Douglas Pfe er and Period 5 of MAC2312 Lecture in Spring ...
PFE Lecture Week4
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PFE / SEM2 2013/2014 Faculty of Manufacturing Universiti Malaysia Pahang Kampus Pekan, Pahang Darul Makmur Tel: +609-424 5800 Fax: +609-4245888 BFM/BHM 2013 PROGRAMMING FOR ENGINEERS Week 4 Pointers and Array • Relationship between pointers and array • Pointer arithmetic • Initialize pointer • Compare pointers • Pointers as function arguments • Pointers to constant & constant pointer
Transcript of PFE Lecture Week4
PFE / SEM2 2013/2014
Faculty of ManufacturingUniversiti Malaysia Pahang
Kampus Pekan, Pahang Darul MakmurTel: +609-424 5800Fax: +609-4245888
BFM/BHM 2013 PROGRAMMING FOR ENGINEERS
Week 4
Pointers and Array• Relationship between
pointers and array• Pointer arithmetic• Initialize pointer
• Compare pointers• Pointers as function
arguments• Pointers to constant &
constant pointer
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9.3 Relationship Between Arrays and Pointers
• Array name can be used as a pointer constantint vals[] = {4, 7, 11};cout << *vals; // displays 4
• Pointer can be used as an array nameint *valptr = vals;cout << valptr[1]; // displays 7
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Program 9-5// This program shows an array name being dereferenced // with the * operator.
#include <iostream.h>
void main(void){
short numbers[] = {10, 20, 30, 40, 50};
cout << "The first element of the array is ";cout << *numbers << endl;
}
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Program Output
The first element in the array is 10
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Pointers in Expressions
• Given:int vals[]={4,7,11};int *valptr = vals;
• What is valptr + 1? • It means (address in valptr) + (1 * size of an int)
cout << *(valptr+1); // displays 7cout << *(valptr+2); // displays 11
• Must use ( ) in expression
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Array Access
Array elements can be accessed in many ways
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Array access method
Example
array name and [ ] vals[2] = 17;
pointer to array and [ ] valptr[2] = 17;
array name and subscript arithmetic
*(vals+2) = 17;
pointer to array and subscript arithmetic
*(valptr+2) = 17;
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Array Access
• Array notation
vals[i]
is equivalent to the pointer notation
*(vals + i)• No bounds checking performed on array
access
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Figure 9-3
numbers
numbers[0] numbers[1] numbers[2] numbers[3] numbers[4]
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Figure 9-4
numbers
numbers[0] numbers[1] numbers[2] numbers[3] numbers[4]
(numbers+1) (numbers+2) (numbers+3) (numbers+4)
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Program 9-6
// This program processes the contents of an array. Pointer// notation is used.#include <iostream.h>
void main(void){
int numbers[5];
cout << "Enter five numbers: ";for (int count = 0; count < 5; count++)
cin >> *(numbers + count);cout << "Here are the numbers you entered:\n";for (int count = 0; count < 5; count++)
cout << *(numbers + count)<< " ";cout << endl;
}
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Program Output with Example Input
Enter five numbers: 5 10 15 20 25 [Enter]Here are the numbers you entered:5 10 15 20 25
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Program 9-7// This program uses subscript notation with a pointer and// pointer notation with an array name.
#include <iostream.h>
void main(void){
float coins[5] = {0.05, 0.1, 0.25, 0.5, 1.0};float *floatPtr; // Pointer to a floatint count; // array index
floatPtr = coins; // floatPtr now points to coins arraycout.precision(2);cout << "Here are the values in the coins array:\n";
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Program continues
for (count = 0; count < 5; count++)cout << floatPtr[count] << " ";
cout << "\nAnd here they are again:\n";for (count = 0; count < 5; count++)
cout << *(coins + count) << " ";cout << endl;
}
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Program Output
Here are the values in the coins array:0.05 0.1 0.25 0.5 1And here they are again:0.05 0.1 0.25 0.5 1
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Program 9-8// This program uses the address of each element in
the array.
