Dr. R. Z. Khan, Associate Professor, Department of Computer Science

Transcription

1 ALIGARH MUSLIM UNIVERSITY Department of Computer Science Course: CSM-102: Programming & Problem Solving Using C Academic Session UNIT-2: Handout-3 Topic: Control Structures (Selection & Repetition) Dr. R. Z. Khan, Associate Professor, Department of Computer Science Objectives: To Know syntax and use of if, if else, Nested if, if else if else, switch selection statements. To learn about relational and logical operators of C language and their use. To learn about order of execution of relational and logical operators with others operators. To learn about use of increment operator ++, and decrement operator --. To learn about syntax of wile, do-while, for statements (loops). To know working of above loops with examples. To learn how to use above concepts in problem solving. Selection Structures: One-Way Selection using the if-statement: Syntax: if (expression) Statement; If the expression is true, then the statement is executed; otherwise the statement is ignored. For example, let marks be an integer variable. To check the value of marks to see if it is more than 75 and print PASS if it is more than 75 we would write if (marks > 75) printf ( PASS\n ) ; If marks is less than or equal to 75 then the print statement is ignored and nothing is printed. 1

2 If suppose that in addition to printing PASS we would like to add 5 bonus marks then we would write if (marks > 75) marks = marks + 5 ; printf ( PASS\n ) ; As seen above if there is more than one statement (compound statement) to be done after checking the expression result, then the statements should be enclosed between parenthesis. If there is only one statement (simple statement) to be executed after if then the parenthesis becomes optional (you can remove it). Note that there is no semicolon at the end of the if or else statement. Two-Way Selection Using the if else statement: Syntax: if (expression) statement 1; else statement 2; If the expression is true, then the statement 1 is executed; otherwise the statement 2 is executed if expression is false. For example, to find the larger of two variables x and y and print the answer and assign the larger value to max variable and smaller value to min variable, we would write : if ( x > y ) max = x ; min = y ; printf ( x is BIG\n ) ; else max = y ; min = x ; printf ( y is BIG\n ) ; Check for the parenthesis used for more than one statement after if and else. 2

3 Relational and Logical operators: The most commonly used relational operators are: > for greater than >= for greater than or equal to < for less than <= for less than or equal to == for equal to!= for not equal to The most commonly used logical operators are: && for binary AND for binary OR! for binary NOT These operators are used in the expression of if statement. The expression having these operators is called Logical expression. Examples: To check if the value of the integer variable x is more than 5 and less than 10 or equal to 15, we write if ( x > 5 && x < 10 x == 15 ) To check if the value of the float variable a is not equal to 1.5 or less than or equal to 2.5 but more than or equal to 1.5, we write if ( a!= 1.5 a <= 2.5 && a >= 1.5 ) 3

4 Multiple Alternative Decisions using if ladder: Syntax: if ( expression 1 ) statement 1; else if ( expression 2 ) statement 2;.. else if ( expression n ) statement n; else statement e; The expressions in a multiple-alternative decision are evaluated in sequence until a true condition is reached. If an expression is true, the statement following it is executed, and the rest of the multiple-alternative decision is skipped. If an expression is false, the statement following it is skipped, and the next expression is checked. If all the expressions are false, the statement e following the final else is executed. For example, to check the value of the variable marks and print the message accordingly, we would write if (marks >= 75) printf ( Distinction\n ) ; else if (marks >= 60) printf ( Pass\n ) ; else if (marks >= 50) printf ( Average\n ) ; else printf ( Fail\n ) ; Nested if statements: There can be if statements inside if and else statements which can be used to implement decisions with several alternatives. For example, if ( x > 2.5 ) p = p + 1 ; if ( x < 3.5 ) n = n + 1; else 4

