CSC 212 Chapter 2, Spring 2013

 2003 Prentice Hall, Inc. All rights reserved. 1 • Before writing a program – Have a thorough understanding of problem – Carefully plan your approach for solving it • While writing a program – Know what “building blocks” are available – Use good programming principles Chapter 2– Control Structures Introduction  2003 Prentice Hall, Inc. All rights reserved. 2 Algorithms • Computing problems – They are solved by executing a series of actions in a specific order • An algorithm is a procedure determining – Actions to be executed – Order to be executed – Example: recipe • Program control – Specifies the order in which statements are executed  2003 Prentice Hall, Inc. All rights reserved. 3 Pseudocode • Pseudocode – Artificial, informal language used to develop algorithms – Similar to everyday English • Not executed on computers – Used to think out a program before coding • Easy to convert into C++ program – Only executable statements • No need to declare variables  2003 Prentice Hall, Inc. All rights reserved. 4 Flowchart • Flowchart – Graphical representation of an algorithm – Special-purpose symbols Start/Stop program Process Running Decision Connecto r Flow Direction  2003 Prentice Hall, Inc. All rights reserved. 5 Control Structures • Sequential execution – Statements executed in order • Transfer of control – Next statement executed not next one in sequence – Eliminate goto • 3 control structures (Bohm and Jacopini) – Sequence structure • Programs executed sequentially by default – Selection structures • if, if/else, switch – Repetition structures • while, do/while, for  2003 Prentice Hall, Inc. All rights reserved. 6 Sequence Structure • Pseudocode Add grade to total Add 1 to counter • Flowchart • C++ total = total + grade; counter = counter + 1; Add grade to total Add 1 to counter  2003 Prentice Hall, Inc. All rights reserved. 7 if Selection Structure • Selection structure – Choose among alternative courses of action – Pseudocode example: If student’s grade is greater than or equal to 60 Print “Passed” – If the condition is true • Print statement executed, program continues to next statement – If the condition is false • Print statement ignored, program continues – Indenting makes programs easier to read • C++ ignores whitespace characters (tabs, spaces, etc.)  2003 Prentice Hall, Inc. All rights reserved. 8 if Selection Structure • Translation into C++ If student’s grade is greater than or equal to 60 Print “Passed” if ( grade >= 60 ) cout << "Passed"; • Diamond symbol (decision symbol) – Indicates decision is to be made – Contains an expression that can be true or false • Test condition, follow path • if structure – Single-entry/single-exit  2003 Prentice Hall, Inc. All rights reserved. 9 if Selection Structure • Flowchart of pseudocode statement true false grade >= 60 print “Passed” A decision can be made on any expression. zero - false nonzero - true Example: 3 - 4 is true  2003 Prentice Hall, Inc. All rights reserved. 10 if/else Selection Structure • if – Performs action if condition true • if/else – Different actions if conditions true or false • Pseudocode if student’s grade is greater than or equal to 60 print “Passed” else print “Failed” • C++ code if ( grade >= 60 ) cout << "Passed"; else cout << "Failed";  2003 Prentice Hall, Inc. All rights reserved. 11 if/else Selection Structure • Ternary conditional operator (?:) – Three arguments (condition, value if true, value if false) • Code could be written: cout << ( grade >= 60 ? “Passed” : “Failed” ); true false print “Failed” print “Passed” grade >= 60 Condition Value if true Value if false  2003 Prentice Hall, Inc. All rights reserved. 12 if/else Selection Structure • Nested if/else structures – One inside another, test for multiple cases – Once condition met, other statements skipped 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”  2003 Prentice Hall, Inc. All rights reserved. 13 if/else Selection Structure • Example if ( grade >= 90 ) // 90 and above cout << "A"; else if ( grade >= 80 ) // 80-89 cout << "B"; else if ( grade >= 70 ) // 70-79 cout << "C"; else if ( grade >= 60 ) // 60-69 cout << "D"; else // less than 60 cout << "F";  2003 Prentice Hall, Inc. All rights reserved. 14 if/else Selection Structure • Compound statement – Set of statements within a pair of braces if ( grade >= 60 ) cout << "Passed.\n"; else { cout << "Failed.\n"; cout << "You must take this course again.\n"; } – Without braces, cout << "You must take this course again.\n"; always executed • Block – Set of statements within braces  2003 Prentice Hall, Inc. All rights reserved. 15 while Repetition Structure • Repetition structure – Action repeated while some condition remains true – Psuedocode while there are more items on my shopping list Purchase next item and cross it off my list – while loop repeated until condition becomes false • Example int product = 2; while ( product <= 1000 ) product = 2 * product;  2003 Prentice Hall, Inc. All rights reserved. 