1 CS1430: Programming in C++ Section 2 Instructor: Qi Yang 213 Ullrich

2 Scope of Variables The region of code where it is legal to reference (use) an identifier. Local Scope Global Scope Class Scope

3 Code Block Between a pair of matching braces. For example, the body of a function int main() { int alpha = 10; // A block for if statement if (alpha > 3) { int n; cin >> n; alpha += 3; } return 0; }

4 Local Scope The scope of an identifier declared inside a block extends from the point of declaration to the end of that block. int main() { int alpha = 10; // A code block if (alpha > 3) { int num; cin >> num; alpha += num; } cout << alpha; // OK? cout << num; // OK? // Run time error! return 0; }

5 Global Scope The scope of an identifier declared outside all functions (and classes) extends from the point of declaration to the end of the entire source file. Programming Rules No global variables!

6 Class Scope Later this semester!

7 Scope of Function Parameters Formal parameters Local scope Same as local variable Cannot reference it outside the function Receive values on function call Actual parameters (no global variables) Local scope Cannot reference it inside the called function

8 Example float DoIt(int num, char op); int main() { int base; float result; char choice; cout << “Enter a number: ”; cin >> base; cout << “C for Cube and S for Square Root: ”; cin >> choice; while (choice != ‘C’ && choice != ‘S’) { cout << “C for Cube and S for Square Root: ”; cin >> choice; } result = DoIt(base, choice); cout << “The result: ” << result; return 0; } // // Precondition: op is ‘C’ or ‘S’ // Postcondition: the cube of // num is computed when op is // ‘C’, and square root of num // is computed when op is ‘S’. // float DoIt(int num, char op) { if (op == ‘C’) result = pow(num, 3); else result = sqrt(num); return result; } // What is wrong? // Result not declared in the // function!

9 Precondition and Postcondition int DoIt(int num, char op); int main() { int base; float result; char choice; cout << “Enter a number: ”; cin >> base; cout << “C for Cube and S for Square: ”; cin >> choice; while (choice != ‘C’ && choice != ‘S’) { cout << “C for Cube and S for Square: ”; cin >> choice; } result = DoIt(base, choice); cout << “The result: ” << result; return 0; } // // Precondition: op is ‘C’ or ‘S’ // Postcondition: the cube of // num is computed when op is // ‘C’, and square root of num // is computed when op is ‘S’. // int DoIt(int num, char op) { float result; if (op == ‘C’) result = pow(num, 3); else result = sqrt(num); return result; } // The two variables // result have the same // name, but different!

10 Parameter Names Meaningful names Formal and actual parameters can have the same name They are different variables in different scopes Normally they have different names

11 Lifetime of a Variable Lifetime The period of time during program execution when an identifier has memory allocated to it. Automatic variables A variable for which memory is allocated and deallocated when control enters and exits the block it is declared. Static variables A variable for which memory remains allocated throughout the execution of the entire program.

12 C++ Functions

// The function computes and returns the gross pay // based on the pay rate and hours. Hours over // 40 will be paid 1.5 times the regular pay rate. float GrossPay(float payRate, float hours) { float total; if (hours > REG_HOURS) total = (hours - REG_HOURS)* OVER_TIME * payRate + REG_HOURS * payRate; else total = hours * payRate; return total; } // No input for payRate and hours // Parameters receive values on function call! 13

Passing Parameters const float REG_HOURS = 40.0; const float OVER_TIME = 1.5; int main() { float hours, rate, gross; cin >> rate >> hours; // rate: 12.5 // hours: 50 gross = GrossPay(rate, hours); // display result return 0; } float GrossPay(float payRate, float hours) { float total; if (hours > REG_HOURS) total = (hours - REG_HOURS) * OVER_TIME * payRate + REG_HOURS * payRate; else total = hours * payRate; return total; } main() GrossPay() Function call Passing parameters Returning to main() With return value 14

