integers

From the reading you have done on Characters and Strings, you have read that char is actually considered to be a type of integer. We can use this property of chars to do integer arithmetic on their values. Every char has a corresponding integer. As you know, we can find out what these values are from an ASCII table.

4. Write a program that will take the given hard-coded character, and print out the character and its corresponding integer value in this example format:

    
   'A' = 65 
   

   Hint: Since characters are integers, you need to only use %d to print the integer.

5. Write a program that takes the name of the robot by using rGetName(), converts all the characters of the name to uppercase (use the string function toupper, if you wish), and then sets the robot's name to the uppercase version, using rSetName().

   Keep in mind that the value of a character is represented by putting single quotes, and string is represented by double quotes.

Control Characters

Characters can represent actions rather than just printing a symbol. Here is a short list of what can be done with some characters:

• '\t' = tab (horizontal tab)
• '\b' = backspace
• '\v' = vertical tab

6. Write a program that:
   • prints out a sentence with tabs in between each word
   • prints out a sentence with vertical tabs between each word
   • illustrates how backspace works (print a word with a few backspace and see how much of the word you can read)

String Functions

7. Write a function with the following prototype;

   void string_reverse (char str[]):
   

   It should reverse the order of the characters in str (except the null character). Note that it will not return a new string, but it will modify the given string.

Homework

Robots with Strings

8. Write a program that beeps once for each uppercase letter in the string and twice for each lowercase letter. If a string contains spaces, punctuation, or digits, those characters should not cause beeps.

   For this program, do not use the string operator strlen().

String Operators

Remember from the reading on characters and strings that:

• strlen(s) = return length of s
• strcmp(s,t) = returns 0 is two strings are the same, a value smaller than 1 if s<t , and a value greater than 1 if t<s .
• strcpy(s,t) = copy the string t to s.

9. The terminal command man can provide helpful information about the standard C functions.

   1. Go to your terminal and type man strcmp. What are the two different ways that you can compare two strings?

   2. In the same manual page, find what the parameters are for strncmp.

      Remember that to quit the man pages, you can simple type q.

10. Here is a link to the C header file for strings, string.h

    1. What does strcat do? Using what you have learned about how strings are stored, and their null characters, explain how strcat works.

    2. Copy the following program; catstr.c. Run the program and observe the PART 1 of the output.

    3. Follow the directions in the comments and fill in the blanks in the program.

    4. What happened in part 2 that caused the output to be what we didn't expect? Hint: Think about how strcat works and the null character.

Strings of Music

11. Write a program that makes the robot beep in the frequencies that corresponds to the musical letters (A,B,C,D,E,F,G - ignore sharps and flats- and H!) that are given in a string. Make the program so that it is not case-sensitive. Here is the header file to define pitches and their corresponding letters: pitches.h Remember that a string is actually an array.

    For example, give it the word "BED" and play the frequencies for B, E, and D.

12. Now make this program work for all the letters in the alphabet. Hint: Use mod to wrap the letters back to the musical letters. For example 'H' would wrap to be 'A' ,'I' would wrap to be 'B', 'J' would wrap to be 'C', and so on.

Feedback Welcome

Development of laboratory exercises is an iteractive process. Prof. Walker welcomes your feedback! Feel free to talk to him during class or stop by his office.