Assoc. Prof. Dr. Tansu FİLİK

Transcription

1 Assoc. Prof. Dr. Tansu FİLİK

2 Previously on Bil 200 Operators Logical and arithmetic operators Control Structures if-else mechanism

3 Low-level I/O They are fast! Might be useful for console applications. But they are a little bit difficult to use properly because of the strange behavior of console stream (keyboard buffer).

4 Low-level I/O - getchar getchar ( ) ; This is a primitive function used for entering a single character. char az; az = getchar ( );

5 Low-level I/O - putchar putchar ( ) ; This is a primitive function used for writing a character. char az; az = getchar ( ); putchar( az );

6 Low-level I/O char c1, c2, c3; c1= g ; c2= < ; c3= \n ; printf( %c%c%c,c1,c2,c3); putchar(c1); putchar(c2); putchar(c3);... > > g< >

7 ... printf( Type 2 characters and press enter: ); scanf( %c%c,&c1,&c2); putchar(c1); putchar(c2); printf( \ntype 2 characters and press enter: ); getchar();/*why? Try without this*/ c1=getchar(); c2=getchar(); /*We read 3 chars from buffer*/ putchar(c1); putchar(c1); Getchar functions do not start execution before an <enter> is pressed.

8 Run: xp<enter> (Type x and p, press enter) Memory block: x p \n Now 2 more getchar() read 2 characters from buffer: x p \n Keyboard buffer position is here! vs<enter> (Type v and s, press enter) Memory block: x p \n v s \n

9 In this position, if we read two more characters: a=getchar(); b=getchar(); x p \n v s \n Caution: You may think that we first read x and p, then v and s. But the last two characters would be <enter> and v! If you read two more chars, they would be s and <enter>! In order to free keyboard buffer after reading (to eliminate <enter> effects, use fflush(stdin).

10 ... printf( \ntype 2 characters and press enter: ); fflush(stdin); /* What?? */ c1=getche(); /* Try with getch() */ c2=getche(); putchar(c1); putchar(c2); /* Works faster than printf( %c%c,c1,c2); and takes less memory */

11 getc() getc ( *file ) ; This function, resembles getchar(), but this one can read from a file, too. char ch; ch = getc (stdin) ; /* input from keyboard */ ch = getc (fileptr) ;/* input from file */

12 putc() putc ( char, *file ) ; This function resembles putchar ( ), but it can write to files, too. char ch; ch = getc (stdin) ; /* read from keyboard */ putc (ch, stdout) ; /* write to screen */ putc (ch, fileptr) ; /* write to file */

13 Control Structures if-else mechanism

14 Control structures: if-else mechanism if else if else Multiple if-else statements can be used as: if (this logical expression1 is true ) statement1 ; else if (this logical expression2 is true ) statement2 ; else if (this logical expression3 is true ) statement3 ; else statement4 ;

15 Control structures: if-else mechanism Example: Number comparison #include <stdio.h> void main ( ) { int a, b ; printf ("a ve b için değer girin ") ; scanf ("%d %d", &a, &b ) ; if ( a < b ) printf ("a küçüktür b'den\n") ; else if ( a = = b ) printf (" a b'ye eşittir\n") ; else printf ("a, b'den büyüktür\n") ;

16 Control structures: if-else mechanism Nested if statements If-else does not have to be regularly listed. The C compiler matches an else statement to the nearest if statement. You have to be careful about logical construction of the program, and using { parentheses. if (nem < 20) if (sicaklik <= 0) printf("soğuk ve kuru bir gün."); if (ruzgar < 10) printf("ne güzel, rüzgar yok"); else printf("nem düşük ve 0 dereceden yüksek"); else if (nem < 60) if (sicaklik <= 0) printf("soguk ve orta nemli."); else printf("0'dan büyük, orta nem oranı.");

17 Introduction to Programming Languages Control structures: if-else mechanism

18 Introduction to Programming Languages Control structures: if-else mechanism

19 Control structures: if-else mechanism Short-cut comparison: sonuc=(condition)? (true statement) : (false statement); Boolean expression if if output else output

20 Control structures: if-else mechanism Short-cut comparison: float diff1,diff2, min, x1=100.0, x2=90.0, x3=10.0; diff1=(x1>x2)? (x1-x2) : (x2-x1); min=(x1>x2)? ( (x2>x3)? x3 : x2 ) : ( (x1>x3)? x3 : x1 )

21 Control structures: switch case structure switch case structure Multiple choice menu selection

22 Control structures: switch case structure Inside an algorithm, if there are more than two possible values of a variable according to which the program will flow, multiple menu selection is the appropriate mechanism. In the algorithm, each different value will invoke a different set of commands. Similar to a list of if-else statements, we have the switch-case structure which is suitable for this.

23 Control structures: switch case structure Layout: switch ( integer test variable ) { case value1 : commands ; case value2 : commands ; case value3 : commands ; default : commands ;

24 Control structures: switch case structure switch phrase shows the beginning of the structure (using something like goto label). It works with integers only. All values in case parts must be integer values. Remember that char and long int are also integers. There must be constant values in case parts. Do not put expressions there. There is no need for { after case.

