There are so many tracks on the retro jukebox that people can’t find the music they are looking for. To help the customers, the guys in the Head First Lounge want you to write another program.

This is the track list:

The list is likely to get longer, so there’s just the first few tracks for now. You’ll need to write a C program that will ask the user which track she is looking for, and then get it to search through all of the tracks and display any that match.

There are several track names that you need to record. You can record several things at once in an array. But remember: each string is itself an array. That means you need to create an array of arrays, like this:

The array of arrays looks something like this in memory:

So now that you know how to record the data in C, what do you need to do with it?

The guys have helpfully given you a spec.

Well, you know how to record the tracks. You also know how to read the value of an individual track name, so it shouldn’t be too difficult to loop through each of them. You even know how to ask the user for a piece of text to search for. But how do you look to see if the track name contains a given piece of text?

Using string.h

The C Standard Library is a bunch of useful code that you get for free when you install a C compiler. The library code does useful stuff like opening files, or doing math, or managing memory. Now, chances are, you won’t want to use the whole of the Standard Library at once, so the library is broken up into several sections, and each one has a header file. The header file lists all of the functions that live in a particular section of the library.

So far, you have only really used the stdio.h header file. stdio.h lets you use the standard input/output functions like printf and scanf.

But the Standard Library also contains code to process strings. String processing is required by a lot of the programs, and the string code in the Standard Library is tested, stable, and fast.

You include the string code into your program using the string.h header file. You add it at the top of your program, just like you include stdio.h.

What’s my Purpose?

See if you can match up each string.h function with the description of what it does.

strchr()	Concatenate two strings.
strcmp()	Find the location of a string inside another string.
strstr()	Find the location of a character inside a string.
strcpy()	Find the length of a string.
strlen()	Compare two strings.
strcat()	Copy one string to another.

Sharpen your pencil

Which of the functions above should you use for the jukebox program? Write your answer below.

______________________________

What’s my Purpose? Solution

You were to match up each string.h function with the description of what it does.

Sharpen your pencil: Solution

You were to write which of the above functions you should use for the jukebox program.

So how exactly does the strstr() function work? Let’s look at an example. Let’s say you’re looking for the string “fun” inside a larger string, “dysfunctional.” You’d call it like this:

The strstr() function will search for the second string in the first string. If it finds the string, it will return the address of the located string in memory. In the example here, the function would find that the fun substring begins at memory location 4,000,003.

But what if the strstr() can’t find the substring? What then? In that case, strstr() returns the value 0. Can you think why that is? Well, if you remember, C treats zero as false. That means you can use strstr() to check for the existence of one string inside another, like this:

char s0[] = "dysfunctional";
char s1[] = "fun";
if (strstr(s0, s1))
  puts("I found the fun in dysfunctional!");

Let’s see how we can use strstr() in the jukebox program.

Pool Puzzle

The guys in the Lounge had already started to write the code to search through the track list, but—oh no!—some of the paper they were writing the code on has fallen into the pool. Do you think you can select the correct pieces of code to complete the search function? It’s been a while since the pool was cleaned, so be warned: some of the code in the pool might not be needed for this program.

Note: the guys have slipped in a couple of new pieces of code they found in a book somewhere.

Note: each thing from the pool can be used only once!

int main()
{
  char search_for[80];
  printf("Search for: ");
  fgets(search_for, 80, stdin);
  search_for[strlen(search_for) - 1] = '';
  find_track();
  return 0;
}

int main()
{
  char search_for[80];
  printf("Search for: ");
  fgets(search_for, 79, stdin);
  search_for[strlen(search_for) - 1] = '';
  find_track(search_for);
  return 0;
}

int main()
{
  char search_for[80];
  printf("Search for: ");
  scanf("%79s", search_for);
  search_for[strlen(search_for) - 1] = '';
  find_track(search_for);
  return 0;
}

int main()
{
  char search_for[80];
  printf("Search for: ");
  scanf("%80s", search_for);
  find_track(search_for);
  return 0;
}

Pool Puzzle Solution

The guys in the Lounge had already started to write the code to search through the track list, but—oh no!—some of the paper they were writing the code on has fallen into the pool. You were to select the correct pieces of code to complete the search function.

Note: the guys have slipped in a couple of new pieces of code they found in a book somewhere.

BE the Compiler Solution

The jukebox program needs a main() function that reads input from the user and calls the find_track() function on the opposite page. Your job was to play like you’re the compiler and say which of the following main() functions is the one you need for the jukebox program.

Let’s bring the code together and review what you’ve got so far:

It’s important that you assemble the code in this order. The headers are included at the top so that the compiler will have all the correct functions before it compiles your code. Then you define the tracks before you write the functions. This is called putting the tracks array in global scope. A global variable is one that lives outside any particular function. Global variables like tracks are available to all of the functions in the program.

Finally, you have the functions: find_track() first, followed by main(). The find_track() function needs to come first, before you call it from main().

Test Drive

It’s time to fire up the terminal and see if the code works.

And the great news is, the program works!

Even though this program is a little longer than any code you’ve written so far, it’s actually doing a lot more. It creates an array of strings and then uses the string library to search through all of them to find the music track that the user was looking for.

man strstr

Code Magnets

The guys are working on a new piece of code for a game. They’ve created a function that will display a string backward on the screen. Unfortunately, some of the fridge magnets have moved out of place. Do you think you can help them to reassemble the code?

Code Magnets Solution

The guys are working on a new piece of code for a game. They’ve created a function that will display a string backward on the screen. Unfortunately, some of the fridge magnets have moved out of place. You were to help them to reassemble the code.

You’ve seen how to use an array of arrays to store a sequence of strings, but another option is to use an array of pointers. An array of pointers is actually what it sounds like: a list of memory addresses stored in an array. It’s very useful if you want to quickly create a list of string literals:

You can access the array of pointers just like you accessed the array of arrays.

C-Cross

Now that the guys have the print_reverse() function working, they’ve used it to create a crossword. The answers are displayed by the output lines in the code.

Across

C-Cross Solution

Now that the guys have the print_reverse() function working, they’ve used it to create a crossword. The answers are displayed by the output lines in the code.

Across

You’ve got Chapter 2.5 under your belt, and now you’ve added strings to your toolbox. For a complete list of tooltips in the book, see Appendix B.