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  Pointers and Switch Statement

Pointers:

The pointer in C is an address of something. This is a rare case certainly whenever we care what the specific address itself is, however pointers are a quite general way to obtain at the contents of something. The unary operator `&' is employed to generate the address of an object, when it has one. Therefore, 

          int a, b;
          b = &a;

place the address of a into b. We cannot do much with it, except print it or pass it to certain other routine, since we haven't given b the right type of declaration. However if we declare that b is certainly a pointer to an integer, we are in good position: 

          int a, *b, c;
          b = &a;
          c = *b;

b includes the address of  a and `c = *b' signifies to employ the value in b as an address, that is, as a pointer. The result is that we get back the contents of a, albeit instead indirectly. (It is always the case which `*&x' is similar as x if x has an address).

The most common use of pointers in C is for walking proficiently all along arrays. However, in the implementation of an array, the array name symbolizes the address of zeroth element of the array; therefore you cannot use it on the left side of an expression. (You cannot modify the address of something by assigning to it). When we state,

char *y;  char x[100];

y is of type pointer to character (though it does not yet point anywhere). We can make y point to an element of x by either of 

      y = &x[0];
       y = x;

As x is the address of x[0] this is consistent and legal.

Now `*y' gives x[0]. More significantly, 

       *(y+1) gives x[1]
       *(y+i) gives x[i]

And the sequence,

y = &x[0];
y++;

departs y pointing at x[1]. 
Let's utilize pointers in a function length which evaluates how long a character array is. Keep in mind that by the convention all character arrays are terminated with a `\0'. (And when they aren't, this program will blow up unavoidably). The old manner: 

       length(s)
          char s[ ]; {
               int n;
               for( n=0; s[n] != '\0'; )
                       n++;
               return(n);
       }

 On rewriting with pointers provides: 

      length(s)
          char *s; {
               int n;
               for( n=0; *s != '\0'; s++ )
                       n++;
               return(n);       }

You can now observe why we have to state what kind of thing s points to - if we are to increment it with s++ we encompass to increment it by the right amount. The pointer version is more proficient (that is, this is almost for all time true) however even more compact is,

for( n=0; *s++ != '\0'; n++ );

The `*s' returns a character; the `++' increases the pointer therefore we will get the next character next time about. As you can observe, as we make things more proficient we as well make them less clear. However `*s++' is an idiom so frequent that you have to know it.

Going a step further, here is our function strcopy which copies a character array s to other t. 

       strcopy(s,t)
          char *s, *t; {
               while(*t++ = *s++);
       }

We omitted the test against `\0', since `\0' is identically zero; you will frequently observe the code this way.

For arguments to the function, and there just, the declarations,

char s[ ];
char *s;

are equal - a pointer to a type, or an array of undetermined size of that type, are similar thing. 

The Switch Statement; Break; Continue:

The switch statement can be employed to substitute the multi-way test we utilized in the last illustration. Whenever the tests are like this: 

       if( c == 'a' ) ...
       else if( c == 'b' ) ...
       else if( c == 'c' ) ...
       else ...

Testing a value against a sequence of constants, the switch statement is frequently clearer and generally gives better code. Utilize it like this: 

       switch( c ) {
       case 'a':
               aflag++;
               break;
       case 'b':
               bflag++;
               break;
       case 'c':
               cflag++;
               break;
       default:               
  printf("%c?\n", c);               
  break;                         
      }

The case statements label different actions we want; default gets done when none of the other cases are pleased. (A default is optional; when it isn't there, and none of the conditions match, you simply fall out the bottom.)

The break statement in this illustration is new. It is there since the cases are merely labels, and subsequent to you do one of them; you fall via to the next unless you take certain explicit action to escape. This is a mixed blessing. On positive side, you can encompass multiple cases on a single statement; we may want to permit both lower and upper.

However what if we just desire to get out after doing case `a' ? We could get out of a case of the switch with a label and a goto, however this is really horrible. The break statement lets us exit devoid of either goto or label.

The break statement as well works in for and while statements; it causes an instant exit from the loop. The continue statement works merely within for's and while's; it causes the subsequent iteration of the loop to be began. This signifies it goes to the increment portion of the for and test part of the while. 

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