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lectures/05_pointers/README.md
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# Lecture 4 --- Pointers, Arrays, & Pointer Arithmetic # Lecture 5 --- Pointers, Arrays, & Pointer Arithmetic
- Pointers store memory addresses. - Pointers store memory addresses.
- They can be used to access the values stored at their stored memory address. - They can be used to access the values stored at their stored memory address.
@@ -7,7 +7,7 @@
- Pointers are also the primitive mechanism underlying vector iterators, which we have used with std::sort and - Pointers are also the primitive mechanism underlying vector iterators, which we have used with std::sort and
will use more extensively throughout the semester. will use more extensively throughout the semester.
## 4.1 Pointer Example ## 5.1 Pointer Example
- Consider the following code segment: - Consider the following code segment:
@@ -27,7 +27,7 @@ because x == 72.0. What’s going on?
![alt text](pointer_init.png "pointer_init") ![alt text](pointer_init.png "pointer_init")
## 4.2 Pointer Variables and Memory Access ## 5.2 Pointer Variables and Memory Access
- x is an ordinary float, but p is a pointer that can hold the memory address of a float variable. The difference - x is an ordinary float, but p is a pointer that can hold the memory address of a float variable. The difference
is explained in the picture above. is explained in the picture above.
@@ -46,7 +46,7 @@ Note: *p is an l-value in the above expression.
- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example1/index.html). - play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example1/index.html).
## 4.3 Defining Pointer Variables ## 5.3 Defining Pointer Variables
- In the example below, p, s and t are all pointer variables (pointers, for short), but q is NOT. You need the * - In the example below, p, s and t are all pointer variables (pointers, for short), but q is NOT. You need the *
before each variable name. before each variable name.
@@ -62,7 +62,7 @@ and may cause your program to crash! (It won’t crash if the uninitialized valu
- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example2/index.html). - play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example2/index.html).
## 4.4 Operations on Pointers ## 5.4 Operations on Pointers
- The unary (single argument/operand) operator * in the expression *p is the “dereferencing operator”. It means - The unary (single argument/operand) operator * in the expression *p is the “dereferencing operator”. It means
“follow the pointer”. *p can be either an l-value or an r-value, depending on which side of the = it appears on. “follow the pointer”. *p can be either an l-value or an r-value, depending on which side of the = it appears on.
@@ -90,7 +90,7 @@ p = 35.1; // Illegal: float assigned to a pointer
useful! Less than and greater than comparisons are also allowed. These are useful only when the pointers are useful! Less than and greater than comparisons are also allowed. These are useful only when the pointers are
to locations within an array. to locations within an array.
## 4.5 Exercise ## 5.5 Exercise
- Draw a picture for the following code sequence. What is the output to the screen? - Draw a picture for the following code sequence. What is the output to the screen?
@@ -108,7 +108,7 @@ cout << x << " " << y << endl;
- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example4/index.html). - play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example4/index.html).
## 4.6 Null Pointers ## 5.6 Null Pointers
- Like the int type, pointers are not default initialized. We should assume it’s a garbage value, leftover from - Like the int type, pointers are not default initialized. We should assume it’s a garbage value, leftover from
the previous user of that memory. the previous user of that memory.
@@ -131,7 +131,7 @@ tests to see if p is pointing somewhere that appears to be useful before accessi
at that location. at that location.
- But don’t make the mistake of assuming pointers are automatically initialized to NULL. - But don’t make the mistake of assuming pointers are automatically initialized to NULL.
## 4.7 Arrays ## 5.7 Arrays
- Here’s a quick example to remind you about how to use an array: - Here’s a quick example to remind you about how to use an array:
```cpp ```cpp
@@ -148,11 +148,11 @@ shrink dynamically in response to the demands of the application.
- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example_arrays_1/index.html). - play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example_arrays_1/index.html).
## 4.8 Stepping through Arrays with Pointers (Array Iterators) ## 5.8 Stepping through Arrays with Pointers (Array Iterators)
- The array code above that uses [] subscripting, can be equivalently rewritten to use pointers. We can re-write it in two ways: - The array code above that uses [] subscripting, can be equivalently rewritten to use pointers. We can re-write it in two ways:
### 4.8.1 First Approach ### 5.8.1 First Approach
```cpp ```cpp
const int n = 10; const int n = 10;
@@ -182,7 +182,7 @@ square root of this value is assigned to \*p.
- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example_arrays_2/index.html). - play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/pointers/example_arrays_2/index.html).
### 4.8.2 Second Approach ### 5.8.2 Second Approach
The first approach uses pointer arithmetic in several places - just to demonstrate the usage of pointer arithmetic; but many students find it confusing and hard to understand. In contrast, the following approach is easier to understand, and **is the recommended way for you to use in tomorrow's lab**; use the first approach only if you truly understand all the pointer arithmetics involved in that approach. The first approach uses pointer arithmetic in several places - just to demonstrate the usage of pointer arithmetic; but many students find it confusing and hard to understand. In contrast, the following approach is easier to understand, and **is the recommended way for you to use in tomorrow's lab**; use the first approach only if you truly understand all the pointer arithmetics involved in that approach.
@@ -199,7 +199,7 @@ for ( i=0; i<n; ++i ){
This second approach also "nicely mimics the subscript notation used in the (original) for loop above, which highlights that fundamentally array subscripts are just pointer arithmetic." - comments by our mentor Eleanor Olson, :smile:. This second approach also "nicely mimics the subscript notation used in the (original) for loop above, which highlights that fundamentally array subscripts are just pointer arithmetic." - comments by our mentor Eleanor Olson, :smile:.
## 4.9 Sorting an Array ## 5.9 Sorting an Array
- Arrays may be sorted using std::sort, just like vectors. Pointers are used in place of iterators. For example, if a is an array of doubles and there are n values in the array, then here’s how to sort the values in the array into increasing order: - Arrays may be sorted using std::sort, just like vectors. Pointers are used in place of iterators. For example, if a is an array of doubles and there are n values in the array, then here’s how to sort the values in the array into increasing order:
@@ -207,6 +207,6 @@ This second approach also "nicely mimics the subscript notation used in the (ori
std::sort( a, a+n ); std::sort( a, a+n );
``` ```
## 4.10 Exercises ## 5.10 Exercises
- [Leetcode problem 26: Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/). Solution: [p26_removeduplicates.cpp](../../leetcode/p26_removeduplicates.cpp) - [Leetcode problem 26: Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/). Solution: [p26_removeduplicates.cpp](../../leetcode/p26_removeduplicates.cpp)