25 Control structures: switch case structure In if-else-if-else-if-else... style, the if part works until the else or. Conversely, once a case is true, all the lines after the case line gets executed. switch(variable) { case 0: case 1: printf("merhaba\n"); printf("%d\n", degisken); scanf("%f", &a); printf("hello\n");...

26 switch case structure - Example Write a program which reads individual letter grades of a student. Consequently, it will display the following corresponding messages: A: "Çok güzel" B: "Güzel" C: "Fena Değil" D: "İyi değil" F: "Çok kötü" Write the program both with if-else, and switchcase.

27 switch case structure - Example Algorithm Print "Notunuzu giriniz" Read not switch(not) { case 'A': yaz "Çok güzel" case 'B': yaz "güzel" case 'C': yaz "fena değil" case 'D': yaz "iyi değil" case 'F': yaz "kötü" #include <stdio.h> void main ( ) { Solution Code char not ; printf ("Notunuzu girin:") ; not = getchar ( ) ; switch(not) { case 'A': printf("çok güzel\n"); case 'B': printf("güzel\n"); case 'C': printf("fena değil\n"); case 'D': printf("iyi değil\n"); case 'F': printf("kötü\n");

28 switch case structure - Example Program Output While running the program, if you enter A, the output is as follows: Çok güzel güzel fena değil iyi değil Kötü What is wrong here? switch(not) { case 'A': printf("çok güzel\n"); case 'B': printf("güzel\n"); case 'C': printf("fena değil\n"); case 'D': printf("iyi değil\n"); case 'F': printf("kötü\n");

29 switch case structure - Example Correction The errors in the program must be eliminated by using a control statement which skips all cases after a case is selected. We use "break". Break goes out of the structure when encountered. The structures include loops, nested loops, switch-cases, etc. It goes out of the next or the next control statement. Usage: break;

30 switch case structure - Example Correction Codes switch(not) { case 'A': printf("çok güzel\n"); break; case 'B': printf("güzel\n"); break; case 'C': printf("fena değil\n"); break; case 'D': printf("iyi değil\n"); break; case 'F': printf("kötü\n"); While running the program, if you enter A, the output is as follows: Çok güzel

31 Control structures: switch case structure Schematic layout true case a case a action(s) break true case b case b action(s) break... false false true case z case z action(s) break false default action(s)

32 switch case structure - Example switch(val) { case 1: case 3: case 5: printf( Tek sayi\n ); break; case 2: case 4: case 6: printf( Cift sayi\n ); break; default: printf( Sayi 1 6 arasinda degil. ); Deliberately missed breaks

33 switch case structure - Exercise The meteorology department wishes to write a program to convert numerical humidity values into meaningful English words. The user enters numbers between 0 and 100 to indicate humidity rate. The following rule converts the rate into words. The program must output the corresponding words. 20% or less Too dry" 21% - 40%: Dry" 41% - 60%: Moderately dry" 61% - 80%: Moderately humid" 81% or more: Humid" Write a program which asks user the humidity rate and writes the words using switch-case structures.

34 Control structures: switch case structure Notes on Switch Case Do not use double or float variables in switch. Only integers are used. After the case label, use as many statements as you can. Using the default: label is helpful in finding the logical mistakes of the program. The most common mistake is forgetting break. Be careful about this (as long as you do not deliberately take it out).

35 Control structures: Loops while, do-while, for loops

36 Control structures: Loops Repeat structures Repeat structures cause the code blocks to re-run. The re-run condition is decided according to the truth of a condition expression. There are number counting loops (for) for running the loop a number of times, or while or do-while types of loops which re-run as long as something is true.

37 Control structures: Loops Repeat structures for and while loops check the truth of a logical expression before entering the loop. do-while loop first enters the loop, and then checks whether a condition is true or not to re-run.

38 Control structures: Loops while loop Similar to if, it can be used in two ways: while ( true logical expression ) statement ; while (true logical expression) { statement1 ; statement2 ; Unlike if, it not only works for once if expression is true, but it works as long as expression stays true..

39 Control structures: Loops while loop examples: #include <stdio.h> main() { int i = 10; while ( i > 0 ) { printf("hello %d\n", i ); i = i -1; #include <stdio.h> Hello 10 Hello 9 Hello 8 Hello 7 Hello 6 Hello 5 Hello 4 Hello 3 Hello 2 Hello 1 main() { int i = 0; do { printf("hello %d\n", i ); i = i -1; while (i > 0)

40 Control structures: Loops #include <stdio.h> void main ( ) { int not ; not = -1 ; while (not!= 0) { printf ("sınav sonucu : >") ; scanf ("%d", &not) ; if (not >= 90) printf ("Notunuz[%d]=A\n", not) ; else if (not >= 80) printf ("Notunuz[%d]=B\n", not) ; else if (not >= 70) printf ("Notunuz[%d]=C\n", not) ; else if (not >= 60) printf ("Notunuz[%d]=D\n", not) ; else if (not!= 0) printf ("Notunuz[%d]=F\n", not) ; else printf ("Çıkıyoruz\n\n") ;

41 Control structures: Loops while loop Exercise: Write program find guessed number that asks user to input numbers until the user determines the number of the computer. Program must guide user by saying less and more.

