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Lecture 12

Lecture 12. What will I learn in this lecture?. Know how if and switch C statements control the sequence of execution of statements. Be able to use relational and logical operators in the conditional part of an if or a switch statement. Related Chapters: ABC Chapter 4.1-4.7 & 4.16.

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Lecture 12

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  1. Lecture 12

  2. What will I learn in this lecture? • Know how if and switch C statements control the sequence of execution of statements. • Be able to use relational and logical operators in the conditional part of an if or a switch statement. • Related Chapters: ABC Chapter 4.1-4.7 & 4.16

  3. C Problem Example 1. Problem Definition Write a program that reads a number and computes the square root if the number is non-negative. 2. Refine, Generalize, Decompose the problem definition (i.e., identify subproblems, I/O, etc.) Input = real number Output=real number 3. Develop Algorithm (processing steps to solve problem)

  4. C Problem Example Print “enter value” Flowchart Read value False True value >= 0.0 Print sqrt(value)

  5. C Problem Example /* C Program to compute the square root of a positive number */ #include <stdio.h>#include <math.h> void main(void) { double value; /* Declare variables. */ /* request user input */ printf("Please enter a non-negative number :"); /* read value */scanf("%lf", &value); /* Output the square root. */ if (value >= 0.0) printf("square_root(%lf) = %lf \n", value , sqrt(value)); }

  6. Selection Structures (Decision Statements) • if Selection Structure if(expression) statement; • if expression evaluates to true, the statement is executed; otherwise execution passes to the next statement in the program. /* Error! Don’t put semicolon here */ /* This is an example of a logical error */ if(value >= 0.0); printf("square_root(%lf) = %lf \n", value,sqrt(value));

  7. C Problem Example (modified) 1. Problem Definition Modify the previous program to notify the user when the input is invalid.

  8. C Problem Example (modified) Print “enter value” Flowchart Read value False True value >= 0.0 Print sqrt(value); Print “invalid input”

  9. C Problem Example (modified) /* C Program to compute the square root of a positive number */ #include <stdio.h>#include <math.h> void main(void) { double value; /* Declare variables. */ /*request user input*/ printf(”Please enter a non-negative number :”); scanf("%lf", &value); /* read value */ /* Output the square root. */ if (value >= 0.0) printf("square_root(%lf) = %lf \n", value,sqrt(value)); else printf("invalid user input, please enter non-negative value\n"); }

  10. Math Library in C - in header filemath.h Arguments (parameters) for each of the following functions are assumed to be of type double. If not, a type double copy is made for use in the function. To compile a program that contains math functions you need to use the -lm (Lm not 1m )option for gcc. > gcc file.c -lm Seenextpage

  11. fabs (x)- |x| (not the same as the abs(x) function) sqrt (x)- square root of x pow (x, a)- xa exp (x)- ex (e = 2.718281828 …) log (x)- ln x = loge x log10 (x)- log10 x sin (x)- sine function (x in radians) cos (x)- cosine function (x in radians) tan (x)- tangent function (x in radians) ceil (x)- smallest integer >= x floor (x)- largest integer <= x

  12. if - else Selection Structure if (expression) statement1; else statement2; • if expression evaluates to true, statement1 is executed and execution skips statement2 • If expression evaluates to false, execution skips statement1 , statement2 is executed

  13. if - else Selection Structure We can also execute multiple statements when a given expression is true: if (expression) { statement1; statementn; } else { statement1; statementm; } if (expression) { statement1; statement2; statementn; } or . .. . .. Example- if(b < a) { temp = a; a = b; b = temp; } (what does this code do?) . ..

  14. C Problem Example 1. Problem Definition Modify the previous program to compute the following:You must check that the value is legal, i.e. value >= 1.0 or value <= -1.0

  15. C Problem Example Print “enter value” Flowchart Read value value >= 1.0 or value <= -1.0 False True Print sqrt(value*value -1.0); Print “invalid input”

  16. C Problem Example (modified) /* Compute the square root of value*value-1.0 */ #include <stdio.h>#include <math.h> void main(void) { double value; /* Declare variables. */ /* request user input*/ printf("Please enter value >= 1.0 or <= -1.0 :"); scanf("%lf", &value); /* read value */ /* Output the square root. */ if ((value >= 1.0) || (value <= -1.0)) printf("square_root(%f) = %f \n", value,sqrt(value*value - 1.0)); else { printf("invalid user input\n"); printf("input should be a value >= 1.0 or <= -1.0 \n"); }}

