Today’s Material

1. Today’s Material • If statement and relational operators • <, <=, >, >=, ==, != • Finding min/max of 2 numbers • Finding the min of 3 numbers • Forming Complex relational expressions • Logical Operators (!, &&, ||) • Cascaded if statement • Conditional Operator • switch statement

2. Generic C Program Structure #include <stdio.h> /* main designates the starting place of our program */ main() { /* Variables hold Data Items Needed in the Program */ Variable Declarations; /* Steps of our program: I/O, computation (expressions) */ Statement1; Statement2; … StatementN; } /* end-of-the program */

3. Start Prompt the user and get the fahrenheit temperature to convert celsius = (fahrenheit-32)/1.8 Print the fahrenheit and celsius degrees End Example C Program • So far our programs consisted of statements all of which are executed in sequence until the end of the program is reached #include <stdio.h> /* Convert fahrenheit to celsius */ main() { float fahrenheit, celsius; printf(“Enter a temp in fahrenheit: “); scanf(“%f”, &fahrenheit); celsius = (fahrenheit-32)/1.8; printf(“%f degrees fahrenheit equals %f degrees celsius\n”, fahrenheit, celsius); }

4. Programs with If Statements • What about programs which need to select between two alternative set of statements? • You make a comparison and execute a different set of statement depending on the outcome of the comparison • Recall our examples with “if” statements • We will consider the problem of computing min/max of 2 numbers next

5. Start Prompt the user and get number1 and number2 number1 < number2 ? yes no min = number1 min = number2 max = number2 max = number1 Print min and max End Computing min and max of 2 numbers • Prompt the user and get number1 and number2 • if (number1 < number2) • 2.1. min = number1; • 2.2. max = number2; • else(i.e., number1 >= number2) • 3.1. min = number2; • 3.2. max = number1; • Print min and max

6. expression Y N statement 1 statement 2 if-else Statement • Allows a program to choose between two alternatives by testing the value of an expression • Syntax: if (expression) statement1; [else statement2;] • If the expression is true (1) statement1 is executed, otherwise statement2 is executed

7. if-else Examples int finalGrade; printf(“Please enter the final grade: “); scanf(“%d”, &finalGrade); if (finalGrade >= 45) printf("Pass \n"); int finalGrade; printf(“Please enter the final grade: “); scanf(“%d”, &finalGrade); if (finalGrade >= 45) printf("Pass!\n"); else printf("Fail!\n");

8. block (compound statement) What if I need to execute more than one statement? int finalGrade; … if(finalGrade >= 45) { printf("Pass!\n"); printf("Congratulations!\n"); } else { printf("Fail!\n"); printf("Better luck next time.\n"); } /* end-else */

9. Curly Brace Location • The location of curly braces is a matter of style • To the compiler it does not matter int finalGrade; … if(finalGrade >= 45){ printf("Pass!\n"); printf("Congratulations!\n"); } else { printf("Fail!\n"); printf("Better luck next time.\n"); } /* end-else */

10. Start Prompt the user and get number1 and number2 number1 < number2 ? yes no min = number1 min = number2 max = number2 max = number1 Print min and max End Finding the min and max of 2 ints int a, b; int min, max; printf(“Enter 2 ints: “); scanf(“%d%d”, &a, &b); if (a < b) { min = a; max = b; } else { min = b; max = a; } /* end-else */ printf(“min of %d and %d is %d\n”, a, b, min); printf(“max of %d and %d is %d\n”, a, b, max);

11. Execution of the Program(1) DATA int a, b; int min, max; printf(“Enter 2 ints: “); scanf(“%d%d”, &a, &b); if (a < b) { min = a; max = b; } else { min = b; max = a; } /* end-else */ printf(“min of %d and %d is %d\n”, a, b, min); printf(“max of %d and %d is %d\n”, a, b, max); a b 56 45 ? ? ? min max 45 56 ? ? 45 56 Enter 2 ints: min of 45 and 56 is 45 max of 45 and 56 is 56

