1 CS 103 Lecture 3 Slides Control Structures Mark Redekopp
2 Announcements • Lab 2 – Due Friday
3 Review • Write a program to ask the user to enter two integers representing hours then minutes. Output the equivalent number of seconds. • To get started… – Go to http://bytes.usc.edu/cs103/in-class-exercises • printseconds – We've started the program for you…look at the • General template for a program with the #includes, using namespace std; and int main() function which returns 0 – We've declared variables where you can store the input and computation results – Now you add code to • Get input from the user • And compute the answer and place it in the 'sec' variable
4 If..else statements MODULE 5: CONDITIONAL STATEMENTS
5 Comparison Operators • Control structures like if, while, and for require conditions to determine what code should execute • To perform comparison of variables, constants, or expressions in C/C++ we can use the basic 6 comparison operators Operator(s) Meaning Example Equality if(x == y) == Inequality if(x != 7) != Less-than if(x < 0) < Greater-than if(y > x) > Less-than OR equal to if(x <= -3) <= Greater-than OR equal to if(y >= 2) >=
6 Logical AND, OR, NOT • Often want to combine several conditions to make a decision • Logical AND => x > 0 && y > 0 • Logical OR => x == 1 || x == 2 • Logical NOT => !(x < 0) • Precedence (order of ops.) => ! then && then || – !cond1 || cond2 && !cond3 – ( ( !cond1 ) || (cond2 && ( !cond3 ) ) ) A B AND A B OR A NOT False False False False False False False True False True False False True True True False True False False True False True True True True True True True
7 Exercise • Which of the following is NOT a condition to check if the integer x is in the range [-1 to 5] • x >= -1 && x <= 5 • -1 <= x <= 5 • !( x < -1 || x > 5) • x > -2 && x < 6
8 bools, ints, and Conditions • Loops & conditional statements require a condition to be evaluated resulting in a true or false result. • In C/C++… – 0 means false / Non-Zero means true – bool type available in C++ => ‘ true ’ and ‘ false ’ keywords can be used but internally • true = non-zero (usually 1) and • false = 0 • Any place a condition would be used a bool or int type can be used and will be interpreted as bool int x = 100; bool done = false; int x=100, y=3, z=0; if(x) while( ! done ) if( !x || (y && !z) ) { x--; } { cin >> done; } { /* code */ } • Example:
9 Conditions and DeMorgans • Write a condition that eats a sandwich if it has neither tomato nor lettuce – if ( !tomato && !lettuce) { eat_sandwich(); } – if ( !(tomato || lettuce) ) { eat_sandwich(); } • DeMorgan's theorem says there is always two ways to express a logic condition – !a && !b !(a || b) – !a || !b !(a && b) • More details in EE 109 and CS 170
10 If..Else Flow Chart if ( condition1 ) { // executed if condition1 is true True False } condition else { // executed if condition1 // above is false } // following statements If Block Else Statements Block Statements Following statements
11 If…Else If…Else • Use to execute only if ( condition1 ) { certain portions of code // executed if condition1 is true } • else if is optional else if ( condition2 ) { – Can have any number of // executed if condition2 is true // but condition1 was false else if statements } • else is optional else if ( condition3 ) { • { … } indicate code // executed if condition3 is true // but condition1 and condition2 associated with the if, // were false } else if, else block else { optional // executed if neither condition if else if else if else // above is true }
12 else if if ( condition1 ) { // executed if condition1 is True } else if ( condition2 ) { // executed if condition2 is True True False condition // but condition1 was False } 1 else These 2 are equivalent { // executed if neither condition True False cond2 condition // above is True } if ( condition1 ) Else If Else If Block { If Block Else Statements Block Statements Statements Statements // executed if condition1 is True Statements } else { if ( condition2 ){ Following // executed if condition2 is True statements // but condition1 was False } else Following { // executed if neither condition Statements // above is True } }