16 The while Repetition Structure • Flowchart of while loop product <= 1000 product = 2 * product true false  2003 Prentice Hall, Inc. All rights reserved. 17 Formulating Algorithms (Counter-Controlled Repetition) • Counter-controlled repetition – Loop repeated until counter reaches certain value • Definite repetition – Number of repetitions 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.  2003 Prentice Hall, Inc. All rights reserved. 18 Formulating Algorithms (Counter- controlled Repetition) • Pseudocode for example: Set total to zero Set grade counter to one While grade counter is less than or equal to ten Input the next grade Add the grade into the total Add one to the grade counter Set the class average to the total divided by ten Print the class average • Next: C++ code for this example  2003 Prentice Hall, Inc. All rights reserved. Outline 19 // Class average program with counter-controlled repetition. #include <iostream> using namespace std; // function main begins program execution int main() { int total; // sum of grades input by user int gradeCounter; // number of grade to be entered next int grade; // grade value int average; // average of grades // initialization phase total = 0; // initialize total gradeCounter = 1; // initialize loop counter  2003 Prentice Hall, Inc. All rights reserved. Outline 20 // processing phase while ( gradeCounter <= 10 ) { // loop 10 times cout << "Enter grade: "; // prompt for input cin >> grade; // read grade from user total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter } // termination phase average = total / 10; // integer division // display result cout << "Class average is " << average << endl; return 0; // indicate program ended successfully } // end function main 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 The counter gets incremented each time the loop executes. Eventually, the counter causes the loop to end.  2003 Prentice Hall, Inc. All rights reserved. 21 Formulating Algorithms (Sentinel- Controlled Repetition) • Suppose 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 program know when to end? • Sentinel value – Indicates “end of data entry” – Loop ends when sentinel input – Sentinel chosen so it cannot be confused with regular input • -1 in this case  2003 Prentice Hall, Inc. All rights reserved. 22 Formulating Algorithms (Sentinel- Controlled Repetition) • Top-down, stepwise refinement – Begin with pseudocode representation of top Determine the class average for the quiz – Divide top into smaller tasks, list in order Initialize variables Input, sum and count the quiz grades Calculate and print the class average  2003 Prentice Hall, Inc. All rights reserved. 23 Formulating Algorithms (Sentinel-Controlled Repetition) • Many programs have three phases – Initialization • Initializes the program variables – Processing • Input data, adjusts program variables – Termination • Calculate and print the final results – Helps break up programs for top-down refinement  2003 Prentice Hall, Inc. All rights reserved. 24 Formulating Algorithms (Sentinel- Controlled Repetition) • Refine the initialization phase Initialize variables goes to Initialize total to zero Initialize counter to zero • Processing Input, sum and count the quiz grades goes 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)  2003 Prentice Hall, Inc. All rights reserved. 25 Formulating Algorithms (Sentinel- Controlled Repetition) • Termination Calculate and print the class average goes to If the counter is not equal to zero Set the average to the total divided by the counter Print the average Else Print “No grades were entered” • Next: C++ program  2003 Prentice Hall, Inc. All rights reserved. Outline 26 // Class average program with sentinel-controlled repetition. #include <iostream> #include <iomanip> using namespace std; // function main begins program execution int main() { int total; // sum of grades int gradeCounter; // number of grades entered int grade; // grade value float average; // number with decimal point for average // initialization phase total = 0; // initialize total gradeCounter = 0; // initialize loop counter Data type float used to represent decimal numbers.  2003 Prentice Hall, Inc. All rights reserved. Outline 27 // processing phase // get first grade from user cout << "Enter grade, -1 to end: "; // prompt for input cin >> grade; // read grade from user // loop until sentinel value read from user while ( grade != -1 ) { total = total + grade; // add grade to total gradeCounter = gradeCounter + 1; // increment counter cout << "Enter grade, -1 to end: "; // prompt for input cin >> grade; // read next grade } // end while // termination phase // if user entered at least one grade ... if ( gradeCounter != 0 ) { // calculate average of all grades entered average = static_cast< float >( total ) / gradeCounter; static_cast<float>() treats total as a float temporarily (casting). Required because dividing two integers truncates the remainder. gradeCounter is an int, but it gets promoted to float.  