  17. Logical Expressions (Relational Operations) In logical expressions (which evaluate to true or false), we can use the following Relational operators: Relational Operator Typeof Test ==equalto (don’t use =) !=notequalto >greaterthan >=greaterthanorequalto <lessthan <=lessthan or equal to

  18. Logical Operators-Truth Tables

  19. Logical Expressions In C the value for False is represented by the value zero and True is represented by any nonzero value. The value False can be any zero value such as the number 0 or 0.0 or null character ‘ \0 ’ or the NULL pointer. Example 1: if ( 5 ) printf("True");/* prints True */ Example 2: int x = 0;/* x declared as an integer variable *//* and initialized to the value 0 */ if (x = 0)/* note the error, == should be used */printf(" x is zero\n");/*message not printed, why?*/

  20. Logical Expressions Avoid using == to test real numbers for equality! Example doublex = .3333333333; /* ten digits of 3s */if (x == 1.0/3.0) printf("equal!\n");else printf(" not equal!\n"); /* prints not equal! */ if ( fabs(x - 1.0/3.0) < 1.0e-10 ) printf("equal!\n"); /* prints equal! */ else printf(" not equal!\n");

  21. Nested if - else Selection Structure 1. Problem Definition Write a program that returns a letter grade based on a quiz score. The input will be the integer score from a 10 point quiz. The letter grades are assigned by:9 - 10 “A”7 - 8 “B”5 - 6 “C”3 - 4 “D”< 3 “F” 2. Refine, Generalize, Decompose the problem definition (i.e., identify subproblems, I/O, etc.) Input = integer score Output=character “grade” 3. Develop Algorithm (processing steps to solve problem)

  22. C Problem Example Print “enter score” Flowchart Read score score == 10 || score == 9 False True Print “A” (continued on next slide) (skip else part of statement)

  23. C Problem Example False score == 8 || score == 7 False True Print “B”; (continued on next slide) (skip else part of statement)

  24. C Problem Example False score == 6 || score == 5 False True Print “C”; (continued on next slide) (skip else part of statement)

  25. C Problem Example False score == 4 || score == 3 False True Print “D” Print “F”

  26. C Problem Example /* C Program to compute the letter grade for a quiz. */ #include <stdio.h> void main(void) { int score; /* Declare variables. */ /* request user input */ printf("Please enter a score :"); scanf("%i", &score); /* read value */ /* Output the grade *//* continued on the next slide */

  27. C Problem Example if ((score == 10) || (score == 9)) printf("A\n");else if ((score == 8) || (score == 7)) printf("B\n"); else if ((score == 6) || (score == 5)) printf("C\n"); else if ((score == 4) || (score == 3)) printf("D\n"); else printf("F\n"); } /* end of program */ Unless { } are used the else matches the first if in the code above.

  28. switch Selection Structure 1. Problem Definition Redo the previous problem by using a switch statement rather than the nested if-else statement. Pseudo-code Algorithm print “enter score” read score switch score score = 9,10 print “A” score = 7,8 print “B” score = 5,6 print “C” score = 3,4 print “D” otherwise print “F”

  29. C Problem Example /* C Program to compute the letter grade for a quiz. */ #include <stdio.h> void main(void) { int score; /* Declare variables. */ /* request user input */ printf("Please enter a score :"); scanf("%i", &score); /* read value */ /* Output the grade */ /* continued on the next slide */

  30. C Problem Example switch (score) { case 10: case 9: printf("A\n"); break; case 8: case 7: printf("B\n"); break; case 6: case 5: printf("C\n"); break; case 4: case 3: printf("D\n"); break; default: printf("F\n"); } /* end of switch */ } /* end of program */

  31. switch Selection Structure The switch structure can be used when we have multiple alternatives based on a value of type int or type char (but not float or double). This structure has the general form: switch (controlling expression) { case label1: label1_statement(s); case label2: label2_statement(s); default: default_statement(s); } (controlling expr. must evaluate to an integer or a character value; each case should end with a break stmt.) . ..

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