5 else m = m + 1; if ( x > 1.5 ) s = s + 1 ; else t = t + 1; In the above example, if x is greater than 2.5 then 1 is added to p and it is checked if x is less than 3.5. If it is so, then 1 is added to n. If x is more than 3.5, 1 is added to m. If x is less than 2.5, the execution comes directly to the else part without doing anything to p, n, m and it is checked if x is more than 1.5. If it is so 1 is added to s else 1 is added to t. Multiple selections using the switch statement: Syntax: switch (expression) case value 1: statement 1; break ; case value 2: statement 2; break ; : : case value n: statement n; break ; default: statement e; Here it is checked whether the value of expression matches any of the case values. For example, to check the value of the character variable color and print the message accordingly, we write switch (color) case R : printf ( red\n ) ; break ; 5

6 case B : printf ( blue\n ) ; break ; case Y : printf ( yellow\n ) ; break ; default: printf ( black\n ) ; break ; The single quotes R, B, Y are used because of the color being character variable. If color is R then red is printed and the other cases are not checked, however if color is not red then other cases are checked until a match is found. If no match is found then the default value of black is printed. Operator Precedence: Operator Precedence! (not), +, -, & (address of) Ist (i.e highest) Note Above are Unary Operators which have a single operand. *, /, % IInd +, _ IIIrd <, <=, >=, > IVth == (equality operator),!= (not equal) Vth && (logical and operator ) VIth (logical OR operator ) VIIth = (assignment operator) VIIIth (i.e Lowest) 6

7 UExamples:U Use of Control Structures (Selection Structures): Example#1: /**************************************************************** Write a program that calculates and displays the reciprocal of an integer, both as a common fraction and a decimal fraction. A typical output line would be: The reciprocal of 2 is 1/2 or The program should display a Reciprocal undefined message for an input of zero. ****************************************************************/ void main () int number; float reciprocal; printf("please Enter an integer number: "); scanf("%d",&number); if (number>0 number<0) reciprocal = 1.0/number; printf("\nthe reciprocal of %d is 1/%d or %0.3f",number,number,reciprocal); else printf("\nreciprocal undefined"); // end of main 7

8 Example#2: /**************************************************************** Write an interactive program that contains a compound if statement and that may be used to compute the area of a square (area=side 2 ) or a triangle (area=base*height/2) after prompting the user to type the first character of the figure name (S or T). ***************************************************************/ using If-statement. void main () int side, height, base, area; char D; printf("if you want area of a square press S or T for area of triangle :"); scanf("%c", &D); if (D=='S') printf("please input the side : "); scanf("%d", &side); area=side*side; printf("the area : %d\n", area); else if (D=='T') printf("enter base and height of the triangle :"); scanf("%d %d", &base, &height); area=(0.5)*base*height; printf("the area of triangle is : %d", area); else printf("dear!, Your choice must be either S or T only "); printf("\nplease try again!"); // end of main 8

9 Using Switch statement: void main( ) char choice; double side, height, base, area; printf("[to find the shape's area choose either 'S' for square, or 'T' for triangle] : "); printf("\nenter your choice to find the area : "); scanf("%c", &choice); switch(choice) case 'S': printf("\nenter its side: "); scanf("%lf", &side); area = (side * side); printf("the area of square is %.2f unit square\n", area); break; case 'T' : printf("\nenter the height and base: "); scanf("%lf %lf", &height, &base); area = (0.5) * height * base; printf(" The area of triangle is %.2f unit square\n", area); break; default: printf("sorry,your choice is not included \nplz try again "); // end of switch // end of main Use of increment operator ++, and decrement operator - - : i++ means i=i+1; i -- means i=i-1; (For above statements in first step initial value of i will be assigned and after that value of i will be incremented or decremented). ++i means i=i+1 ; -- i means i=i-1 ; (But in these, in first step initial value of i will be incremented or decremented and then value of i will be assigned ). 9

10 Examples: main() int i=2, r1 ; r1=i++; printf("value of r1 is : %d", r1); Sample Output: void main() int i=2, r1 ; r1 = ++i; printf("value of r1 is : %d", r1); Sample Output: 10

