Decisions

1. Decisions If statements in C

2. Decision Making: Equality and Relational Operators • Executable statements • Perform actions (calculations, input/output of data) • Perform decisions • May want to print "pass" or "fail" given the value of a test grade • if control structure • If a condition is true, then the body of the if statement executed • 0 is false, non-zero is true • Control always resumes after the if structure • Keywords • Special words reserved for C • Cannot be used as identifiers or variable names

3. Decision Making: Equality and Relational Operators

4. Decision Making: Equality and Relational Operators

5. Control Structures • Sequential execution • Statements executed one after the other in the order written • All programs written in terms of 3 control structures • Sequence structures: Built into C. Programs executed sequentially by default • Selection structures: C has three types: if, if/else, and switch • Repetition structures: C has three types: while, do/while and for

6. The if Selection Structure • Selection structure: • Used to choose among alternative courses of action • Pseudocode: If student’s grade is greater than or equal to 80Print “Passed” • If condition true • Print statement executed and program goes on to next statement • If false, print statement is ignored and the program goes onto the next statement • Indenting makes programs easier to read • C ignores whitespace characters

7. The if Selection Structure • Pseudocode statement in C: if ( grade >= 60 ) printf( "Passed\n" ); • C code corresponds closely to the pseudocode

8. true false print “Passed” grade >= 60 The if Selection Structure • if structure is a single-entry/single-exit structure A decision can be made on any expression. zero - false nonzero - true Example: 3 - 4 istrue

9. The if/else Selection Structure • if • Only performs an action if the condition is true • if/else • Specifies an action to be performed both when the condition is true and when it is false • Psuedocode: If student’s grade is greater than or equal to 60Print “Passed” elsePrint “Failed” • Note spacing/indentation conventions

10. The if/else Selection Structure • C code: if ( grade >= 60 ) printf( "Passed\n"); else printf( "Failed\n"); • Ternary conditional operator (?:) DO NOT USE! • Takes three arguments (condition, value if true, value if false) • Our pseudocode could be written: printf( "%s\n", grade >= 60 ? "Passed" : "Failed" ); • Or it could have been written: grade >= 60 ? printf( “Passed\n” ) : printf( “Failed\n” );

11. false true print “Passed” print “Failed” grade >= 60 The if/else Selection Structure • Flow chart of the if/else selection structure • Nested if/else structures • Test for multiple cases by placing if/else selection structures inside if/else selection structures • Once condition is met, rest of statements skipped • Deep indentation usually not used in practice

12. The if/else Selection Structure • Pseudocode for a nested if/else structure If student’s grade is greater than or equal to 90 Print “A”else If student’s grade is greater than or equal to 80 Print “B” else If student’s grade is greater than or equal to 70 Print “C” else If student’s grade is greater than or equal to 60 Print “D” else Print “F”

13. The if/else Selection Structure • Compound statement: • Set of statements within a pair of braces • Example: if ( grade >= 60 ) printf( "Passed.\n" ); else { printf( "Failed.\n" ); printf( "You must take this course again.\n" );} • Without the braces, the statement printf("You must take this course again.\n"); would be executed automatically

14. The if/else Selection Structure • Block: • Compound statements with declarations • Syntax errors • Caught by compiler • Logic errors: • Have their effect at execution time • Non-fatal: program runs, but has incorrect output • Fatal: program exits prematurely

15. isdigit() Function The isdigit() function is part of the character-handling library <ctype.h> #include <stdio.h> #include <ctype.h> main() { char cResponse = '\0'; printf("\nPlease enter a letter: "); scanf("%c", &cResponse); if ( isdigit(cResponse) == 0 ) printf("\nThank you\n"); else printf("\nYou did not enter a letter\n"); }

16. INTRODUCTION TO BOOLEAN ALGEBRA Boolean algebra is named after George Boole, a mathematician in the nineteenth century. Boole developed his own branch of logic containing the values true and false and the operators and, or, and not to manipulate the values. Even though Boole’s work was before the advent of computers, his research has become the foundation of today’s modern digital circuitry in computer architecture.

