6. Pointer Arrays; Pointers to Pointers
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| 6.1 Pointer |
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 Address stored by pointer also data in memory |
| Can address location of address in memory – pointer to that pointer |
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int n= 3;
int ∗pn =&n; /∗ pointer to n ∗/
int ∗∗ppn =&pn; /∗ pointer to address of n ∗/
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| Many uses in C: pointer arrays, string arrays |
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| Example: What does the difference? |
void swap( int ∗∗a, int ∗∗b)
{
int ∗temp = ∗a;
∗a = ∗b;
∗b = temp ;
}
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void swap(int *a, int *b)
{
int temp;
temp = *a;
*a = *b;
*b = temp;
}
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| 6.2 Pointer Arrays |
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Pointer array – array of pointers
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int ∗arr[20]; // an array of pointers to int’s char
∗arr[10]; // an array of pointers to char’s
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Pointers in array can point to arrays themselves
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char ∗strs [10]; // an array of char arrays (or strings)
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| Example |
– Compute the sorted array from the int arr[100].
– Can declare a pointer array int ∗sorted_array[100]; containing pointers to elements of arr and sort the pointers instead of the numbers themselves |
/∗ iterate until out−of−order element found; shift the element, and continue iterating ∗/
void insertion_sort(void)
{
unsigned int i, len = array_length(arr);
for (i=1; i<len; i++) {
if (∗sorted_array[i] < ∗sorted_array[i−1])
shift_element(i);
}
/∗ move previous elements down until insertion point reached ∗/
void shift_element(unsigned int i)
{
int ∗pvalue ;
/∗ guard against going outside array ∗/
for (pvalue = sorted_array[i];
i && ∗sorted_array[i−1] > ∗pvalue; i−−) {
/∗ move pointer down ∗/
sorted_array[i] = sorted_array[i−1];
}
sorted_array[i] = pvalue; /∗ insert pointer ∗/
}
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| 6.3 String Arrays |
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| An array of strings, each stored as a pointer to an array of chars |
| Each string may be of different length |
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char str1[] = "hello" ; /∗ length = 6 ∗/
char str2[] = "goodbye" ; /∗ length = 8 ∗/
char str3[] = "ciao" ; /∗ length = 5 ∗/
char strArray[] = {str1, str2, str3};
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| – Note that strArray contains only pointers, not the characters themselves! |
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