13 Single Statement Bodies • The Rule: Place code for an if (x == 5) y += 2; if, else if, or else construct in else curly braces { … } y -= 3; • The Exception: – An if or else construct with a if (x == 5) single statement body does not y += 2; else require { … } if(x < 5) – Another if counts as a single y = 6; statement else • Prefer { … } even in single y = 0; statement bodies so that editing later does not introduce bugs
14 PROBLEM SOLVING IDIOMS
15 Rule/Exception Idiom • Name : Rule/Exception // Default action • Description : Perform a default if( /* Exceptional Case */ ) { action and then us an ‘if’ to // Code to apply to // exceptional case correct for exceptional cases } • Structure : Default action code Structure followed by if statement with code to correct the exceptional bool primeMember = /* set somehow */; case double shippingFee = 7.99; if( primeMember == true ) • Example(s) : { shippingFee = 0; – Shipping for "members" } Example
16 Look-up Table Idiom • Name : Look-up Table (Parallel cases) if( /* Condition 1 */ ) { – A table can describe the mapping of input // Case 1 code to output } else if( /* Condition 2 */ ) • Description : Break input into { // Case 2 code mutually exclusive cases, taking some } else if( /* Condition 3 */ ) action or producing some output in { each case // Case 3 code } • Structure : Single level else { /* Default */ // Default code 'if..else if..else' statement } Score Grade Look-up Table Structure (input) (output) if( weather == "hot" ) { > 90 A clothing = "t-shirt"; Weather Dress } 80-89 B else if( weather == "mild" ) { Hot T-shirt 70-79 C clothing = "long sleeves"; } Mild Long Sleeves 55-69 D else { /* Default */ clothing = "sweater"; Cold Sweater < 55 F }
17 Decision Tree (Subcase) Idiom • Name : Decision Tree (Subcase) if( /* Condition 1 */ ) { • Description : The result of one // Case 1 code condition determines which if( /* Subcondition 1a */ ) { // Subcase 1a code condition (subcase) to check next } else { • Structure : Nested 'if' statements // Subcase 1b code } } else if( /* Condition 2 */ ) Customer Service Call Menu { // Case 2 code Top-level Account Hours of if( /* Subcondition 2a */ ) { Cases: Operation Issues // Subcase 2a code } } Sub-Cases: Balance Cancel
18 Exercises • Conditionals In-Class Exercises – discount – weekday – nth
19 The Right Style • Is there a difference int x; between the following cin >> x; two code snippets if( x >= 0 ) { cout << "Positive"; } • Both are equivalent but if( x < 0 ) { cout << "Negative"; } – Two if statements implies both can execute – An if..else implies a int x; cin >> x; mutually exclusive relationship where only 1 if( x >= 0 ) { cout << "Positive"; } can execute else { cout << "Negative"; } • For mutually exclusive cases, use if..else for clarity sake
20 Find the bug // What’s the problem below • What's the problem with int x; this code… cin >> x; if (x = 1) { cout << "x is 1" << endl; } else • Common mistake is to { cout << "x is not 1" << endl; } use assignment '=' rather than equality comparison '==' operator // What’s the problem below int x; • Assignment puts 1 into x cin >> x; and then uses that value if (x = 1) // x == 1 of x as the "condition" { cout << "x is 1" << endl; } else – 1 = true so we will { cout << "x is not 1" << endl; } always execute the if portion
21 Switch (Study on own) • Again used to execute only switch(expr) // expr must eval to an int certain blocks of code { • Cases must be a constant case 0: // code executed when expr == 0 • Best used to select an action break; case 1: when an expression could be 1 // code executed when expr == 1 of a set of constant values break; case 2: • { … } around entire set of cases case 3: case 4: and not individual case // code executed when expr is • Computer will execute code // 2, 3, or 4 break; until a break statement is default: // code executed when no other encountered // case is executed – Allows multiple cases to be break; combined } • Default statement is like an else statement
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