learn-c-the-hard-waych14.txt

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Chapter 14
Exercise 13: Switch Statement

   In other languages like Ruby you have a switch-statement that can take
   any expression. Some languages like Python just don't have a
   switch-statement since an if-statement with boolean expressions is
   about the same thing. For these languages, switch-statements are more
   alternatives to if-statements and work the same internally.

   The switch-statement is actually entirely different and is really a
   "jump table". Instead of random boolean expressions, you can only put
   expressions that result in integers, and these integers are used to
   calculate jumps from the top of the switch to the part that matches
   that value. Here's some code that we'll break down to understand this
   concept of "jump tables":
     __________________________________________________________________

   Source 35: ex13.c
   1  #include <stdio.h>
   2
   3  int main(int argc, char *argv[])
   4  {
   5      if(argc != 2) {
   6          printf("ERROR: You need one argument.\n");
   7          // this is how you abort a program
   8          return 1;
   9      }
   10
   11      int i = 0;
   12      for(i = 0; argv[1][i] != '\0'; i++) {
   13          char letter = argv[1][i];
   14
   15          switch(letter) {
   16              case 'a':
   17              case 'A':
   18                  printf("%d: 'A'\n", i);
   19                  break;
   20
   21              case 'e':
   22              case 'E':
   23                  printf("%d: 'E'\n", i);
   24                  break;
   25
   26              case 'i':
   27              case 'I':
   28                  printf("%d: 'I'\n", i);
   29                  break;
   30
   31              case 'o':
   32              case 'O':
   33                  printf("%d: 'O'\n", i);
   34                  break;
   35
   36              case 'u':
   37              case 'U':
   38                  printf("%d: 'U'\n", i);
   39                  break;
   40
   41              case 'y':
   42              case 'Y':
   43                  if(i > 2) {
   44                      // it's only sometimes Y
   45                      printf("%d: 'Y'\n", i);
   46                  }
   47                  break;
   48
   49              default:
   50                  printf("%d: %c is not a vowel\n", i, letter);
   51          }
   52      }
   53
   54      return 0;
   55  }
     __________________________________________________________________

   In this program we take a single command line argument and print out
   all of the vowels in an incredibly tedious way to demonstrate a
   switch-statement. Here's how the switch-statement works:
    1. The compiler marks the place in the program where the
       switch-statement starts, let's call this location Y.
    2. It then evaluates the expression in switch(letter) to come up with
       a number. In this case the number will be the raw ASCII code of the
       letter in argv[1].
    3. The compiler has also translated each of the case blocks like
       case 'A': into a location in the program that is that far away. So
       the code under case 'A' is at Y+'A' in the program.
    4. It then does the math to figure out where Y+letter is located in
       the switch-statement, and if it's too far then it adjusts it to
       Y+default.
    5. Once it knows the location, the program "jumps" to that spot in the
       code, and then continues running. This is why you have break on
       some of the case blocks, but not others.
    6. If 'a' is entered, then it jumps to case 'a', there's no break so
       it "falls through" to the one right under it case 'A' which has
       code and a break.
    7. Finally it runs this code, hits the break then exits out of the
       switch-statement entirely.

   This is a deep dive into how the switch-statement works, but in
   practice you just have to remember a few simple rules:
    1. Always include a default: branch so that you catch any missing
       inputs.
    2. Don't allow "fall through" unless you really want it, and it's a
       good idea to add a comment //fallthrough so people know it's on
       purpose.
    3. Always write the case and the break before you write the code that
       goes in it.
    4. Try to just use if-statements instead if you can.

14.1 What You Should See

   Here's an example of me playing with this, and also demonstrating
   various ways to pass the argument in:
     __________________________________________________________________

   Source 36: ex13 output
   1$ make ex13
   2cc -Wall -g    ex13.c   -o ex13
   3$ ./ex13
   4ERROR: You need one argument.
   5$
   6$ ./ex13 Zed
   70: Z is not a vowel
   81: 'E'
   92: d is not a vowel
   10$
   11$ ./ex13 Zed Shaw
   12ERROR: You need one argument.
   13$
   14$ ./ex13 "Zed Shaw"
   150: Z is not a vowel
   161: 'E'
   172: d is not a vowel
   183:   is not a vowel
   194: S is not a vowel
   205: h is not a vowel
   216: 'A'
   227: w is not a vowel
   23$
     __________________________________________________________________

   Remember that there's that if-statement at the top that exits with a
   return 1; when you don't give enough arguments. Doing a return that's
   not 0 is how you indicate to the OS that the program had an error. Any
   value that's greater than 0 can be tested for in scripts and other
   programs to figure out what happened.

14.2 How To Break It

   It is incredibly easy to break a switch-statement. Here's just a few of
   the ways you can mess one of these up:

    1. Forget a break and it'll run two or more blocks of code you don't
       want it to run.
    2. Forget a default and have it silently ignore values you forgot.
    3. Accidentally put in variable into the switch that evaluates to
       something unexpected, like an int that becomes weird values.
    4. Use uninitialized values in the switch.

   You can also break this program in a few other ways. See if you can
   bust it yourself.

14.3 Extra Credit

    1. Write another program that uses math on the letter to convert it to
       lowercase, and then remove all the extraneous uppercase letters in
       the switch.
    2. Use the ',' (comma) to initialize letter in the for-loop.
    3. Make it handle all of the arguments you pass it with yet another
       for-loop.
    4. Convert this switch-statement to an if-statement. Which do you like
       better?
    5. In the case for 'Y' I have the break outside the if-statement.
       What's the impact of this and what happens if you move it inside
       the if-statement. Prove to yourself that you're right.

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