מערכים (arrays) 02 דצמבר 15 02 דצמבר 15 02 דצמבר 1502 דצמבר 15 02...
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Transcript of מערכים (arrays) 02 דצמבר 15 02 דצמבר 15 02 דצמבר 1502 דצמבר 15 02...
מערכים(arrays)
21/04/23 07:36
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Department of Computer Science-BGU
Problems with simple variables
Hard to give up values for high number of variables.
Complex to sort a high number of variables by value.
Impossible to use loops .
21/04/23 07:36
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Department of Computer Science-BGU
Arrays
A block of many variables of the same type.
Array can be declared for any type. E.g. int A[10] is an array of 10 integers.
Examples: list of students’ marks series of numbers entered by user vectors matrices
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Department of Computer Science-BGU
Arrays in Memory
Sequence of variables of specified typeThe array variable itself holds the address in
memory of beginning of sequenceExample: double S[10];
The k-th element of array A is specified by A[k-1] (0 based)
0 1 2 3 4 5 6 7 8 9
S
……
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Department of Computer Science-BGU
Constant integer type
Example - reverse
#include <stdio.h>void main() { int i, A[10];
printf("please enter 10 numbers:\n"); for(i=0; i<10; i++) scanf("%d",&A[i]);
printf("numbers in reversed order:\n"); for(i=9; i>=0; i--) printf("%d\n",A[i]);}
Constant integer type
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Department of Computer Science-BGU
#define
#define defines a symbolic name.
During preprocessing phase, symbolic names are replaced by the replacement text.
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Department of Computer Science-BGU
Reverse with #define
/* get 10 integers from the user and printing them in reversed order*/
#include <stdio.h>#define NUM 10
void main() { int i; int A[NUM];
printf(“Please enter %d numbers:\n",NUM); for(i=0; i<NUM; i++) scanf("%d",&A[i]);
printf("numbers in reversed order:\n"); for(i=NUM-1; i>=0; i--) printf("%d\n",A[i]);} 21/04/23 07:36
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Department of Computer Science-BGU
Will it work?
Example :#include <stdio.h>#define NUM 10
void main() { int i = 10; int A[i];
… NO !! Need a constant integer type , no
variable !
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Department of Computer Science-BGU
Initialization
Like in the case of regular variables, we can initialize the array during declaration.
The number of initializers cannot be more than the number of elements in the array
But it can be lessin which case, the remaining elements are initialized to 0
The next declarations:int array1[5] = {0};
int array2[8] = {2, 4, 6, 8}; meaning:
int array1[5] = {0,0,0,0,0};int array2[] = {2, 4, 6, 8,0,0,0,0};
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Department of Computer Science-BGU
Initialization (cont.)
If you like, the array size can be inferred from the number of initializers.
Leaving the square brackets empty .So these are identical declarations :
int array1 [8] = {2, 4, 6, 8, 10, 12, 14, 16}; int array2 [] = {2, 4, 6, 8, 10, 12, 14, 16};
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Department of Computer Science-BGU
Bubble Sort
#include <stdio.h> void main() {
int array[] = {78,73,63,62,58,45,34,11}; int i,temp, n = 8 , flag; do {
flag = 0;for ( i = 0 ; i < n-1 ; i++) {
if ( array[i] > array[i+1] ) { temp = array[i]; array[i] = array[i+1]; array[i+1] = temp; flag = 1;
} }
} while (flag); }
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Department of Computer Science-BGU
Insertion Sort
#include <stdio.h> int main() {
int array[]= {12 ,3,34,14,98,33,9,17}; int temp,loops = 0 ,j, i, n = 8; for ( i = 1 ; i < n ; i++) {
temp = array[i];j = i-1;loops++;while (( j >=0) && ( temp < array[j])) {
array[j+1] = array[j];j--;loops++;
}array[j+1] = temp;}
}
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Department of Computer Science-BGU
Selection Sort
void selection_sort(int list[], int n) { int i, j, min; for (i = 0; i < n - 1; i++) { min = i; for (j = i+1; j < n; j++) { if (list[j] < list[min]) { min = j; } } temp = list[i]; list[i] = list[min]); list[min] = list[i]; } }
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Department of Computer Science-BGU
Two Dimensional Arrays
We sometimes want to keep an inherent Two-Dimensional structure of data.
Example: We can define a two-dimensional 3x3 matrix
by
double A[3][3];
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Department of Computer Science-BGU
Two Dimensional Arrays
Array of arrays:int A[2][3] = { {1, 2, 3},
{4, 5, 6} }; Means an array of 2 integer arrays, each of
length 3. Access: j-th element of the i-th array is
A[i][j]
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Department of Computer Science-BGU
Initialization Two Dimensional Arrays
When initializing the array, it is necessary to indicate the size of the second dimension:
double B[][2] = {{1,2}, {2,3}, {3,4}};
Filling with zeros (0) is similar to one dimension array.