12. Execution of the Program(2) DATA int a, b; int min, max; printf(“Enter 2 ints: “); scanf(“%d%d”, &a, &b); if (a < b) { min = a; max = b; } else { min = b; max = a; } /* end-else */ printf(“min of %d and %d is %d\n”, a, b, min); printf(“max of %d and %d is %d\n”, a, b, max); a b 22 77 ? ? ? min max 22 77 ? ? 77 22 Enter 2 ints: min of 77 and 22 is 22 max of 77 and 22 is 77

13. expression Y N statement 1 statement 2 Operator Meaning == equal to != not equal < less than <= less than or equal to > greater than >= greater than or equal to Relational Expression in if Statement • Expression in if statement compares two values and produces either True (1) or False (0) • Called a relational expression • Formed using relational operators • C does not have an explicit boolean type • So integers are used instead. The general rules is: • “Zero is false, any non-zero value is true”

14. Expression Interpretation Value a < b True 1 (a + b) >= c True 1 (b + c) > (a + 5) False 0 c != 3 False 0 b == 2 True 1 Relational Expression Examples • Assume a = 1, b = 2, and c = 3

15. Flowchart for finding the min of 3 ints(1) Start Prompt the user and get number1, number2 and number3 number1 < number2 ? yes no number2 < number3 ? yes number1 < number3 ? yes no no min = number3 min = number1 min = number2 min = number2 Print min End

16. Code for finding the min of 3 ints (1) • Problem: Find the minimum of 3 integers int a, b, c; int min; printf(“Enter 3 ints: “); scanf(“%d%d%d”, &a, &b, &c); if (a < b){ if (a < c) min = a; else min = c; } else { if (b < c) min = b; else min = c; } /* end-else */ printf(“Min of %d, %d, %d is %d\n”, a, b, c, min);

17. Flowchart for finding the min of 3 ints(2) Start Prompt the user and get number1, number2 and number3 min = number1 yes number2 < min? no min = number2 yes number3 < min? no min = number3 Print min End

18. Code for finding the min of 3 ints (2) • Problem: Find the minimum of 3 integers /* Here is a simpler alternative implementation */ int a, b, c; int min; printf(“Enter 3 ints: “); scanf(“%d%d%d”, &a, &b, &c); min = a; /* Assume that a is the minimum */ if (b < min) min = b; /* Is b smaller? */ if (c < min) min = c; /* Is c smaller? */ printf(“Min of %d, %d, %d is %d\n”, a, b, c, min);

19. The need for Logical Operators • In certain cases, you may want to form more complex relational expressions • (x is equal to 5) OR (x is equal to 8) • (x == 5) OR (x == 8) • (x is greater than 5) AND (x is less than 10) • (x > 5) AND (x < 10) • (x is less than y) AND (y is not equal to 20) • (x < y) AND (y != 20) • C provides 3 logical operators to form such complex relational expressions • AND (&&), OR (||), NOT (!)

20. Symbol Meaning && AND || OR ! NOT Logical Operators • Used to combine relational expressions that are either True (1) or False (0) • Using logical operators you can form complex relational expressions and use them inside if statements

21. Expressions using Logical Operators(1) • Suppose that a is an integer variable whose value is 7 and ch is a character variable holding the character 'q':

22. Expressions using Logical Operators(2) int temp = 75; double rain = 0.35; /* probability */ printf(“warm? %d\n”, temp > 70 && temp < 85); printf(“nice? %d\n”, temp > 70 && rain < 0.4); printf(“hot/cold? %d\n”, temp < 50 || temp > 85); printf(“cloudy? %d\n”, rain > 0.3 && rain < 0.7); warm? 1 nice? 1 hot/cold? 0 cloudy? 1

23. Expressions using Logical Operators(3) /* If a equals 4 OR a equals 10 */ if (a == 4||a == 10){ ... } else { ... } /* end-else */ /* x is in between 2 AND 20 */ if (x >= 2&&x <= 20){ ... } /* end-if */ /* y is greater than 20 AND x is NOT equal to 30 */ if (y > 20&& x != 30){ ... } /* end-if */

24. Precedence Operator Associativity 1 -, ++, -- right 2 *, /, % left 3 +, - left 4 >, >=, <, <= left ==, != left 5 &&, || left 6 7 =, +=, -=, .. right Associativity & Precedence Rules • Here is the associativity and precedence rules for all operators provided by C • Again, the best thing to do is to parenthesize the expression to avoid ambiguity