2003 Prentice Hall, Inc. All rights reserved. Outline 28 // display average with two digits of precision cout << "Class average is " << setprecision( 2 ) <<setiosflags(ios::fixed|ios:showpoint) << average << endl; } // end if part of if/else else // if no grades were entered, output appropriate message cout << "No grades were entered" << endl; return 0; // indicate program ended successfully } // end function main 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 setprecision(2)prints two digits past decimal point if used with fixed (rounded to fit precision). Programs that use this must include <iomanip> fixed forces output to print in fixed point format (not scientific notation). Also, showpoint forces trailing zeros and decimal point to print.  2003 Prentice Hall, Inc. All rights reserved. 29 Nested Control Structures • Problem statement 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 – Program processes 10 results • Fixed number, use counter-controlled loop – Two counters can be used • One counts number that passed • Another counts number that fail – Each test result is 1 or 2 • If not 1, assume 2  2003 Prentice Hall, Inc. All rights reserved. 30 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  2003 Prentice Hall, Inc. All rights reserved. 31 Nested Control Structures • Refine Input the ten quiz grades and count passes and failures to While student counter is less than or equal to ten Input the next exam result If the student passed Add one to passes Else Add one to failures Add one to student counter  2003 Prentice Hall, Inc. All rights reserved. 32 Nested Control Structures • 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” • Program next  2003 Prentice Hall, Inc. All rights reserved. Outline 33 // Analysis of examination results. #include <iostream> using namespace std; // function main begins program execution int main() { // initialize variables in declarations int passes = 0; // number of passes int failures = 0; // number of failures int studentCounter = 1; // student counter int result; // one exam result // process 10 students using counter-controlled loop while ( studentCounter <= 10 ) { // prompt user for input and obtain value from user cout << "Enter result (1 = pass, 2 = fail): "; cin >> result;  2003 Prentice Hall, Inc. All rights reserved. Outline 34 // if result 1, increment passes; if/else nested in while if ( result == 1 ) // if/else nested in while passes = passes + 1; else // if result not 1, increment failures failures = failures + 1; // increment studentCounter so loop eventually terminates studentCounter = studentCounter + 1; } // end while // termination phase; display number of passes and failures cout << "Passed " << passes << endl; cout << "Failed " << failures << endl; // if more than eight students passed, print "raise tuition" if ( passes > 8 ) cout << "Raise tuition " << endl; return 0; // successful termination } // end function main  2003 Prentice Hall, Inc. All rights reserved. 35 Assignment Operators • Assignment expression abbreviations – Addition assignment operator c = c + 3; abbreviated to c += 3; • Statements of the form variable = variable operator expression; can be rewritten as variable operator= expression; • Other assignment operators d -= 4 (d = d - 4) e *= 5 (e = e * 5) f /= 3 (f = f / 3) g %= 9 (g = g % 9)  2003 Prentice Hall, Inc. All rights reserved. 36 Increment and Decrement Operators • Increment operator (++) c++ can be used instead of c += 1 • Decrement operator (--) c-- can be used instead of c -= 1 • Preincrement • When the operator is used before the variable (++c or ––c) • Variable is changed, then the expression it is in is evaluated. • Postincrement • When the operator is used after the variable (c++ or c--) • Expression the variable is in executes, then the variable is changed.  2003 Prentice Hall, Inc. All rights reserved. 37 Increment and Decrement Operators • If c = 5, then – cout << ++c; • c is changed to 6, then printed out – cout << c++; • Prints out 5 (cout is executed before the increment) • c then becomes 6  2003 Prentice Hall, Inc. All rights reserved. 38 Increment and Decrement Operators • When variable not in expression – Preincrementing and postincrementing have same effect ++c; cout << c; and c++; cout << c; are the same  2003 Prentice Hall, Inc. All rights reserved. Outline 39 // Preincrementing and postincrementing. #include <iostream> using namespace std; // function main begins program execution int main() { int c; // declare variable // demonstrate postincrement c = 5; // assign 5 to c cout << c << endl; // print 5 cout << c++ << endl; // print 5 then postincrement cout << c << endl << endl; // print 6 // demonstrate preincrement c = 5; // assign 5 to c cout << c << endl; // print 5 cout << ++c << endl; // preincrement then print 6 cout << c << endl; // print 6 return 0; // indicate successful termination } // end function main  2003 Prentice Hall, Inc. All rights reserved. 