#include <iostream.h>#include <iomanip.h>
void main(void){
float coins[5] = {0.05, 0.1, 0.25, 0.5, 1.0};float *floatPtr; // Pointer to a floatint count; // array indexcout.precision(2);cout << "Here are the values in the coins array:\n";
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Program continues
for (count = 0; count < 5; count++){
floatPtr = &coins[count];cout << *floatPtr << " ";
}cout << endl;
}
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Program Output
Here are the values in the coins array:0.05 0.1 0.25 0.5 1
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9.4 Pointer Arithmetic
• Some mathematical operations may be performed on pointers.– The ++ and – operators may be used to increment
or decrement a pointer variable.– An integer may be added to or subtracted from a
pointer variable. This may be performed with the +, - +=, or -= operators.
– A pointer may be subtracted from another pointer.
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Pointer Arithmetic
Assume the variable definitions int vals[]={4,7,11}; int *valptr = vals; Examples of use of ++ and -- valptr++; // points at 7 valptr--; // now points at 4
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More on Pointer Arithmetic
Assume the variable definitions:
int vals[]={4,7,11};
int *valptr = vals;
Example of the use of + to add an int to a pointer:
cout << *(valptr + 2)
This statement will print 11
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Assume the variable definitions:
int vals[]={4,7,11};
int *valptr = vals;
Example of use of +=:
valptr = vals; // points at 4
valptr += 2; // points at 11
More on Pointer Arithmetic
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More on Pointer Arithmetic
Assume the variable definitions
int vals[] = {4,7,11};
int *valptr = vals;
Example of pointer subtraction
valptr += 2;
cout << valptr - val;
This statement prints 2: the number of
ints between valptr and val
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Program 9-9
// This program uses a pointer to display the contents// of an integer array.#include <iostream.h>
void main(void){
int set[8] = {5, 10, 15, 20, 25, 30, 35, 40};int *nums, index;nums = set;cout << "The numbers in set are:\n";for (index = 0; index < 8; index++){
cout << *nums << " ";nums++;
}
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Program continues
cout << "\nThe numbers in set backwards are:\n";for (index = 0; index < 8; index++){
nums--;cout << *nums << " ";
}}
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Program Output
The numbers in set are:5 10 15 20 25 30 35 40The numbers in set backwards are:40 35 30 25 20 15 10 5
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10.5 Initializing Pointers
• Can initialize to NULL or 0 (zero) int *ptr = NULL;• Can initialize to addresses of other variables
int num, *numPtr = #int val[ISIZE], *valptr = val;
• Initial value must have correct typefloat cost;int *ptr = &cost; // won't work
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9.6 Comparing Pointers
• If one address comes before another address in memory, the first address is considered “less than” the second. C++’s relational operators maybe used to compare pointer values.
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10.6 Comparing Pointers• Relational operators can be used to compare
addresses in pointers• Comparing addresses in pointers is not the
same as comparing contents pointed at by pointers: if (ptr1 == ptr2) // compares // addresses if (*ptr1 == *ptr2) // compares
// contents
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Figure 9-5
0x5A00
array[0] array[1] array[2] array[3] array[4]
0x5A04 0x5A08 0x5A0C 0x5A0F
(Addresses)
An array of five integers
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Program 9-10// This program uses a pointer to display the contents// of an integer array.#include <iostream.h>
void main(void){
int set[8] = {5, 10, 15, 20, 25, 30, 35, 40};int *nums = set; // Make nums point to set
cout << "The numbers in set are:\n";cout << *nums << " "; // Display first elementwhile (nums < &set[7]){
nums++;cout << *nums << " ";
}
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Program continues
cout << "\nThe numbers in set backwards are:\n";cout << *nums << " "; // Display last elementwhile (nums > set){
nums--;cout << *nums << " ";
}}
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Program Output
The numbers in set are:5 10 15 20 25 30 35 40The numbers in set backwards are:40 35 30 25 20 15 10 5
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9.7 Pointers as Function Parameters
• A pointer can be used as a function parameter. It gives the function access to the original argument, much like a reference parameter does.
