C Tutorial, Part V

Pointer Arithmetic

Look at the following code segment found in arrayPtr.c

char *wordArray[MAX_WORDS];  /* compile-time array of char* */
char **wordArray2;  /* pointer to char* */
char matrix[MAX_WORDS][30];

wordArray2 = (char**)malloc(MAX_WORDS * sizeof(char*));
/* wordArray2 is now a MAX_WORDS-size array of char*'s */

The semantics of using the names wordArray and wordArray2 are as follows:

• wordArray - a synonym for the value of the address of the first element of wordArray
• wordArray2 - a variable that stores the address of the first element of wordArray2
• wordArray + i - the value of the address of the i'th element of wordArray
• wordArray2 + i - the value of the address of the i'th element of wordArray2
• wordArray2[i] - dereferences wordArray2 and produces the value stored in the i'th position
• thus, wordArray2[i] is equivalent to *(wordArray2 + i)

Which leads us to the "Fundamental Theorems" of Pointer Arithmetic:

For any array, arrayName, integer, i, and pointer variable, ptr:


• arrayName[i]   ==   *(arrayName + i)

  AND

• ptr + i   ==   ptr + (i * sizeof( *ptr ))
      /* recall that *ptr is the object that ptr points to */

An interesting corollary (which we'll see below):


• (char *)ptr + 4   ==   (int *)ptr + 1

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Lets spend some time passing various arguments to the different array functions and, in particular, see the warning generated by the call of the printMatrix routine. The correct prototype is void printMatrix(char arr[][30]); or printMatrix(char (*arr)[30]);.

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Applications of Pointer Arithmetic to strings

Here are some small programs that illustrate common uses of pointer arithmetic:

• In ptrArith_1.c we simply dereference the array's name to access the first char of the array.

• In ptrArith_2.c we use pointer arithmetic to copy from one string to another.

• In ptrArith_3.c we use a slightly more compact syntax to copy from one string to another.

• In ptrArith_4.c we condense the copy to its minimal form. Get used to reading this kind of code!

• In ptrArith_5.c we just move the copy line into a function.

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Pointer Arithmetic and Casting

In ptrCast.c we mix casting with pointer arithmetic to do something funky...