40 Essentials of Counter-Controlled Repetition • Counter-controlled repetition requires – Name of control variable/loop counter – Initial value of control variable – Condition to test for final value – Increment/decrement to modify control variable when looping  2003 Prentice Hall, Inc. All rights reserved. Outline 41 // Counter-controlled repetition. #include <iostream> using namespace std; // function main begins program execution int main() { int counter = 1; // initialization while ( counter <= 10 ) { // repetition condition cout << counter << endl; // display counter ++counter; // increment } // end while return 0; // indicate successful termination } // end function main  2003 Prentice Hall, Inc. All rights reserved. 42 for Repetition Structure • General format when using for loops for ( initialization; LoopContinuationTest; increment ) statement • Example for( int counter = 1; counter <= 10; counter++ ) cout << counter << endl; – Prints integers from one to ten No semicolon after last statement  2003 Prentice Hall, Inc. All rights reserved. Outline 43 // Counter-controlled repetition with the for structure. #include <iostream> using namespace std; // function main begins program execution int main() { // Initialization, repetition condition and incrementing // are all included in the for structure header. for ( int counter = 1; counter <= 10; counter++ ) cout << counter << endl; return 0; // indicate successful termination } // end function main  2003 Prentice Hall, Inc. All rights reserved. 44 for Repetition Structure • for loops can usually be rewritten as while loops initialization; while ( loopContinuationTest){ statement increment; } • Initialization and increment – For multiple variables, use comma-separated lists for (int i = 0, j = 0; j + i <= 10; j++, i++) cout << j + i << endl;  2003 Prentice Hall, Inc. All rights reserved. Outline 45 // Summation with for. #include <iostream> using namespace std; // function main begins program execution int main() { int sum = 0; // initialize sum // sum even integers from 2 through 100 for ( int number = 2; number <= 100; number += 2 ) sum += number; // add number to sum cout << "Sum is " << sum << endl; // output sum return 0; // successful termination } // end function main Sum is 2550  2003 Prentice Hall, Inc. All rights reserved. 46 Examples Using the for Structure • Program to calculate compound interest • Possible case: A person invests $1000.00 in a savings account yielding 5 percent interest. Assuming that all interest is left on deposit in the account, calculate and print the amount of money in the account at the end of each year for 10 years. Use the following formula for determining these amounts: a = p(1+r) • p is the original amount invested (i.e., the principal), r is the annual interest rate, n is the number of years and a is the amount on deposit at the end of the nth year • The program presented here will input the amount invested, the interest rate and the number of years. n  2003 Prentice Hall, Inc. All rights reserved. Outline 47 // Calculating compound interest. #include <iostream> #include <iomanip> #include <cmath> // enables program to use function pow using namespace std; // function main begins program execution int main() { double amount; // amount on deposit double principal ; // starting principal double rate ; // interest rate int year; cout << "Enter the Principal:\n"; cin >> principal; cout << "The rate:\n"; cin >> rate; cout << "the number of years:\n"; cin >> year; cout << "Year" << setw(21) << "Amount on deposit" << endl; cout << setiosflags(ios::fixed|ios::showpoint) << setprecision(2);  2003 Prentice Hall, Inc. All rights reserved. Outline 48 // calculate amount on deposit for each of the years for ( int count = 1; count <=year ; count++ ) { // calculate new amount for specified year amount = principal * pow( 1.0 + rate, count ); // output one table row cout << setw( 4 ) << count << setw( 21 ) << amount << endl; } // end for return 0; // indicate successful termination } // end function main  2003 Prentice Hall, Inc. All rights reserved. 49 switch Multiple-Selection Structure • switch – Test variable for multiple constant values – Series of case labels and optional default case switch ( variable ) { case value1: // taken if variable == value1 statements break; // necessary to exit switch case value2: case value3: // taken if variable == value2 or == value3 statements break; default: // taken if variable matches no other cases statements break; }  2003 Prentice Hall, Inc. All rights reserved. 50 switch Multiple-Selection Structure true false . . . case a case a action(s) break case b case b action(s) break false false case z case z action(s) break true true default action(s)  2003 Prentice Hall, Inc. All rights reserved. 