Computer Programming 5th Week loops (do-while, for), Arrays, array operations, C libraries

Transcription

1 Computer Programming 5th Week loops (do-while, for), Arrays, array operations, C libraries Hazırlayan Asst. Prof. Dr. Tansu Filik

2 Computer Programming Previously on Bil 200 Low-Level I/O getchar, putchar, getc, putc Control Structures if-else, switch-case, loops (while, do-while, for loops)

3 Computer Programming Exam Examples #include <stdio.h> main(void) { int i = 0, x = 0; for (i = 1; i < 10; ++i) { if( i % 2 == 1) x += i; else x--; printf("%d-", x); printf("\nx = %d", x); Write output of the program x = 21 Rewrite the equivalent of the following for loop using while loop i = 1; while (i < 10) { if( i % 2 == 1) x += i; else x--; printf("%d ", x); ++i;

4 Computer Programming Exam Examples #include <stdio.h> main(void) { int i, j, k, x=0, y=0; for (i = 0; i < 5; ++i) for (j = 0; j<i; ++j) { switch (i + j - 1) { case -1: case 0: y +=++x; x += y; break; case 1: case 2: case 3: x += 2; default: x += 3; printf("%d ", x); printf("\nx = %d\n", x); Write output of the program #include <stdio.h> main(void) { int i, j, k, x=0, y=0; x = 21 Rewrite the program by using if-else statement instead of switch statement which will give the same output. for (i = 0; i < 5; ++i) for (j = 0; j<i; ++j) { if ((i + j - 1) <= 0 ){ y += ++x; x += y; else if ((i + j - 1) > 0 && (i + j - 1) <= 3) { x += 2; x += 3; else x += 3; printf("%d ", x); printf("\nx = %d\n", x);

5 Computer Programming Exam Examples

6 Computer Programming Control structures: Loops Example: Nested for loops Consider formula: x 2 + y 2 = z 2. Here, x, y, and z are integers. As an example, for 3,4,5 triangle, = 5 2. We wish to obtain others. Brute force method: We may check all integer x-y couples and see if an integer z can be obtained.

7 Computer Programming Example: Nested for loops #include <stdio.h> #define N 20 void main(void) { int i,j,k, karetoplam; Control structures: Loops for (i=1; i <= N; i++) { for (j=1; j <= N; j++) { karetoplam = i * i + j * j; /* x2 + y2 */ /* is there a suitable z for the x and y? */ for(k = 1 ; k <= N; k++ ) { if (karetoplam == k * k) { printf("%5d %5d %5d\n", i, j, k);

8 Computer Programming Example: Nested for loops Control structures: Loops

9 Computer Programming Exercises: Control structures: Loops Can you write the previous program using only two nested loops? Think of a way to eliminate repetitions in the loop. In this way, once is determined, the program will not output

10 Computer Programming Exercises: Control structures: Loops Write a program in which you can enter a string into a string variable using for loop and getchar. Whenever an enter is pressed, the new string must be available for printing with printf( %s,..). Find a simple program that converts lowercase letters to uppercase ones inside a string. Do not use long lists of if-else or switch-case. Do not use ready C functions either.

11 Computer Programming Exercises: Control structures: Loops Can you write the previous program using only two nested loops? Think of a way to eliminate repetitions in the loop. In this way, once is determined, the program will not output

12 Computer Programming Exercises: Control structures: Loops Write a program in which you can enter a string into a string variable using for loop and getchar. Whenever an enter is pressed, the new string must be available for printing with printf( %s,..). Find a simple program that converts lowercase letters to uppercase ones inside a string. Do not use long lists of if-else or switch-case. Do not use ready C functions either. You do not need to know ASCII codes of letter, either. Be smart.

13 Computer Programming Arrays Arrays List of variables: [ ]

14 Computer Programming Arrays Arrays It is the declaration of a list of same type of variables under a single name. An example can be the days of week. This is a 1-D array. The first element is Monday, the last is Sunday.

15 Computer Programming Arrays Arrays Another example is the days of a month. This can be considered as a 1-D array, or a 2-D array whose horizontal elements are days of weeks, and vertical elements are the weeks. Days of a year, therefore, can be considered as a 3-D array. First dimension is days of week, second dimension is weeks of month, third dimension is months of year.

16 Computer Programming Arrays Array declarations A 1-D array is declared by a variable name and number of elements inside a square bracket. Example: int gun [ 7 ]; gun corresponds to the array name, whose elements are of int type. There are seven elements. Similarly, a 2-D array is declared by two square brackets: int ay[ 4 ][ 7 ]; There are 4 rows and 7 columns of ay.

17 Computer Programming Arrays Array usage Each element of an array is reached by its index written in square parantheses. In C, the first element is with index 0 (zero). For example, the gun array has first 0th, and last 6th element. The numbers inside the square parantheses are called index numbers.

