UNIT 2: CONTROL STRUCTURES. IF ELSE, FOR, WHILE TEXT LOGISTICS - PowerPoint PPT Presentation

CS103L SPRING 2017 UNIT 2: CONTROL STRUCTURES. IF… ELSE, FOR, WHILE

TEXT LOGISTICS ▸ Lab #3 due Friday - must go if you haven’t checked it off yet ▸ HW#2 due Thursday - note updated due dates

TEXT LEARNING OBJECTIVES ▸ Understand control flow statements: ▸ if, else-if, else ▸ while, do-while ▸ for

TEXT QUICK WARMUP ▸ Write a program to ask the user to enter two integers representing hours then minutes ▸ In class exercises ▸ http://bits.usc.edu/cs103/in-class-exercises/in-class-exercises-set1/ ▸ printseconds ▸ Skeleton code including variables is already started

TEXT COMPARISONS AND LOGICAL OPERATORS ▸ Often we want to check if a condition is true or false ▸ Is x == 100? Is y < 35.5? Is x > z? ▸ Control flow statements require conditions ▸ In C/C++ the integer value 0 means ‘false’. Any value other than 0 is ‘true’ ▸ Have bool type and true/false constants to help ▸ Behind the scenes true == 1, false == 0

TEXT CONDITIONS ▸ Conditions usually arrive from comparisons and logical operators ▸ Comparisons: ▸ ==, !=, >, <, >=, <= ▸ Equals to, not equals, greater than, less than, greater than or equal, less than or equal

TEXT • • LOGICAL OPERATORS • • • ▸ Combine the result of expressions ‘logically’ – – ( ( !x ) || (y && ( !z ) ) ) … if x=100, y= 3, z=0 then this expression is… ▸ Logical AND: && operator expr_1 && expr_2 • – ▸ x == 1 && y <= 2 – A B AND A B OR A NOT ▸ Logical OR: || operator expr_1 || expr_2 False False False False False False False True False True False False True True True False ▸ x >= 5 || x < 0 True False False True False True True True True True True True ▸ Logical NOT: ! operator !expr_1 ▸ !(x >= 5 || x < 0) ▸ !x && y < 0 ▸ Operator precedence: ! then && then ||

TEXT LOGICAL OPERATOR PRACTICE ▸ x=100;y=-3;z=0 ▸ !x || y && !z ▸ With parens: ((!x) || ( y && (!z)))

TEXT LOGICAL OPERATOR PRACTICE ▸ Which of the following does *not* check if an integer x is in the range [-1 to 5] (inclusive) ▸ x >= -1 && x <=5 ▸ -1 <= x <= 5 ▸ !(x < -1 || x > 5) ▸ x > -2 && x < 6

TEXT IF STATEMENTS ▸ Controlling the flow of your code

TEXT CONTROL STATEMENT #1: IF…ELSE IF…ELSE ▸ Used to control what code executes based on one or more conditions if ( condition1 ) ▸ Form 1: basic if statement { // executed if condition1 is True } ▸ else block is optional else True False { condition // executed if neither condition // above is True } // following statements If Block Else Statements Block Statements Following statements

TEXT If…Else If…Else IF STATEMENT • if ( condition1 ) ▸ Use if to execute only parts of your code { // executed if condition1 is True } else if ( condition2 ) ▸ else if is optional, can have as many as you • { need // executed if condition2 is True – // but condition1 was False } else if ( condition3 ) ▸ final else is also optional, will execute if *none* { • of the conditions are true // executed if condition3 is True // but condition1 and condition2 • { … } indicate code // were False } ▸ Use {} brackets to associate code with the else condition { // executed if neither condition // above is True }

TEXT FULL IF STATEMENT FLOW CHART if ( condition1 ) { // executed if condition1 is True True False } condition else if ( condition2 ) 1 { // executed if condition2 is True True False cond2 // but condition1 was False condition } else { Else If Else If Block // executed if neither condition If Block Else Statements Statements Statements Block Statements // above is True Statements } // following statements Following statements Following Statements

TEXT IN CLASS EXCERCISES ▸ discount ▸ weekday ▸ N-th

TEXT STYLE NOTES ▸ Code “style” ▸ What do we mean by that? ▸ Often there are many, many different ways to write even simple code ▸ Try to choose version that is clear, shows intent