25. Cascaded If Statements • We often need to test a series of conditions stopping as soon as one of them is true • E.g. We want to test whether “n” is equal to 0, less than 0 or greater than 0 if (n < 0) printf(“n < 0\n”); else { if (n == 0) printf(“n == 0\n”); else printf(“n > 0\n”); } /* end-else */ • Instead of nesting the second if statement inside the else, we often write it as follows, called the cascaded if if (n < 0) printf(“n < 0\n”); else if (n == 0) printf(“n == 0\n”); else printf(“n > 0\n”);

26. Cascaded if Syntax if (expression1) statement1; [else if (expression2) statement2; else if (expression3) statement3; … else statementN;]

27. Cascaded if Example int finalGrade; … if (finalGrade >= 90) printf(“Passed: Your grade is A \n”); else if (finalGrade >= 85) printf(“Passed: Your grade is A- \n”); else if (finalGrade >= 80) printf(“Passed: Your grade is B+ \n”); else if (finalGrade >= 75) printf(“Passed: Your grade is B \n”); else if (finalGrade >= 70) printf(“Passed: Your grade is B- \n”); else if (finalGrade >= 55) printf(“Passed: Your grade is C \n”); else printf(“Failed \n”);

28. a = (b >= 0) ? b : -b; a gets the absolute value of b min = (a < b) ? a : b; min gets the minimum of a & b Conditional Operator • You will encounter statements of the following sort in many place in your code: if (expression) statement1; else statement2; • There is a shorter way to write such statements using the tertiary conditional operator • condition ? expr1 : expr2 • Examples:

29. switchStatement • Often we need to compare an expression against a series of values to see which one matches • We can implement such code using cascaded if as follows if (grade == 5) printf(“Excellent\n”); else if (grade == 4) printf(“Good\n”); else if (grade == 3) printf(“Pass\n”); else if (grade == 2) printf(“Poor\n”); else if (grade == 1) printf(“Fail\n”); else printf(“Illegal grade\n”); • As an alternative to this kind of cascaded if, C provides the “switch” statement

30. switchStatement Example • Here is the re-implementation of the previous cascaded if statements using the switch statement switch(grade){ case 5: printf(“Excellent\n”); break; case 4: printf(“Good); break; case 3: printf(“Pass\n”); break; case 2: printf(“Poor\n”); break; case 1: printf(“Fail\n”); break; default: printf(“Illegal grade\n”); break; } /* end-switch */

31. switch Statement Syntax • Selects a sequence of one or more instructions based on the result of comparing the value of an expression to a specific value switch(expression) { case value1: statement1; … [break;] case value2: statement2; … [break;] default: statementN; … [break;] } /* end-switch */

32. Another switch Example • int main(){ • char operator; • int a, b; • printf(“Enter an expression: “); • scanf(“%d%c%d”, &a, &operator, &c); • switch(operator) { • case ‘+’: • printf(“%d+%d = %d\n”, a, b, a+b); • break; • case ‘-’: • printf(“%d-%d = %d\n”, a, b, a-b); • break; • case ‘*’: • printf(“%d*%d = %d\n”, a, b, a*b); • break; • case ‘/’: • if(value == 0) { • printf(“Error: Divide by zero \n”); • printf(“ Operation ignored \n”); • } else • printf(“%d/%d = %d\n”, a, b, a/b); • break; • default: • printf(“Unknown operator \n”); • break; • } /* end-switch */ • return 0; • } /* end-main */

33. The role of break inside switch • Notice that “break” takes us out of the switch statement • If break is omitted after a case, the control continues with the first statement of the next case switch(grade){ case 5: printf(“Excellent\n”); case 4: printf(“Good); case 3: printf(“Pass\n”); case 2: printf(“Poor\n”); case 1: printf(“Fail\n”); default: printf(“Illegal grade\n”); } /* end-switch */ • If grade == 3, this will print • Pass Poor Fail Illegal grade

34. The role of break inside switch • Falling through the case may be what is really intended in your code. • If that’s the case, put a comment for your own benefit • Example: switch(grade){ case 5: case 4: case 3: case 2: num_passing++; /* Fall through */ case 1: num_total++; break; } /* end-switch */