17. Truth table for and (&&)

18. Truth table for or (||)

19. Solve the following Boolean algebra problems, given x == 5, y == 3, and z == 4 1. x > 3 and z == 4 2. y >= 3 or z > 4 3. NOT(x == 4 or y < z) 4. (z == 5 or x > 3) and (y == z or x < 10)

20. C syntax for compound if statements if ( 3 > 5 && 5 < 5 ) printf("The entire expression is false\n"); if ( 3 > 5 || 6 < 5 ) printf("The entire expression is false\n");

21. Control statement – replaces multiple if/else statements The switch statement in c

22. Example switch statement switch(n) { case 0: printf("You typed zero.\n"); break; case 1: printf("n is a perfect square\n"); break; case 2: printf("n is an even number\n"); case 3: case 7: printf("n is a prime number\n"); break; case 4: printf("n is a perfect square\n"); case 6: printf("n is an even number\n"); break; default: printf("Only single-digit numbers are allowed\n"); break; }

23. switch (using pre-processor) #define STATE_READY 1 #define STATE_SET 2 #define STATE_GO 3 #define STATE_FAIL 4 switch( state ) { case STATE_READY: state = STATE_SET; if( x < 0 ) state = STATE_FAIL; break; case STATE_SET: state = STATE_GO; if( y > 0 ) state = STATE_FAIL; break; case STATE_GO: printf( "go!\n" ); break; case STATE_FAIL: exit( -1 ); }

24. Random Numbers int iRandomNum = 0; srand(time(0)); iRandomNum = (rand() % 4) + 1;

25. Assignment • Finish the 5 problems on page 48 Decisions:

26. 3.8 Formulating Algorithms(Counter-Controlled Repetition) • Counter-controlled repetition • Loop repeated until counter reaches a certain value • Definite repetition: number of repetitions is known • Example: A class of ten students took a quiz. The grades (integers in the range 0 to 100) for this quiz are available to you. Determine the class average on the quiz • Pseudocode: Set total to zeroSet grade counter to one While grade counter is less than or equal to tenInput the next gradeAdd the grade into the totalAdd one to the grade counter Set the class average to the total divided by tenPrint the class average

27. 1 /* Fig. 3.6: fig03_06.c 2 Class average program with 3 counter-controlled repetition */ 4 #include <stdio.h> 5 6 int main() 7 { 8 int counter, grade, total, average; 9 10 /* initialization phase */ 11 total = 0; 12 counter = 1; 13 14 /* processing phase */ 15 while ( counter <= 10 ) { 16 printf( "Enter grade: " ); 17 scanf( "%d", &grade ); 18 total = total + grade; 19 counter = counter + 1; 20 } 21 22 /* termination phase */ 23 average = total / 10; 24 printf( "Class average is %d\n", average ); 25 26 return 0; /* indicate program ended successfully */ 27 } 1. Initialize Variables 2. Execute Loop 3. Output results

28. Enter grade: 98 Enter grade: 76 Enter grade: 71 Enter grade: 87 Enter grade: 83 Enter grade: 90 Enter grade: 57 Enter grade: 79 Enter grade: 82 Enter grade: 94 Class average is 81 Program Output

29. 3.9 Formulating Algorithms with Top-Down, Stepwise Refinement • Problem becomes: Develop a class-averaging program that will process an arbitrary number of grades each time the program is run. • Unknown number of students • How will the program know to end? • Use sentinel value • Also called signal value, dummy value, or flag value • Indicates “end of data entry.” • Loop ends when user inputs the sentinel value • Sentinel value chosen so it cannot be confused with a regular input (such as -1 in this case)

30. 3.9 Formulating Algorithms with Top-Down, Stepwise Refinement • Top-down, stepwise refinement • Begin with a pseudocode representation of the top: Determine the class average for the quiz • Divide top into smaller tasks and list them in order: Initialize variablesInput, sum and count the quiz gradesCalculate and print the class average • Many programs have three phases: • Initialization: initializes the program variables • Processing: inputs data values and adjusts program variables accordingly • Termination: calculates and prints the final results