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Department of Computer Science-BGU
Exercise
jkBkiAjiCn
k
*1
0
Write a program that defines 3 matrices A,B,C of size 3x3 with float elements; initialize the first two matrices (A and B)
Compute the matrix multiplication of A and B and store it in C (i.e. C = A*B)
Matrix Multiplication:
Print all the matrices on the screen
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Department of Computer Science-BGU
Solution
#include <stdio.h>#define N 3void main(){
int a[N][N],b[N][N],c[N][N]={0};int i,j,k;for(i=0;i<N;i++) for(j=0;j<N;j++) scanf("%d%d",&a[i][j],&b[i][j]);for(i=0;i<N;i++) for(j=0;j<N;j++)
for(k=0;k<N;k++) c[i][j]+= a[i][k]*b[k][j];
printf("\n\n");
for(i=0;i<N;i++){for(j=0;j<N;j++)
printf("%4d",a[i][j]);printf("\n");
{
for(i=0;i<N;i++){for(j=0;j<N;j++)
printf("%4d",b[i][j]);printf("\n");
}for(i=0;i<N;i++){
for(j=0;j<N;j++)printf("%4d",c[i][j]);
printf("\n");{
{
21/04/23 07:36Department of Computer Science-BGU
18
Strings in C
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Department of Computer Science-BGU
String
A sequence of characters.Stored, as might be expected, in an array
of chars.Another way to initialize: char A[]=“blabla”;
Problem : It may be much shorter
than the array where it’s stored
How can we know where a string end ?!
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Department of Computer Science-BGU
The Terminator
Strings terminate with NULL character, signed by ‘\0’ (ASCII code 0).
This is a convention used to know where the string ends.
It means that in order to hold a string of 7 chars we need an array of length at least 8
So the previous initialization : char A[]=“blabla”;
is equivalent to char A[] = {‘b’, ‘l’, ‘a’, ‘b’, ‘l’, ‘a’, ‘\
0’};
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Department of Computer Science-BGU
Printing strings
printf allows printing whole strings at once using %s –
char str[200];/* … */printf(“%s\n”, str);
This will print the contents of str ,cell by cell, until a ‘\0’ is encountered this may be less than 200, but also more
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Department of Computer Science-BGU
The fool on the null
#include <stdio.h>void main() { char str[]="I'm a full string"; printf("%s\n",str); str[7]='o'; str[8]='o'; printf("%s\n",str); str[11]='\0'; printf("%s\n",str); str[11] = ‘s’; printf("%s\n", str);}
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Department of Computer Science-BGU
Reading-in strings
There are several ways of accepting strings as input from the user.
The obvious way to go is read character by character using getchar()
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Department of Computer Science-BGU
Reading-in strings - scanf
A simpler way is to use scanfTo read in a string to a variable str we can use:
scanf(“%s”, str); Note there’s no ‘&’ sign!!!
Scanf reads-in letters until a space or newline is encountered
The maximum length can be stated in the parentheses – scanf(“%10s”, str); This will read in 10 letters, plus the ‘\0’ sign (so str should
have place for 11 characters)
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Department of Computer Science-BGU
Reading-in strings – gets)(
An other simpler way is to use gets()To read in a string to a variable str we can use:
gets( str); Note there’s no ‘&’ sign too!!!
gets reads-in letters until a newline (pressing enter on the keyboard) is encountered.
The maximum length can be stated for the array of characters declaration minus 1.
gets change the new line for the `\0` sign as last character in the string.
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Department of Computer Science-BGU
Writing out strings – puts)(
A simpler way to write out a string is to use puts()To write out a string that is into variable str we
can use: puts( str);
puts write out letters until a `\0` sign is encountered.
puts change the `\0` sign for new line as the last character in the string.
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Department of Computer Science-BGU
Comparing strings
We cannot just compare strings’ contents by char A[7]=“Hello”; char B[7]=“Hello”; if(A==B) { … }Because A and B are addresses of A[0] and B[0]
A==B only if A and B are the same string in memory In order to compare the contents we must scan char by
char
‘H’
‘e’
‘l’ ‘l’ ‘o’
\‘0’ .…
B
….
….
‘H’
‘e’
‘l’ ‘l’ ‘o’
\‘0’ .…
A
….
….
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Department of Computer Science-BGU
String library
Like in the case of stdio.h and math.h, we have a special library for handling strings
We should #include <string.h>Functions:
strlen(s) – returns the length of s strcmp(s1, s2) – compares s1 with s2 strcpy(s1, s2) – copies to contents of s2 to s1 strcat(s1 , s2) – add the s1 at the end of s2 and more…
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Department of Computer Science-BGU