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1 Arrays and Pointers CSCI 112: Programming in C
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Arrays and Pointers
CSCI 112: Programming in C
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Quick review
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• A pointer is a variable that …
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Quick review
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• A pointer is a variable that takes the address of another variable as a value
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Quick review
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• A pointer is a variable that takes the address of another variable as a value
• The name of an array is …
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Quick review
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• A pointer is a variable that takes the address of another variable as a value
• The name of an array is a pointer to the first element in the array.
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
In memory…
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
In memory…
• The base address of this array is 1000
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
In memory…
• The base address of this array is 1000
• Where does scores point to?
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
In memory…
• The base address of this array is 1000
• Where does scores point to?
scores
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
In memory…
• The base address of this array is 1000
• Where does scores point to?
• What is the value of scores?
scores
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An example
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int scores[6] = {85, 79, 100, 95, 68, 89};
In C…
In memory…
• The base address of this array is 1000
• Where does scores point to?
• What is the value of scores?
• (It’s 1000, the address of the first element)
scores
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Another example
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We’ve declared an int pointer and an int array.
scores points to the first element.
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
scores ptr
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Another example
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We set ptr to the address of the first element of scores
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
scores ptr
ptr = &scores[0];
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Another example
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We set ptr to the address of the first element of scores
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
scores ptr
ptr = &scores[0];
Evaluate this statement:ptr == scores
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Another example
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What is the value of t?int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
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scores ptr
ptr = &scores[0];
int t = *ptr;
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Another example
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What is the value of t?
We follow ptr back to the first element in arr– so 1!
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
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scores ptr
ptr = &scores[0];
int t = *ptr;
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Another example
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Which variables does this affect?
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
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scores ptr
ptr = &scores[0];
int t = *ptr;
*ptr = 0;
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Another example
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Which variables does this affect?
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
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scores ptr
ptr = &scores[0];
int t = *ptr;
*ptr = 0;
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Another example
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Which variables does this affect?
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
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scores ptr
ptr = &scores[0];
int t = *ptr;
*ptr = 0;
ptr = scores;
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Another example
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Which variables does this affect?
None!Itsetsptr toaddressoffirstelementinscores…nochange
int scores[6] = {1, 2, 3, 4, 5, 6};int *ptr;
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scores ptr
ptr = &scores[0];
int t = *ptr;
*ptr = 0;
ptr = table;
// Equivalent to writing ptr = &scores[0];
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Pointer arithmetic: some powerful stuff
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• Pointer arithmetic is an alternative to array indexing.
• If ptr is a pointer to a particular element in an array, then ptr+1 points to the next element.
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ptr ptr+1
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Pointer arithmetic: some powerful stuff
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• Why does this work?
• When dealing with pointers, any arithmetic you do is in context of the size of that variable.
• For example, if we have an char pointer (a char is 1 byte) then writing ptr + 1 moves us 1 byte forward in memory.
• If we have an int pointer (where an int is often 4 bytes) then writing ptr + 1 moves us 4 bytes forward in memory.
• So, ptr + 1 always moves one “cell” forward.
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Pointer arithmetic: simple exampleUsing subscripts, we’d set all elements in an array my_arr to 0 as follows:
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for (i=0; i < SIZE; i++)my_arr[i] = 0;
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Pointer arithmetic: simple exampleUsing subscripts, we’d set all elements in an array my_arr to 0 as follows:
The same can be done using pointer arithmetic:
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for (i=0; i < SIZE; i++)my_arr[i] = 0;
for (i=0; i < SIZE; i++)*(my_arr+i) = 0;
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Let’s break that last line apart…
table+i moves from the first element of the array (which table points to) to the 1+ith. For example, when i =1, table+i points to the second element.
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*(table+i) = 0;
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Let’s break that last line apart…
table+i moves from the first element of the array (which table points to) to the 1+ith. For example, when i =1, table+i points to the second element.
Because table+i is a pointer, we need to dereference it with the * operator before we try and assign a value to it. (This gets us to the actual int value pointed to.)
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*(table+i) = 0;
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Iterating over an array using only pointers
Thisdoesthesamethingastheprevioustwoexamples,butdoesn’tneedaloopcontrolvariable(i inthelastexample.)
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for (ptr = my_arr; ptr < &my_arr[SIZE]; ptr++)*ptr = 0;
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Key DifferenceThe one difference between an array name and a pointer is that the latter is a variable but the former is a constant. So ptr = array and ptr++ are valid operations, but constructs like array = ptr or array++ are illegal.
This is because an array name is a constant pointer, set at compile time. It’s not to be changed!
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