11 Looping Structures: While statement: Syntax: while (expression) statement A conditional loop is one whose continued execution depends on the value of a logical expression. In the above syntax as long as the expression is true the statement (or statements if enclosed within braces) is executed. When the expression evaluates to false the execution of the statement(s) stops and program continues with the other following statements. Example: Let number be an integer variable. while (number > 100) printf ( Enter a number\n ) ; scanf ( %d, &number) ; In the above example, as long as the entered value of number is greater than 100 the loop continues to ask to enter a value for number. Once a number less than or equal to 100 is entered it stops asking. If in the beginning itself the value of number is less than or equal to 100, then the loop is not at all executed. Note: In the while loop the expression is tested before the loop body statements are entered. So, if the expression is false in the beginning the loop body is never entered. do-while statement : Syntax: do Statement; while (expression) ; Here, the expression is tested after the statement(s) are executed. So, even if the expression is false in the beginning the loop body is entered at least once. Here also like the while statement, as long as the expression is true the statement (or statements if enclosed within braces) is executed. When the expression evaluates to false the execution of the statement(s) stops and program continues with the other following statements. 11

12 Example: The example of while loop can be written with do while loop as : do printf ( Enter a number\n ) ; scanf ( %d, &number) ; while (number > 100) ; In the above example, as long as the entered value of number is greater than 100 the loop continues to ask to enter a value for number. Once a number less than or equal to 100 is entered it stops asking. Note: In the conditional looping using while and do while the number of times the loop will be executed is not known at the start of the program. For-statement: Syntax: for (expression 1; expression 2; expression 3) Statement; In the iterative looping using for loop the number of times the loop will be executed is known at the start of the program. In the above syntax, the expression 1 is the initialization statement which assigns an initial value to the loop variable. The expression 2 is the testing expression where the loop variable value is checked. If it is true the loop body statement(s) are executed and if it is false the statements(s) are not executed. The expression 3 is usually an increment or decrement expression where the value of the loop variable is incremented or decremented. Example: for (count = 0 ; count < 10 ; count++) printf ( Enter a value\n ) ; scanf ( %d, &num) ; num = num + 10 ; printf ( NUM = %d\n, num) ; 12

13 In the above example the integer variable count is first assigned a value of 0, it is initialized. Then the value of count is checked if it is less than 10. If it is < 10 the loop body asks to enter a value for num. Then the value of count is incremented by 1 and again count is checked for less than 10. If it is true the loop body is again executed and then the count is incremented by 1 and checked for < 10. It keeps on doing like this until the count is >= 10. Then the loop stops. So, here the loop is executed 10 times. Note: The increment operation is written as ++ and decrement operation is written as --. The increment increases the value by 1 and decrement decreases the value by 1. How to generate number from 1 to 10 Using different loops: for while do-while for(n=1; n<=10; ++n) n=1; n=1; while(n<=10) do n=n+1; n=n+1; while(n<=10); Examples using different loops: Example#1: for-loop : void main() int num,square,limit; printf("please Input value of limit : "); scanf("%d", &limit); for (num=0; num < limit; ++num) square = num*num; printf("%5d %5d\n", num, square); // end of for // end of main 13

14 Sample Output: Example#2: /********************************************************** Program Showing a Sentinel-Controlled for loop. To compute the sum of a list of exam scores. ***********************************************************/ #include <stdio.h> #define SENTINEL -99 int main() int sum = 0, /* sum of scores input so far */ score; /* current score */ printf("enter first score (or %d to quit)> ", SENTINEL); for(scanf("%d", &score); score!= SENTINEL; scanf("%d", &score)) sum += score; printf("enter next score (%d to quit)> ", SENTINEL); // end of for loop printf("\nsum of exam scores is %d\n", sum); return (0); // end of main Sample Output: 14

15 Example#3: while-loop : /*Conditional loop Using while Statement */ #define RIGHT_SIZE 10 void main() int diameter; printf("please Input Balloon's diameter > "); scanf("%d",&diameter); while(diameter < RIGHT_SIZE) printf("keep blowing! \n"); printf("please Input new Balloon's diameter > "); scanf("%d", &diameter); // end of while loop printf("stop blowing! \n"); // end of main Sample Output: Example#4: do-while loop : /* do-while loop */ void main() 15

