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מערכים (arrays)

מערכים (arrays). 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. Arrays. A block of many variables of the same type. Array can be declared for any type.

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מערכים (arrays)

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  1. מערכים(arrays) Department of Computer Science-BGU

  2. 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 . Department of Computer Science-BGU

  3. 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 Department of Computer Science-BGU

  4. Arrays in Memory • Sequence of variables of specified type • The array variable itself holds the address in memory of beginning of sequence • Example: double S[10]; • The k-th element of array A is specified by A[k-1] (0 based) Constant integer type … 0 1 2 3 4 5 6 7 8 9 … S Department of Computer Science-BGU

  5. 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 Department of Computer Science-BGU

  6. #define • #definedefines a symbolic name. • During preprocessing phase, symbolic names are replaced by the replacement text. Department of Computer Science-BGU

  7. 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]); } Department of Computer Science-BGU

  8. 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 ! Department of Computer Science-BGU

  9. 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}; Department of Computer Science-BGU

  10. 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}; Department of Computer Science-BGU

  11. 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);  } Department of Computer Science-BGU

  12. Insertion Sort #include <stdio.h>  int main() { int array[]= {12 ,3,34,14,98,33,9,17};  inttemp,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; }  } Department of Computer Science-BGU

  13. 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]; } } Department of Computer Science-BGU

  14. 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]; Department of Computer Science-BGU

  15. 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] Department of Computer Science-BGU

  16. 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. Department of Computer Science-BGU

  17. 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: Exercise • Print all the matrices on the screen Department of Computer Science-BGU

  18. Solution #include <stdio.h> #define N 3 void 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"); { { Department of Computer Science-BGU

  19. חיפוש במערך - Search • מערך מאפשר חיפוש של איבר בתוך המערך . • במידה שהאיבר קיים וגם במקרה שהאיבר לא קיים במערך. • ניתן להשתמש בכמה שיטות בתלות במצב האיברים במערך: • חיפוש סדרתי במערך לא לממויין • חיפוש סדרתי במערך ממויין • חיפש בינרי במערך ממויין המכללה האקדמית להנדסה סמי שמעון

  20. חיפוש בינרי void main(){ int a[]={3,5,8,12,23,34,56,78,99}; int left = 0, right = 8, middle; int x= 36; while (left <= right) { middle = (left + right)/2; if (a[middle] > x) right = middle - 1; else if (a[middle] < x) left = middle + 1; else break; { if (left > right) printf("The number %d is not in the array\n",x); else printf("The number %d is in the array\n",x); } המכללה האקדמית להנדסה סמי שמעון

  21. Strings in C Department of Computer Science-BGU

  22. 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 ?! Department of Computer Science-BGU

  23. 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’}; Department of Computer Science-BGU

  24. 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 Department of Computer Science-BGU

  25. 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); } Department of Computer Science-BGU

  26. 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() Department of Computer Science-BGU

  27. Reading-in strings - scanf • A simpler way is to use scanf • To 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) Department of Computer Science-BGU

  28. 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. Department of Computer Science-BGU

  29. 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. Department of Computer Science-BGU

  30. …. ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’ …. …. …. ‘H’ ‘e’ ‘l’ ‘l’ ‘o’ ‘\0’ …. …. A B 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 Department of Computer Science-BGU

  31. 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… Department of Computer Science-BGU

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