IN Parameter The value of the actual parameter is passed into the function and assigned to the formal parameters. float GrossPay(float payRate, float hours); int main() {... cin >> rate >> hours; // rate: 12.5 // hours: 45.5 gross = GrossPay(rate, hours);... } 15

IN Parameters int Largest(int num1, int num2, int num3); cin >> score1 >> score2 >> score3; max = Largest(score1, score2, score3); void DisplayResult(float avg, float max, float min); // Input scores and compute the highest, // lowest and average DisplayResult(avg, highest, lowest); // The function does output, but IN parameters! float sqrt(float x); 16

Function Parameters In The value of the actual parameter is passed into the function and assigned to the formal parameter. Out The value of formal parameter is passed out of the function and assigned to the actual parameter. InOut Both In and Out. 17

float GrossPay(float payRate, float hours); int main() { float hours, rate, gross; // Input values in main() cin >> rate >> hours; gross = GrossPay(rate, hours); // display result return 0; } // Q: Can we use a function to input rate and hours? 18

Write a function to input rate and hours Function Prototype Name: GetInput Type: void Cannot pass two values using the return statement Parameters: rate (payRate), hours (hoursWorked) type: float (Passing values back to calling function) (OUT parameters!) (&) void GetInput(float& payRate, float& hoursWorked); 19

float GrossPay(float payRate, float hours); void GetInput(float& payRate, float& hoursWorked); int main() { float hours, rate, gross; // Call function GetInput() to get two values GetInput(rate, hours); // Call function GrossPay to get one value gross = GrossPay(rate, hours); // display result return 0; } 20

Function Definition // // The function inputs payRate and hoursWorked and // pass both values back to the calling function. // Parameters: (out, out) // void GetInput(float& payRate, float& hoursWorked) { cout << "Enter pay rate: "; cin >> payRate; cout << "Enter hours: "; cin >> hoursWorked; return; } // The function does input with prompt, but OUT parameters! // How can it pass two values back? // Out Parameters: & // Statement return can pass only one value back! 21

// // The function computes and returns the gross pay // based on the pay rate and hours. Hours over // 40 will be paid 1.5 times the regular pay rate. // Parameters: (in, in) // float GrossPay(float payRate, float hours) { if (hours > REG_HOURS) payRate = (hours - REG_HOURS) * OVER_TIME * payRate + REG_HOURS * payRate; else payRate = hours * payRate; return payRate; } // No local variable // payRate is used to store the result 22

Reference and Value Parameters float GrossPay(float payRate, float hours); void GetInput(float& payRate, float& hoursWorked); Value parameter: No & The value of actual parameter is passed to the formal parameter Reference Parameter: & The address of actual parameter is passed to the formal parameter Does the actual parameter change its value when the corresponding formal parameter changes its value? Value parameter (no &): NO Reference parameter (&): YES 23

Value and Reference Parameters const float REG_HOURS = 40.0; const float OVER_TIME = 1.5; int main() { float hours, rate, gross; GetInput(rate, hours); gross = GrossPay(rate, hours); // display result return 0; } float GrossPay(float payRate, float hours) { if (hours > REG_HOURS) payRate = (hours - REG_HOURS) * OVER_TIME * payRate + REG_HOURS * payRate; else payRate = hours * payRate; return payRate; } main() GetInput() void GetInput(float& payRate, float& hoursWorked) { cout << "Enter pay rate: "; cin >> payRate; cout << "Enter hours: "; cin >> hoursWorked; return; } GrossPay() Passing parameters Passing parameters Addresses of rate and hours Return control With value Return control 24

Tracing Functions Input: GetInput(rate, hours); // Reference parameters gross = GrossPay(rate, hours);// Value parameters main() GetInput() GrossPay() hours rate gross & payRate & hoursWorked hours payRate ? ? ? ? ? ? ? Addr of Addr of rate hours

Schedule Program 2 Grace Time: 10 PM, Friday Style! Name conversion Quiz 4-2 Due 10 pm Thursday (Lab5: next week) Program 3 –Optional groups –Two students each group –Sign group by Friday 26