CSCI 2212: Intermediate Programming / C Working with Pointers

Transcription

1 ... 1/26 CSCI 2212: Intermediate Programming / C Working with Pointers Alice E. Fischer November 13, 2015

2 ... 2/26 Outline Using Pointers with Arrays Pointers to Functions

3 ... 3/26 Using Pointers with Arrays Using Pointers with Arrays Head Pointers, Scanners, and Sentinels Input and Output with Pointers Array Processing with Pointers Insertion Sort

4 ... 4/26 Using Pointers with Arrays Head Pointers, Scanners, and Sentinels Three pointers are used to process an array sequentially: When you declare an array, the name of the array acts as a head pointer. Or you can declare a head pointer and initialize it malloc. An sentinel pointer sits JUST PAST the end of the array and marks the end. A scanning pointer (or cursor) starts at the head and moves one step at a time toward the end. A for loop initializes and increments the cursor and processes the contents of each array slot. When random access to the array is needed, use subscripts, not pointers.

5 ... 5/26 Using Pointers with Arrays Looping with Subscripts Here, we use subscripts for input into a declared array. void Flex::readData( istream& ins ){ int k; for( k=0; ; ++k) { if (k==max) grow(); } ins >> data[k] ; if(! ins.good()) break; } n = k; // Actual # of items read, stored.

6 ... 6/26 Using Pointers with Arrays Looping with Subscripts The flexible array grows each time it becomes overfull. k n max data k n max k n max k n max k n max k n max k n max

7 ... 7/26 Using Pointers with Arrays Looping with Pointers This illustrates using pointers to output a flex array. ostream& Flex::print ( ostream& outs ){ BT* cursor = data; BT* end = cursor + n; // an off-board sentinel for( ; cursor<end; ++cursor) outs <<*cursor << \n ; return outs; }

8 ... 8/26 Using Pointers with Arrays Looping with Pointers Start by setting an off-board pointer to the end of the data in the array. data cursor end n max data cursor n max end Continue by incrementing the pointer.

9 ... 9/26 Using Pointers with Arrays Ending the Scan Stop when the pointer reaches the end pointer: data cursor end n max data cursor end n max

10 ... 10/26 A Call Chart for Insertion Sort Insertion Sort Logic Insertion Sort Inner Loop Insertion Sort Outer Loop

11 ... 11/26 Insertion Sort Logic sort end = data + n newcomer = data + 1 < end ++ false true newcomer = * = return false > sort.data -- true * = newcomer * = *(-1) false *(-1) <= newcomer true break

12 ... 12/26 Insertion Sort Execution preparation To begin the sort, we initialize to subscript 1. The outer loop will execute N-1 times, with increasingly longer segments of the array. The element in slot 0 is colored tan to indicate that it is in the sorted part of the array. On each, the element under the pointer will be inserted into the sorted part of the array. before sorting data end

13 ... 13/26 Insertion Sort Execution 1 The element under the pointer is called the newcomer. We start by removing it from the array, leaving a. The pointer starts at the pointer, and moves backwards through the array until it reaches the proper insertion slot for the newcomer. In this example, the loop ends after the first comparison. When the loop ends, the newcomer moves into the newcomer newcomer

14 ... 14/26 Insertion Sort Execution 2 At each step, one array element is moved one slot to the right, making the move leftward. This time, two elements are moved to the right and the insertion slot is found at the left end of the array. When the loop ends, the newcomer moves into the newcomer

15 ... 15/26 Insertion Sort Execution 3 Now we are on 4 and the newcomer is 109. The inner loop exits on the first comparison and the 109 is put back into the same it started from newcomer

16 ... 16/26 Insertion Sort Execution 4 Now we are on 5 and the newcomer is.83. The inner loop will execute four times and shift four elements rightward. The (and the newcomer) end up at slot 0 of the array newcomer

17 ... 17/26 Insertion Sort Execution 5 This time the newcomer belongs in the middle of the sorted part of the array. The loop stops at slot 3 and the newcomer is stored there newcomer

18 ... 18/26 Insertion Sort Execution 6 On the next, two elements are moved to the right when 5.4 become the newcomer newcomer

19 ... 19/26 Insertion Sort Execution 7 Then it happens again: two elements are moved to the right when 6.9)become the newcomers newcomer

20 ... 20/26 Insertion Sort Execution 8 Now 1.7 is the newcomer and causes 7 elements to be moved. On the last, 132 is the newcomer and the loop exits without moving anything. The 132 is stored back into the slot it came from newcomer

21 ... 21/26 Insertion Sort Execution 9 On the last, 41 is the newcomer Two things are moved, 41 is stored in the, and the sort is finished. It takes 9 es to sort a list of 10 items newcomer

22 ... 22/26 Pointers to Functions Pointers to Functions Function Types and Typedefs Pointing at a Function Example: Menu Processing

23 ... 23/26 Pointers to Functions Function Types Sometimes, we want one function to take another function as a parameter. A thing must have a type before it can be a parameter to a function. Therefore, we need to be able to write the type of a function. The syntax for the type of a function is somewhat confusing, so we often use typedef to give a simple name to a function type. A function prototype is a combination of the name of the function and its type.

24 ... 24/26 Pointers to Functions Function prototype and typdef return type name parameter types double sin ( double, double ); typedef return type parameter types double (* dubl_2dubl )( double, double ); new type name

25 ... 25/26 Pointers to Functions Pointing at a Function To set a function pointer, use the function name without parens. typedef double (* dubl_2dubl)( double, double ); dubl_2dubl fp = pow; dubl_2dubl gp = fmod; fp double pow( double, double ) gp double fmod( double, double ) See the code example named func.c

26 ... 26/26 Pointers to Functions Example: Menu Processing This is the array of function pointers that is used in the code example named menu.c function [0] [1] [2] [3] [4] [5] [6]. cos() log() exp() sin() tan() log_2()