18 Computer Programming Arrays int gun[7]; gun[0] gun[1] gun[2] gun[3] gun[4] gun[5] gun[6] Usage examples: gun[5] = 1; if( gun[5] == 4 ) break; gun[5] = gun[6] - 1;

19 Computer Programming Arrays Example #include <stdio.h> void main() { int gun [ 7 ]; int i; for( i = 0 ; i < 7 ; i++ ) { gun[ i ] = 0;

20 Bil-200 Arrays int c[12]; c[0]=-45; c[1]=0; c[2]=6;... c[11]=78; c[0] c[1] c[2] c[3] c[4] c[5] c[6] c[7] c[8] c[9] c[10] c[11] The array index starts from 0. and ends at N-1

21 Bil-200 Arrays x=2; c[x] == c[5-3] == c[2] == 6; typical mistakes: c(4) c[1]... c[12] float f=1.0; c[f];

22 Computer Programming Arrays Initializations of arrays We know how to initialize single variables while declaration. The situation is similar for arrays. Just use a list o values inside a block separated by, s. #include <stdio.h> void main() { int gun[7] = { 0,2,4,6,8,10,11 ;...

23 Computer Programming Arrays Initializations of arrays If the array size is too big, declarative initialization is difficult. In that case, make initial assignments inside a program using loops. If we still want to make initializations, we may initialize the first few, and the rest will be zero. As an example: int sayilar [ 10 ] = { 2 ; double dizi [ 250 ] = { 0 ;

24 Computer Programming Arrays - Example #include <stdio.h> void main() { int gun[7] = { 2,3,4,5,6,7,8 ; int i; for (i = 0 ; i < 7 ; i++ ) { printf ("gün[%d] Şubat %d, 2004\n", i, gun[i] ) ;

25 Computer Programming Arrays - Example Write a program which generates random numbers as many as required by the user. The random number generation function exists in a standard library called "stdlib.h". The function name is rand(). It generates numbers between 0 and In order to randomize the generation, use fhe following function: "srand(time(null));". The function time( ) requires including "time.h".

26 Computer Programming Arrays Example: Solution #include <stdio.h> #include <stdlib.h> #include <time.h> #define UZUNLUK 10 void main ( ) { int i, sayilar [UZUNLUK] ; srand(time(null)); for (i = 0 ; i < UZUNLUK ; i++ ) { sayilar[ i ] = rand ( ) ; printf ( "sayi [ %d ] = %d\n", i, sayilar[ i ]) ;

27 Computer Programming Arrays Exercise How do we generate 100 random values using the previous program structure? Extend the program so that it finds and writes the maximum and minimum values inside the random array. Furthermore, the program must find the average, too.

28 Exercise Write the following program and see that the result is NOT #include <stdio.h> void main() { float top = 0.0; int i; It will display on the screen. for(i=0;i<10000;i++) { top += ; printf("toplam = %f\n", top);

29 Why? Floating point numbers are not exactly real numbers. Floating point numbers are represented by inverse powers of 2 (binary representation). Each will not be precisely , so when we add them times, the result will be different. For a better precision, use double. But it will cost more memory.

30 Rounding errors In floating points, never use equality comparisons. for (x = 0.0; x!= 10.0; x += 0.1) { Use near comparisons for floating points. In some computers, the following code will result in different number of loops! for (x = 0; x < 10.0; x += 0.1) { For loops, prefer integers.

31 int and char int (32 bit) and char (8 bit) may also be casted to each other. int char_kod; for (char_kod = (int) 'A'; char_kod <= (int) 'Z'; char_kod++) { printf("%c", (char) char_kod); This program prints out ABCDEFGHIJKLMNOPQRSTUVWXYZ on the screen.

32 Array operations Array search and sorting

33 Array operations Array operations are very frequently used in various programs. In a sorted array, search can be done by simply checking each and every element. However, there are faster ways to do it. A common fast method is called binary search.

34 Sequential search array a list of values a_size array size target the searched value num 1 found false While (num <= a_size) VE (found = false) { if target equals array[num] found true else num num + 1

35 Sequential search #include <stdio.h> #define UZUNLUK 10 void main() { int dizi[uzunluk], bulundu = 0; int sayi=uzunluk, hedef, num = 1; If the value is at the end of the array, it takes very long time. /* target and array[] is read from file */ while( (num <=sayi) && (!bulundu) ) { if(hedef == dizi[num]) bulundu = 1; else num++;

36 Binary search Remember the random number generating and guessing program. The smart guess strategy is to begin with 50. If smaller, the next is 25, otherwise, the next guess is 75i etc. This is a best way to converge to the guessed number. The binary search resembles this one. At each search step, it goes to the middle of the two possible boundaries.