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10.7 Pointers as Function Parameters
• A pointer can be a parameter• Works like a reference parameter to allow
change to argument from within function• A pointer parameter must be explicitly
dereferenced to access the contents at that address
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Pointers as Function Parameters
Requires: 1) asterisk * on parameter in prototype and heading
void getNum(int *ptr); 2) asterisk * in body to dereference the pointer
cin >> *ptr; 3) address as argument to the function
getNum(&num);
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Pointers as Function Parameters
void swap(int *x, int *y) { int temp;
temp = *x; *x = *y; *y = temp;
} int num1 = 2, num2 = -3; swap(&num1, &num2);
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Program 9-11
// This program uses two functions that accept addresses of// variables as arguments.#include <iostream.h>
// Function prototypesvoid getNumber(int *);void doubleValue(int *);
void main(void){
int number;getNumber(&number) // Pass address of number to getNumberdoubleValue(&number); // and doubleValue.cout << "That value doubled is " << number << endl;
}
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Program continues// Definition of getNumber. The parameter, Input, is a pointer.// This function asks the user for a number. The value entered// is stored in the variable pointed to by Input.
void getNumber(int *input){
cout << "Enter an integer number: ";cin >> *input;
}
// Definition of doubleValue. The parameter, val, is a pointer.// This function multiplies the variable pointed to by val by// two.
void doubleValue(int *val){
*val *= 2;}
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Program Output with Example Input
Enter an integer number: 10 [Enter]That value doubled is 20
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Program 9-12// This program demonstrates that a pointer may be used as a // parameter to accept the address of an array. Either subscript// or pointer notation may be used.#include <iostream.h>#include <iomanip.h>
// Function prototypesvoid getSales(float *);float totalSales(float *);
void main(void){
float sales[4];
getSales(sales);cout.precision(2);
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Program continues
cout.setf(ios::fixed | ios::showpoint);cout << "The total sales for the year are $";cout << totalSales(sales) << endl;
}
// Definition of getSales. This function uses a pointer to accept// the address of an array of four floats. The function asks the // user to enter the sales figures for four quarters, and stores// those figures in the array. (The function uses subscript// notation.)
void getSales(float *array){
for (int count = 0; count < 4; count++){
cout << "Enter the sales figure for quarter ";cout << (count + 1) << ": ";cin >> array[count];
}}
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Program continues// Definition of totalSales. This function uses a pointer to// accept the address of an array of four floats. The function // gets the total of the elements in the array and returns that// value. (Pointer notation is used in this function.)
float totalSales(float *array){
float sum = 0.0;
for (int count = 0; count < 4; count++){
sum += *array;array++;
}return sum;
}
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Program Output with Example Input
Enter the sales figure for quarter 1: 10263.98 [Enter]Enter the sales figure for quarter 2: 12369.69 [Enter]Enter the sales figure for quarter 3: 11542.13 [Enter]Enter the sales figure for quarter 4: 14792.06 [Enter]The total sales for the year are $48967.86
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10.8 Ponters to Constants and Constant Pointers
• Pointer to a constant: cannot change the value that is pointed at
• Constant pointer: address in pointer cannot change once pointer is initialized
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Ponters to Constant
• Must use const keyword in pointer definition:const double taxRates[] =
{0.65, 0.8, 0.75};
const double *ratePtr;• Use const keyword for pointers in
function headers to protect data from modification from within function
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Pointer to Constant – What does the Definition Mean?
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Constant Pointers
• Defined with const keyword adjacent to variable name:int classSize = 24;int * const classPtr = &classSize;
• Must be initialized when defined• Can be used without initialization as a function
parameter– Initialized by argument when function is called– Function can receive different arguments on different calls
• While the address in the pointer cannot change, the data at that address may be changed
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Constant Pointer – What does the Definition Mean?
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