16 int num; do printf("please Input Positive numbers between ( < 1 or >= 10 ) : "); scanf("%d",&num); // end of do loop while (num < 1 num >=10) ; printf("dear, Sorry! Your number is out of specified range in program"); // end of main Sample Output: Example#5: Use of Increment and Decrement operators: /* Explanation on Increment, Decrement operators */ void main() int n, m, p, i = 3, j = 9; n = ++i * --j; printf("from above First given expression calculated n=%d i=%d j=%d\n", n, i, j); m = i + j--; printf("from above second given expression calculated m=%d i=%d j=%d\n",m,i,j); p = i + j; printf("from above Third given expression calculated p=%d i=%d j=%d\n", p, i, j); // end of main 16

17 Sample Output: Exercises Problem#1: Trace output of following program manually (Without using Computer): main( ) int j=0, x=0; switch(j-1) case 0 : case -1 : x += 1; break; case 1: case 2: case 3: x += 2; break; default: x += 3; // end of switch printf("x = %d\n", x); // end of main Problem#2: Write a program that reads three integer numbers and prints the highest among them. Problem#3: Write a program to find the roots of a quadratic equation. Consider the case of imaginary roots. 17

18 Problem#4: Write a program that prompts the user for the day number (0 to 6) and prints the day name (Saturday to Friday) using the switch statement. Problem#5: An electric power distribution company charges its domestic consumers as follows: Consumption Units Rate of Charge Rs per unit Rs. 100 plus Rs per unit excess of Rs. 230 plus Rs per unit excess of and above Rs. 390 plus Rs per unit excess of 600 Write C Program to read the customer number and power consumed and prints the amount to be paid by the customer. Problem#6: Write C-Program to read exam score of a student of AMU, Aligarh from keyboard and calculate latter grades based on following grading scheme: Score Grade >= 90 A >= 85 B+ >= 80 B >= 75 C+ >= 70 C >= 65 D+ >= 60 D <60 F 18

19 Problem#7: Write a program that reads a set of grades (between 0 to 100) and finds the following: a. Sum b. Average (Note: Solve using do-while). Problem#8: Write a program to generate Square, Cube, Square_Root of positive numbers from 1 to any Limit Entered by user. (Note: Solve using for loop). Problem#9: Write a program that reads the time and duration of a phone call in minutes and calculates the cost. Each minute costs 1.5 Rs. A discount of 20% is given if the time of the call is between 22:00 1:00 and a discount of 40% is given if the time of the call is between 1:00 8:00. Check the correctness of the time (0:00-23:59) and duration. Problem#10: There are 9870 people in a town of Farrukhabad District whose population is increases by 10% each year. Write a program that contents a loop that display the annual (yearly) population and determine how many years (count_year) it will take for the population just above 30,000 people. Problem#11: Write a program that reads the values of the variables a, b, and d then finds and prints the value of x using the following formula: a x = 5 + 3d b Problem#12: Find output of the following programs: 1). int main() 19

20 int x,i=1; for(x=-1; x<=10; x++) i++; if(x < 5) continue; else break; printf("khan"); printf( i=%d\n,i); return 0; 2). int main() int j=1; while(j <= 255) printf("%d\n", j); j++; return 0; 3). int main() int i=0; for(; i<=5; i++); printf("%d,", i); return 0; 4). int main() char str[]="c-program"; int a = 5; printf(a >10?"Ps\n":"%s\n", str); return 0; 20

21 5). int main() unsigned int i = 65535; /* Assume 2 byte integer*/ while(i++!= 0) printf("%d",++i); printf("\n"); return 0; 6). What will be the output of the program, if a short int is 2 bytes wide? int main() short int i = 0; for(i<=5 && i>=-1; ++i; i>0) printf("%u,", i); return 0; 7). int main() unsigned int i = 65536; /* Assume 2 byte integer*/ while(i!= 0) printf("%d",++i); printf("\n"); return 0; 8). 21

