Lecture 18: The Elegant, Abstract World of Computing
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Lecture 18: The Elegant, Abstract World of Computing
description
Lecture 18: The Elegant, Abstract World of Computing. Arrays and Pointers. Arrays and pointers closely related Array name like a constant pointer Arrays passed to a function by reference Pointers can do array subscripting operations. Relationship between Pointers and Arrays. - PowerPoint PPT Presentation
Transcript of Lecture 18: The Elegant, Abstract World of Computing
Lecture 18: The Elegant, Abstract World of Computing
Arrays and PointersArrays and pointers closely related
Array name like a constant pointerArrays passed to a function by referencePointers can do array subscripting operations
Relationship between Pointers and Arrays
Define an array b[5] and a pointer bPtr int b[5]; int *bPtr;
To set them equal to one another usebPtr = b;
The array name (b) is actually the address of first element of the array.
bPtr = &b[ 0 ];Explicitly assigns the address of first element of b to bPtr.
Element b[ 3 ]Can be accessed by *(bPtr + 3) --- pointer/offset notation
where 3 is called the offset.Can be accessed by bPtr[ 3 ] --- pointer/subscript notationbPtr[ 3 ] same as b[ 3 ]Can also be accessed by *(b+3) --- view b as a constant pointer
*b accesses to b[0]
Relationship between Pointers and ArraysWhat is the difference?
int b[5], a[5]; int *objPtr;
A pointer can be assigned to point to different objects.objPtr = b;objPtr = a;
Array name likes a constant pointer.objPtr = b;a = objPtr;b = a;
PointersDereferencing a NULL pointer is NOT allowed.
int *aPtr = NULL, *bPtr = 0;int x;x = *aPtr;x = * bPtr;
Dereferencing a pointer not pointing to an object is NOT allowed.int *aPtr , x;x = *aPtr;
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k]; }return;}
……
…a[0]
0xBFFFF82C1
RAM
In main(), just before call into sumRef()
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k]; }return;}
……
…a[0]
0xBFFFF82C1
RAM
In main(), while calling into sumRef()
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
0
0
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), just after execute “*sum = 0;”
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), …
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
0
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), after execute “*sum += x[k];”
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
0
1
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), increment k
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
0
1
1
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), perform addition
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
1
1
1
3
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), increment k
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
1
3
2
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), perform addition
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
0
3
2
6
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), right after the for loop
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0xBFFFF840
k0xBFFFF85C
05
15
Passing Arrays by Reference
Array name like a constant pointer
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int *sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, &total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int *sum ) { int k;
*sum = 0; for (k = 0; k < size; k++) { *sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In main(), right after call sumRef()
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
15
Call-by-Value via Call-by-ReferencePassing array by referencePassing integer/char/float by value#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k]; }return;}
……
…a[0]
0xBFFFF82C1
RAM
In main(), just before call into sumRef()
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k]; }return;}
……
…a[0]
0xBFFFF82C1
RAM
In main(), while calling into sumRef()
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0
k0xBFFFF85C
0
Passing array by referencePassing integer/char/float by value
0
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), just after execute “sum = 0;”
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
0
k0xBFFFF85C
0
Passing array by referencePassing integer/char/float by value
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), …
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
k0xBFFFF85C
0
0
0
Passing array by referencePassing integer/char/float by value
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), after execute “sum += x[k];”
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
k0xBFFFF85C
0
01
Passing array by referencePassing integer/char/float by value
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), increment k
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
k0xBFFFF85C
01
1
Passing array by referencePassing integer/char/float by value
1
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), perform addition
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
k0xBFFFF85C
01
3
Passing array by referencePassing integer/char/float by value
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In sumRef(), right after the for loop
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
x0xBFFFF850
0xBFFFF82C
size0xBFFFF854
5
sum0xBFFFF858
k0xBFFFF85C
05
15
Passing array by referencePassing integer/char/float by value
Call-by-Value via Call-by-Reference
#include<stdio.h>
#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d\n”, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k];}return;}
……
…a[0]
0xBFFFF82C1
RAM
In main(), right after call sumRef()
2
3
4
5
a[1]0xBFFFF830
a[2]0xBFFFF834
a[3]0xBFFFF838
a[4]0xBFFFF83C
total0xBFFFF840
0
Passing array by referencePassing integer/char/float by value
In-class Programming Assignment
#include <stdio.h>#define SIZE 3000#define WINDOW 10
void readin(double sonar[SIZE]);void denoising(double *sonarPtr, double *rPtr);
int main (){double sonar[SIZE];double denoisedSonar[SIZE];
readin(sonar);denoising(sonar, denoisedSonar);return 0;
}
void denoising(double *sonarPtr, double *rPtr){ /* You need to finish this function */ For every possible starting index j of the sliding window, sum = 0; compute the total sum of the sonar data inside the sliding window; rPtr[j] = sum / WINDOW;
}
void readin(double sonar[SIZE]){}
7.034 0.361 2.550 11.588 19.166 18.317 9.821 1.490 0.983 8.766 17.684 19.537 12.622 3.296 0.134 6.043 15.590 19.998 15.214 5.636 0.071 3.634 13.050
j
Practice Question
Q. What is the output of the following program?
#include<stdio.h>#define SIZE 5void sumRef( int *x, int size, int sum );
int main( void ) { int a[SIZE] = {1, 2, 3, 4, 5}; int total = 0; sumRef( a, SIZE, total ); printf(“%d %d\n”, *a, total); return 0; }
void sumRef( int *x, int size, int sum ) { int k;
sum = 0; for (k = 0; k < size; k++) { sum += x[k]; }for (k = 0; k < size; k++) { x[k] *= 3; }}
A. 3 15B. 1 15C. 3 0D. 1 0
Solu
tion
: C
Practice Question
Q. To assign the third element b[2] of an array b to the variable x, which of the following is WRONG?
int x, b[5] = {1, 2, 3, 4, 5};int *bPtr;
A. bPtr = b;x = *(bPtr+2);
B. bPtr = b;x = bPtr[2];
C. x = *(b+2);
D. b = b + 2;x = *b;
E. bPtr = b;bPtr += 2;x = *bPtr; S
olu
tion
: D