37 Binary search left 1 right LENGTH found 0 (false) while not found and left <= right { mid center of left and right If target equals array[mid] then found 1 (true) else if target < array[mid] then right mid 1 else if target > array[mid] then left mid + 1

38 Binary search Lets search for 33. At first, left 0, right 26, found 0: Left Right since found= false, left<= right, we say mid= (Sol+ Sağ) / 2 : Left Mid Right

39 Binary search target (33) < array[mid] (61), right mid - 1: Left Right Mid found is false, since left <= right, mid (left + right) / 2: Left Mid Right

40 Binary search target (33) < array[mid] (47), right mid - 1: left right mid found is false and left <= right, mid (left + right) / 2: left mid right

41 Binary search target (33) > array[mid] (18), left mid + 1: mid left right found is false and left <= right, mid (left + right) / 2: left mid right

42 Binary search target (33) == array[mid] (33), found true Sol Orta Sağ found is true. So, we return mid as the returned search result.

43 Exercise: Binary search Create a file of 10 ascending ordered number in Notepad. In your program, read these numbers to an array. As the user to enter one of these 10 numbers as the target. Implement the binary search to find the location of the target in the array (and in the file). Write the result on the screen. Think of a way to overcome the not exist problem.

44 Exercise Write a program in which you somehow enter two very large (up to 30 digits) numbers and multiply them (resulting a number with up to 60 digits. Notice that: There are no built in numeric types that can hold such big integer numbers (with full precision). Therefore you have to: Enter the digits into an array (use getche) and multiply them using the algorithm they teach for high school arithmetic.

45 Introduction to Programming Languages C Libraries C Liabraries "math.h", "stdlib.h", "string.h"

46 Introduction to Programming Languages C Libraries The main C language only consists of expressions and grammar rules. C, itself, has NO built in function! Most C compilers come with the standard libraries containing several useful functions. In order to be able to use these functions, you must use #include directive to include the associated library containing the useful function.

47 Introduction to Programming Languages C Libraries Function Libraries <stdio.h> - printf(), fprintf(), scanf(), fscanf(), fopen(), putchar(), getchar(),... <math.h> - pow(), sqrt(), fabs()... <ctype.h> - toupper(), tolower(), isalpha(), isdigit(),... <stdlib.h> - rand(), srand(), exit(),...

48 Introduction to Programming Languages C Libraries Function Libraries : math.h Many mathematical function (prototypes) exist here. fabs (x) : absolute value of float x sqrt (x) : square root of x exp (x) : e to the power x (e = ) log (x) : ln x log10 (x) : base-10 logarithm pow(x, y) : x to the power y sin (x) : sine (radians) cos (x) : cosine (radians) tan (x) : tangent fmod (a, b) : remainder of a/b ceil (x) : least integer >= x floor (x) : greatest integer <= x

49 Introduction to Programming Languages C Libraries Function Libraries : stdlib.h exit(val) : used for terminating program. if (fp == NULL) exit(1); /* if file cannot open, quit*/ rand(void) : generates a random number for(i=0; i < 100; ++i) /* 100 random numbers*/ printf("%d ", rand()); srand(value) : seed for the randomizer srand(100); /* starting point of random sequence*/

50 Introduction to Programming Languages C Libraries Function Libraries : time.h time(null): Numbe of seconds passed since 01/01/1970. int secs; secs = time(null);

51 Introduction to Programming Languages C Libraries Example: Approximate pi value Pi is a very commonly used constant in mathematical expressions. This number has infinite decimal points. Special applications require as many of these decimals as possible. Simple applications may be OK with 22/7. In the following example, we will see how near to pi the value 22/7 is. In C, asin(1) is approximately equal to pi.

52 Approximate value of pi #include <stdio.h> #include <math.h> int main(void) { double pi, yak, hata; asin, is the arc-sine function. pi = 2 * asin(1); /* realistic pi value*/ yak = (double) 22 / 7; /* approximate pi */ hata = fabs(yak pi) / pi; /* find error */ printf("pi = %.15f\n", pi); /* write result */ printf("yaklaşık = %.15f\n", yak); printf("hata = %.5f", hata * 100); return 0;

53 pi = yaklaşık = hata = Press any key to continue

54 time(null) Gives number of seconds since 01/01/1970 At 13:40 on 17/11/2015, its value was. zaman: #include<stdio.h> #include<time.h> void main(){ int t; t = time(null); printf("zaman: %d \n", t);

55 time(null) Before using you should write: #include <time.h> Usage: int t; t = time(null); Since each time instant is different, it is a good randomization seed.

56 Clock: tik-tak #include <stdio.h> #include <time.h> void main(void) { int i, t; for(i=0; i <= 10; ++i) { /* just for 10 sec */ t = time(null) + 1; /* t next sec */ while (time(null) < t); /* wait */ printf("tik\n"); /* write tik */ t = time(null) + 1; /* wait 1 sac */ while(time(null) < t); /* wait */ printf("tak\n"); /* write tak */