22 int main() int x=1, y=1; for(; y; printf("%d %d\n", x, y)) y = x++ <= 5; printf("\n"); return 0; 9). int main() int i = 5; while(i-- >= 0) printf("%d,", i); i = 5; printf("\n"); while(i-- >= 0) printf("%i,", i); while(i-- >= 0) printf("%d,", i); return 0; 10). int main() int i=3; switch(i) case 1: printf("hello\n"); case 2: printf("hi\n"); case 3: continue; default: printf("bye\n"); return 0; 22

23 11). int main() char j=1; while(j < 5) printf("%d, ", j); j = j+1; printf("\n"); return 0; 12). int main() int x, y, z; x=y=z=1; z = ++x ++y && ++z; printf("x=%d, y=%d, z=%d\n", x, y, z); return 0; 13). #include <stdio.h> int main(void) int x = 2; switch (x) case 1: printf( one\n ); case 2: printf( two\n ); case 3: printf( three\n ); return (0); 23

24 14). #include <stdio.h> int main (void) int x; for ( x = 1; x <= 3; x++) printf("amu\n"); printf("2012 1\n"); return (0); 15). #include <stdio.h> int main (void) int x; x = 30; if (x < 0 x > 15) printf("wrong\n"); else if (x < 10) printf("fail\n"); else printf("pass\n"); return (0); 16). void main() int i=4,j=-1, k=0, w, x, y, z; w = i j k; x = i && j && k; y = i j && k; z = i && j k; printf("\nw=%d x=%d y=%d z=%d",w, x, y,!z); 24

25 17). void main() int i, x=2; for(i=1;i<5;++i) if(i%2 == 1) x +=i; else x--; printf("x= %d\n",x);; printf("value of x is = %d\n", x); Answers: 1). i=8 2) ). 6 Explanation: Step 1: int i = 0; here variable i is an integer type and initialized to '0'. Step 2: for(; i<=5; i++); variable i=0 is already assigned in previous step. The semicolon at the end of this for loop tells, "there is no more statement is inside the loop". 25

26 Loop 1: here i=0, the condition in for(; 0<=5; i++) loop satisfies and then i is incremented by '1'(one) Loop 2: here i=1, the condition in for(; 1<=5; i++) loop satisfies and then i is incremented by '1'(one) Loop 3: here i=2, the condition in for(; 2<=5; i++) loop satisfies and then i is incremented by '1'(one) Loop 4: here i=3, the condition in for(; 3<=5; i++) loop satisfies and then i is increemented by '1'(one) Loop 5: here i=4, the condition in for(; 4<=5; i++) loop satisfies and then i is incremented by '1'(one) Loop 6: here i=5, the condition in for(; 5<=5; i++) loop satisfies and then i is incremented by '1'(one) Loop 7: here i=6, the condition in for(; 6<=5; i++) loop fails and then i is not incremented. Step 3: printf("%d,", i); here the value of i is 6. Hence the output is '6'. 4). C-program Explanation: Step 1: char str[]="c-program"; here variable str contains "C-program". Step 2: int a = 5; here variable a contains "5". Step 3: printf(a >10?"Ps\n":"%s\n", str); this statement can be written as if(a > 10) printf("ps\n"); else printf("%s\n", str); Here we are checking a > 10 means 5 > 10. Hence this condition will be failed. So it prints variable str. Hence the output is "C-program". 5). Infinite loop Explanation: 26

27 Here unsigned int size is 2 bytes. It varies from 0,1,2,3,... to Step 1:unsigned int i = 65535; Step 2: Loop 1: while(i++!= 0) this statement becomes while(65535!= 0). Hence the while(true) condition is satisfied. Then the printf("%d", ++i); prints '1'(variable 'i' is already increemented by '1' in while statement and now increemented by '1' in printf statement) Loop 2: while(i++!= 0) this statement becomes while(1!= 0). Hence the while(true) condition is satisfied. Then the printf("%d", ++i); prints '3'(variable 'i' is already increemented by '1' in while statement and now increemented by '1' in printf statement) The while loop will never stops executing, because variable i will never become '0'(zero). Hence it is an 'Infinite loop'. 6) Explanation: for(i<=5 && i>=-1; ++i; i>0) so expression i<=5 && i>=-1 initializes for loop. expression ++i is the loop condition. expression i>0 is the increment expression. In for( i <= 5 && i >= -1; ++i; i>0) expression i<=5 && i>=-1 evaluates to one. Loop condition always get evaluated to true. Also at this point it increases i by one. An increment_expression i>0 has no effect on value of i.so for loop get executed till the limit of integer (ie ) 7. No output. Explanation: Here unsigned int size is 2 bytes. It varies from 0,1,2,3,... to Step 1:unsigned int i = 65536; here variable i becomes '0'(zero). because unsigned int varies from 0 to Step 2: while(i!= 0) this statement becomes while(0!= 0). Hence the while(false) condition is not satisfied. So, the inside the statements of while loop will not get executed. 27