31. 3.9 Formulating Algorithms with Top-Down, Stepwise Refinement • Refine the initialization phase from Initialize variables to: Initialize total to zero Initialize counter to zero • Refine Input, sum and count the quiz grades to Input the first grade (possibly the sentinel)While the user has not as yet entered the sentinel Add this grade into the running total Add one to the grade counter Input the next grade (possibly the sentinel)

32. 3.9 Formulating Algorithms with Top-Down, Stepwise Refinement • Refine Calculate and print the class average to If the counter is not equal to zero Set the average to the total divided by the counter Print the averageelse Print “No grades were entered”

33. 1 /* Fig. 3.8: fig03_08.c 2 Class average program with 3 sentinel-controlled repetition */ 4 #include <stdio.h> 5 6 int main() 7 { 8 float average; /* new data type */ 9 int counter, grade, total; 10 11 /* initialization phase */ 12 total = 0; 13 counter = 0; 14 15 /* processing phase */ 16 printf( "Enter grade, -1 to end: " ); 17 scanf( "%d", &grade ); 18 19 while ( grade != -1 ) { 20 total = total + grade; 21 counter = counter + 1; 22 printf( "Enter grade, -1 to end: " ); 23 scanf( "%d", &grade ); 24 } 1. Initialize Variables 2. Get user input 2.1 Perform Loop

34. 25 26 /* termination phase */ 27 if ( counter != 0 ) { 28 average = ( float ) total / counter; 29 printf( "Class average is %.2f", average ); 30 } 31 else 32 printf( "No grades were entered\n" ); 33 34 return 0; /* indicate program ended successfully */ 35 } 3. Calculate Average 3.1 Print Results Program Output Enter grade, -1 to end: 75 Enter grade, -1 to end: 94 Enter grade, -1 to end: 97 Enter grade, -1 to end: 88 Enter grade, -1 to end: 70 Enter grade, -1 to end: 64 Enter grade, -1 to end: 83 Enter grade, -1 to end: 89 Enter grade, -1 to end: -1 Class average is 82.50

35. 3.10 Nested control structures • Problem • A college has a list of test results (1 = pass, 2 = fail) for 10 students • Write a program that analyzes the results • If more than 8 students pass, print "Raise Tuition" • Notice that • The program must process 10 test results • Counter-controlled loop will be used • Two counters can be used • One for number of passes, one for number of fails • Each test result is a number—either a 1 or a 2 • If the number is not a 1, we assume that it is a 2

36. 3.10 Nested control structures • Top level outline Analyze exam results and decide if tuition should be raised • First Refinement Initialize variables Input the ten quiz grades and count passes and failures Print a summary of the exam results and decide if tuition should be raised • Refine Initialize variables to Initialize passes to zero Initialize failures to zero Initialize student counter to one

37. 3.10 Nested control structures • Refine Input the ten quiz grades and count passes and failures to While student counter is less than or equal to tenInput the next exam result If the student passed Add one to passeselse Add one to failures Add one to student counter • Refine Print a summary of the exam results and decide if tuition should be raised to Print the number of passes Print the number of failures If more than eight students passed Print “Raise tuition”

38. 1 /* Fig. 3.10: fig03_10.c 2 Analysis of examination results */ 3 #include <stdio.h> 4 5 int main() 6 { 7 /* initializing variables in declarations */ 8 int passes = 0, failures = 0, student = 1, result; 9 10 /* process 10 students; counter-controlled loop */ 11 while ( student <= 10 ) { 12 printf( "Enter result ( 1=pass,2=fail ): " ); 13 scanf( "%d", &result ); 14 15 if ( result == 1 ) /* if/else nested in while */ 16 passes = passes + 1; 17 else 18 failures = failures + 1; 19 20 student = student + 1; 21 } 22 23 printf( "Passed %d\n", passes ); 24 printf( "Failed %d\n", failures ); 25 26 if ( passes > 8 ) 27 printf( "Raise tuition\n" ); 28 29 return 0; /* successful termination */ 30 } 1. Initialize variables 2. Input data and count passes/failures 3. Print results

39. Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 2 Enter Result (1=pass,2=fail): 2 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 2 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 1 Enter Result (1=pass,2=fail): 2 Passed 6 Failed 4 Program Output