42 Control structures: Loops do-while loop do statement; while ( true logical expression ); do { statement1 ; statement2 ; while ( true logical expression );

43 Control structures: Loops #include <stdio.h> void main ( ) { int not; do { printf ("sınav sonucu : >") ; scanf ("%d", &not) ; if (not >= 90) printf ("Notunuz[%d]=A\n", not) ; else if (not >= 80) printf ("Notunuz[%d]=B\n", not) ; else if (not >= 70) printf ("Notunuz[%d]=C\n", not) ; else if (not >= 60) printf ("Notunuz[%d]=D\n", not) ; else if (not!= 0) printf ("Notunuz[%d]=F\n", not) ; else printf ("Çıkıyoruz\n\n") ; while (not!= 0)

44 Control structures: Loops for loop (generic) A disturbingly general loop structure. statement and expression can be anything. The whole program, no matter how ridiculous, can be written inside a single for loop.

45 Control structures: Loops for loop for( initial statements;logical expression;updates) { statements; initial statements can be used as the first commands and assignments before running the loop. These statements work only for once. You can use several statements separated by,.

46 Control structures: Loops for loop for( initial statements;logical expression;updates) { statements; updates get executed after the statements in the block are executed. There may be several updates separated by,.

47 Control structures: Loops for loop for( initial statements;logical expression;updates) { statements; After the updates, the logical expression is checked. If it is true, the loop continues.

48 Control structures: Loops for loop for (statement1;statement2;statement3) { statements ; statement1 initializes the for loop. It may be a mathematical assignment, or several commands. It runs only once.

49 Control structures: Loops for loop for (statement1;statement2;statement3) { statements ; statement2 checks for the truth of an expression. That expression can be anything that determines the continuity or stop condition.

50 Control structures: Loops for loop for (statement1;statement2;statement3) { statements ; statement3 makes the final changes to the control variables. These changes and variables can be completely different than the ones in statement1, or the body statements.

51 int gun, saat, dakika; for(gun=1; gun<=3; gun++) printf( gun=%d\n,gun); for(saat=5; saat>2; saat--) { dakika=60*saat; printf( S:%d D:%d\n,saat,dakika);

52 Control structures: Loops for loop : Notes for is an extremely flexible structure. The statement1-3 parts may be completely empty, leaving everything inside the body. Yet, do not forget to use ;. Skipping the first and third statements actually turn the for loop into a while loop. Without the second statement (in the middle), you cannot determine when to terminate the loop. In this case, you must use break somewhere inside the body to check for a termination situation. The first and third statements can contain multiple commands. Using such a style, you can write single line for loops, even without a body part.

53 Control structures: Loops for loop : Example for(;;) { < - > printf( Bir sayi girin (çıkış için 0): ); scanf( %d,&number); if(number==0) break; for(number=1;number!=0;) { printf( Bir sayi girin (çıkış için 0): ); scanf( %d,&number); < - > for(number=1;number!=0; printf( Bir sayi girin (çıkış için 0): ),scanf( %d,&number));

54 Control structures: Loops for while statement1 ; while ( expression2 ) { statements ; statement3 ; for ( statement1;expression2;statement3 ) { statements ;

55 Control structures: Loops for loops : Example #include <stdio.h> void main ( ) { int k, n ; for( k = 1, n = 12 ; k<9 && n>6 ; k++, n--) { printf ("k=%d, n=%d\n", k, n ) ;

56 Control structures: Loops Exercises: What is the value of x when the following statement is complete? for (x = 0 ; x < 100 ; x++) ; What is the value of ctr when the following statement is complete? for (ctr = 2 ; ctr < 10 ; ctr += 3) ; How many X s does the following print? for (x = 0; x < 10; x++) for (y = 5; y > 0; y--) printf("x");

57 Control structures: Loops Loop termination The keyword break can be used for terminating the loop. If the loop is nested, it terminates the current block. The keyword continue is used for skipping until the end of the loop block, but the block does not terminate. Depending on the algorithm, you may use combinations of these keywords.

58 Control structures: Loops Loop termination: break: exit loop In the following program, break causes the loop to finish when value of x becomes 5. void main() { int x = 0; for( ; ; ) { /* infinite loop */ if(x == 5) break; printf("x = %d\n", x); x++;

59 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.

60 Control structures: Loops Example: Nested for loops #include <stdio.h> #define N 20 void main(void) { int i,j,k, karetoplam; 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);

61 Control structures: Loops Example: Nested for loops

62 Control structures: Loops Exercises: 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

63 Control structures: Loops Exercises: 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.