28 Hence there is no output. Note: Don't forget that the size of int should be 2 bytes. If you run the above program in GCC it may run infinite loop, because in Linux platform the size of the integer is 4 bytes. 8) ). 4, 3, 2, 1, 0, -1 4, 3, 2, 1, 0, -1 Explanation: Step 1: Initially the value of variable i is '5'. Loop 1: while(i-- >= 0) here i = 5, this statement becomes while(5-- >= 0) Hence the while condition is satisfied and it prints '4'. (variable 'i' is decremented by '1'(one) in previous while condition) Loop 2: while(i-- >= 0) here i = 4, this statement becomes while(4-- >= 0) Hence the while condition is satisfied and it prints '3'. (variable 'i' is decremented by '1'(one) in previous while condition) Loop 3: while(i-- >= 0) here i = 3, this statement becomes while(3-- >= 0) Hence the while condition is satisfied and it prints '2'. (variable 'i' is decremented by '1'(one) in previous while condition) Loop 4: while(i-- >= 0) here i = 2, this statement becomes while(2-- >= 0) Hence the while condition is satisfied and it prints '1'. (variable 'i' is decremented by '1'(one) in previous while condition) Loop 5: while(i-- >= 0) here i = 1, this statement becomes while(1-- >= 0) Hence the while condition is satisfied and it prints '0'. (variable 'i' is decremented by '1'(one) in previous while condition) Loop 6: while(i-- >= 0) here i = 0, this statement becomes while(0-- >= 0) Hence the while condition is satisfied and it prints '-1'. (variable 'i' is decremented by '1'(one) in previous while condition) Loop 7: while(i-- >= 0) here i = -1, this statement becomes while(-1-- >= 0) Hence the while condition is not satisfied and loop exits. The output of first while loop is 4,3,2,1,0,-1 28

29 Step 2: Then the value of variable i is initialized to '5' Then it prints a new line character(\n). See the above Loop 1 to Loop 7. The output of second while loop is 4,3,2,1,0,-1 Step 3: The third while loop, while(i-- >= 0) here i = -1(because the variable 'i' is decremented to '-1' by previous while loop and it never initialized.). This statement becomes while(-1-- >= 0) Hence the while condition is not satisfied and loop exits. Hence the output of the program is 4,3,2,1,0,-1 4,3,2,1,0,-1 10). Error: Misplaced continue Explanation: The keyword continue cannot be used in switch case. It must be used in for or while or do while loop. If there is any looping statement in switch case then we can use continue. 11). 1, 2, 3, 4 12). x=2, y=1, z=1 Explanation: Step 1: x=y=z=1; here the variables x,y, z are initialized to value '1'. Step 2: z = ++x ++y && ++z; becomes z = ( (++x) (++y && ++z) ). Here ++x becomes 2. So there is no need to check the other side because (Logical OR) condition is satisfied.(z = (2 ++y && ++z)). There is no need to process ++y && ++z. Hence it returns '1'. So the value of variable z is '1' Step 3: printf("x=%d, y=%d, z=%d\n", x, y, z); It prints "x=2, y=1, z=1". here x is increemented in previous step. y and z are not increemented. 13). one two 29

30 three 14). AMU AMU AMU ). wrong 16). w=1 x=0 y=1 z=0 17). x= 3 x= 2 x= 5 x= 4 Value of x is = 4 30