40. 3.11 Assignment Operators • Assignment operators abbreviate assignment expressions c = c + 3; can be abbreviated as c += 3; using the addition assignment operator • Statements of the form variable=variableoperatorexpression; can be rewritten as variableoperator=expression; • Examples of other assignment operators: d -= 4 (d = d - 4) e *= 5 (e = e * 5) f /= 3 (f = f / 3) g %= 9 (g = g % 9)

41. 3.12 Increment and Decrement Operators • Increment operator (++) • Can be used instead of c+=1 • Decrement operator (--) • Can be used instead of c-=1 • Preincrement • Operator is used before the variable (++c or --c) • Variable is changed before the expression it is in is evaluated • Postincrement • Operator is used after the variable (c++ or c--) • Expression executes before the variable is changed

42. 3.12 Increment and Decrement Operators • If c equals 5, then printf("%d", ++c); • Prints 6 printf("%d", c++); • Prints 5 • In either case, c now has the value of 6 • When variable not in an expression • Preincrementing and postincrementing have the same effect ++c; printf( “%d”, c ); • Has the same effect as c++; printf( “%d”, c );

43. Chapter 4 - Program Control Outline 4.1 Introduction 4.2 The Essentials of Repetition 4.3 Counter-Controlled Repetition 4.4 The For Repetition Structure 4.5 The For Structure: Notes and Observations 4.6 Examples Using the For Structure 4.7 The Switch Multiple-Selection Structure 4.8 The Do/While Repetition Structure 4.9 The break and continue Statements 4.10 Logical Operators 4.11 Confusing Equality (==) and Assignment (=) Operators 4.12 Structured Programming Summary

44. 4.1 Introduction • This chapter introduces • Additional repetition control structures • for • do/while • switch multiple selection structure • break statement • Used for exiting immediately and rapidly from certain control structures • continue statement • Used for skipping the remainder of the body of a repetition structure and proceeding with the next iteration of the loop

45. 4.2 The Essentials of Repetition • Loop • Group of instructions computer executes repeatedly while some condition remains true • Counter-controlled repetition • Definite repetition: know how many times loop will execute • Control variable used to count repetitions • Sentinel-controlled repetition • Indefinite repetition • Used when number of repetitions not known • Sentinel value indicates "end of data"

46. 4.3 Essentials of Counter-Controlled Repetition • Counter-controlled repetition requires • The name of a control variable (or loop counter) • The initial value of the control variable • A condition that tests for the final value of the control variable (i.e., whether looping should continue) • An increment (or decrement) by which the control variable is modified each time through the loop

47. 4.3 Essentials of Counter-Controlled Repetition • Example: int counter = 1; // initialization while ( counter <= 10 ) { // repetition condition printf( "%d\n", counter ); ++counter; // increment } • The statement int counter = 1; • Names counter • Declares it to be an integer • Reserves space for it in memory • Sets it to an initial value of 1

48. 4.4 The for Repetition Structure • Format when using for loops for ( initialization; loopContinuationTest; increment ) statement • Example: for( int counter = 1; counter <= 10; counter++ ) printf( "%d\n", counter ); • Prints the integers from one to ten No semicolon (;) after last expression

49. 4.4 The for Repetition Structure • For loops can usually be rewritten as while loops: initialization;while( loopContinuationTest ) { statement; increment;} • Initialization and increment • Can be comma-separated lists • Example: for (int i = 0, j = 0; j + i <= 10; j++, i++) printf( "%d\n", j + i );

50. 4.5 The for Structure: Notes and Observations • Arithmetic expressions • Initialization, loop-continuation, and increment can contain arithmetic expressions. If x equals 2 and y equals 10 for ( j = x; j <= 4 * x * y; j += y / x ) is equivalent to for ( j = 2; j <= 80; j += 5 ) • Notes about the for structure: • "Increment" may be negative (decrement) • If the loop continuation condition is initially false • The body of the for structure is not performed • Control proceeds with the next statement after the for structure • Control variable • Often printed or used inside for body, but not necessary