51 switch Multiple-Selection Structure • Example upcoming – Program to read grades (A-F) – Display number of each grade entered • Details about characters – Single characters typically stored in a char data type • char a 1-byte integer, so chars can be stored as ints – Can treat character as int or char • 97 is the numerical representation of lowercase ‘a’ (ASCII) • Use single quotes to get numerical representation of character cout << "The character (" << 'a' << ") has the value " << static_cast< int > ( 'a' ) << endl; Prints The character (a) has the value 97  2003 Prentice Hall, Inc. All rights reserved. Outline 52 // Counting letter grades. #include <iostream> using namespace std; // function main begins program execution int main() { int grade; // one grade int aCount = 0; // number of As int bCount = 0; // number of Bs int cCount = 0; // number of Cs int dCount = 0; // number of Ds int fCount = 0; // number of Fs cout << "Enter the letter grades." << endl << "Enter the EOF character to end input." << endl;  2003 Prentice Hall, Inc. All rights reserved. Outline 53 // loop until user types end-of-file key sequence while ( ( grade = cin.get() ) != EOF ) { // determine which grade was input switch ( grade ) { // switch structure nested in while case 'A': // grade was uppercase A case 'a': // or lowercase a ++aCount; // increment aCount break; // necessary to exit switch case 'B': // grade was uppercase B case 'b': // or lowercase b ++bCount; // increment bCount break; // exit switch case 'C': // grade was uppercase C case 'c': // or lowercase c ++cCount; // increment cCount break; // exit switch cin.get() uses dot notation . This function gets 1 character from the keyboard (after Enter pressed), and it is assigned to grade. cin.get() returns EOF (end-of- file) after the EOF character is input, to indicate the end of data. EOF may be ctrl-d or ctrl-z, depending on your OS. Compares grade (an int) to the numerical representations of A and a. break causes switch to end and the program continues with the first statement after the switch structure. Assignment statements have a value, which is the same as the variable on the left of the =. The value of this statement is the same as the value returned by cin.get(). This can also be used to initialize multiple variables: a = b = c = 0;  2003 Prentice Hall, Inc. All rights reserved. Outline 54 case 'D': // grade was uppercase D case 'd': // or lowercase d ++dCount; // increment dCount break; // exit switch case 'F': // grade was uppercase F case 'f': // or lowercase f ++fCount; // increment fCount break; // exit switch case '\n': // ignore newlines, case '\t': // tabs, case ' ': // and spaces in input break; // exit switch default: // catch all other characters cout << "Incorrect letter grade entered." << " Enter a new grade." << endl; break; // optional; will exit switch anyway } // end switch } // end while Notice the default statement, which catches all other cases. This test is necessary because Enter is pressed after each letter grade is input. This adds a newline character that must be removed. Likewise, we want to ignore any whitespace.  2003 Prentice Hall, Inc. All rights reserved. Outline 55 // output summary of results cout << "\n\nTotals for each letter grade are:" << "\nA: " << aCount // display number of A grades << "\nB: " << bCount // display number of B grades << "\nC: " << cCount // display number of C grades << "\nD: " << dCount // display number of D grades << "\nF: " << fCount // display number of F grades << endl; return 0; // indicate successful termination } // end function main  2003 Prentice Hall, Inc. All rights reserved. Outline 56 Enter the letter grades. Enter the EOF character to end input. a B c C A d f C E Incorrect letter grade entered. Enter a new grade. D A b ^Z Totals for each letter grade are: A: 3 B: 2 C: 3 D: 2 F: 1  2003 Prentice Hall, Inc. All rights reserved. 57 do/while Repetition Structure • Similar to while structure – Makes loop continuation test at end, not beginning – Loop body executes at least once • Format do { statement } while ( condition ); true false action(s) condition  2003 Prentice Hall, Inc. All rights reserved. Outline 58 // Using the do/while repetition structure. #include <iostream> using namespace std; // function main begins program execution int main() { int counter = 1; // initialize counter do { cout << counter << " "; // display counter } while ( ++counter <= 10 ); // end do/while cout << endl; return 0; // indicate successful termination } // end function main 1 2 3 4 5 6 7 8 9 10 Notice the preincrement in loop-continuation test.  2003 Prentice Hall, Inc. All rights reserved. 59 break and continue Statements • break statement – Immediate exit from while, for, do/while, switch – Program continues with first statement after structure • Common uses – Escape early from a loop – Skip the remainder of switch  2003 Prentice Hall, Inc. All rights reserved. Outline 60 // Using the break statement in a for structure. #include <iostream> using namespace std; // function main begins program execution int main() { int x; // x declared here so it can be used after the loop // loop 10 times for ( x = 1; x <= 10; x++ ) { // if x is 5, terminate loop if ( x == 5 ) break; // break loop only if x is 5 cout << x << " "; // display value of x } // end for cout << "\nBroke out of loop when x became " << x << endl; return 0; // indicate successful termination } // end function main Exits for structure when break executed.