TEXT STYLE QUESTION ▸ Which is better? ▸ Why? int x; int x; cin >> x; cin >> x; if( x >= 0) { if( x >= 0) { cout << “Positive”; cout << “Positive”; } } if( x < 0 ) { else { cout << “Negative”; cout << “Negative”; } }

TEXT COMMON BUG ▸ What’s wrong here? int x; cin >> x; if( x = 1) { cout << “x is 1!” << endl; } else { cout << “x is not 1.” << endl; }

TEXT = VS == ▸ Common mistake to use = (assignment) instead of == (equals to) ▸ The = operator returns the value assigned, so ( x = 1) returns 1 ▸ if( x = 1) will always get 1 → true int x; ▸ if() block will always execute! cin >> x; if( x = 1) { cout << “x is 1!” << endl; } else { cout << “x is not 1.” << endl; }

TEXT = VS == FIXED ▸ Fixed code int x; cin >> x; if( x == 1) { cout << “x is 1!” << endl; } else { cout << “x is not 1.” << endl; }

TEXT ? OPERATOR ▸ Often we find ourselves writing code of the form: if (x>0) { z = 2; } else { z = 1; } ▸ ? is a short cut: z = x > 0 ? 2 : 1; ▸ Syntax: condition ? expr_if_true : expr_if_false;

TEXT LOOPS ▸ Doing something more than once

TEXT for Repetition #include <iostream> WHY DO WE NEED LOOPS? • using namespace std; t a int main() se { • ▸ Almost *all* programs have at least one loop… cout << 1 << endl; cout << 2 << endl; ... – 100 cout << 100 << endl; ▸ Print out numbers 1 - 100 return 0; – } – ▸ Play a game of rounds until there is a winner – • Imagine the game of 'war'…it ▸ Can we do this without loops? #include <iostream> Assume this produces • • Imagine the game of 'war'…it a true/false result using namespace std; indicating if the game is over after without loops, but… int main() ▸ Maybe? performing a turn { • bool gameOver; gameOver = take_turn(); without loops, but… if( ! gameOver ){ gameOver = take_turn(); if( ! gameOver ) { ... { } }

TEXT LOOP TYPE #1: WHILE LOOP ▸ While loop ▸ Do something while a condition is true ▸ Two forms: ▸ while(condition){ //something } ▸ do { //something } while(condition)

TEXT WHILE LOOP • ▸ condition is evaluated first // While Type 1: – while ( condition ) { ▸ If condition is true, body is executed – // code to be repeated // (should update condition) ▸ After body, condition is checked again } • ▸ Body *should* update condition – – ▸ Why? – ▸ How many times will body execute?

TEXT • – DO-WHILE LOOP – ▸ Body is executed • ▸ Then condition is checked, if true, body is executed again – // While Type 2: – ▸ Body should update condition do { – // code to be repeated ▸ How many times will body execute? // (should update condition) } while ( condition );

TEXT WHILE LOOP = REPEATING IF // guessing game bool guessedCorrect = false; • if( !guessedCorrect ) { guessedCorrect = guessAgain(); ▸ While loop is like a repeating if statement } t // want to repeat if cond. check again if( !guessedCorrect ) • ▸ If your description of the problem (algorithm) { guessedCorrect = guessAgain(); } // want to repeat if cond. check again contains: An if-statement will only execute once ▸ “until <xxx> is true” ' // guessing game bool guessedCorrect = false; ▸ “as long as <yyy> is bigger than <x> while( !guessedCorrect ) { guessedCorrect = guessAgain(); } ▸ “while the player’s guess is wrong” A 'while' loop acts as a repeating 'if' ▸ Then you need a while loop statement

TEXT IN CLASS EXERCISES ▸ countodd

TEXT FOR LOOP ▸ Very common looping structure ▸ Anatomy: conditional expression or Loop Initialization statement update statement for(init stmt; cond; update stmt) { • // body of loop } • • • • Outputs 0 5 10 15 … •

TEXT FOR LOOP or Loop ▸ Initialization statement: for(init stmt; cond; update stmt) ▸ Any valid statement, executed first { • // body of loop ▸ Conditional Expression: } • ▸ Expression evaluated next • ▸ Body: • ▸ Executed if conditional expression is true • Outputs 0 5 10 15 … ▸ Update statement: • ▸ Any valid statement, executed after body ▸ Condition is checked again after Update and body is executed if true… until condition is false