Math2220 Lecture 7

Type Casting

A type cast is a way of changing a value of one type to the value of another type.

Explicit type casting

3 methods

type(expression)
(type)expression
static_cast<type>(expression)

Examples:
char(75);
(double)3;
static_cast<double>(3);

Example

#include<iostream>
using namespace std;

int main() {
    int a, b;
    cin >> a >> b;
    cout << double(a)/b << endl;

    char line[100];
    cin.getline(line,100);  
    cin.getline(line,100); 
}

Output

4 7
0.571429

Question: What happens if you don't use type casting?

Inplicit type casting

ints are type cast as doubles when used with other doubles.
char and bool can be typed cast as int. Similarly, int can be type cast as char or int.

Example

#include<iostream>
using namespace std;

int main() {
    bool b = 3;
    cout << b << endl; // 1

    char ch = 72;
    cout << ch << endl; // H

    ch='A';
    cout << int(ch) << endl; // 65, check ascii table

    int num = 99;
    cout << char(num) << endl; // c, check ascii table

    char line[100];
    cin.getline(line,100); 
}

Sample output


1
H
65
c

The ascii value of H is 72. Refer to ASCII table at wikipedia.

Function

Question

Input : user's midterm and final score
Output : user's grade.
Final score is calculated by the maximum of 50% midterm + 50% final and 40% midterm + 60% final.
Grade is calculated by : A : 90-100, B : 80-90, C: 65-80, D: 50-65, F : under 50.

Example

#include<iostream>
using namespace std;

void main() {
    double midterm, exam; // score for midterm and final exam
    double final; // final result
    cout << "Please enter your midterm and final score: ";
    cin >> midterm >> exam;
    
    double result1 = 0.5*midterm + 0.5*exam;
    double result2 = 0.4*midterm + 0.6*exam;
    // the final score is the max of result1 and result2
    if(result1 >= result2) 
       final = result1;
    else 
       final = result2;
    
    if (final >= 90) {
        cout << "Your grade is A.\n";
    } else if (final >= 80) {
        cout << "Your grade is B.\n";
    } else if (final >= 65) {
        cout << "Your grade is C.\n";
    } else if(final >= 50) {
        cout << "Your grade is D.\n";
    } else {
        cout << "Your grade is F.\n";
    }

    char line[100];
    cin.getline(line,100); 
    cin.getline(line,100); 
}

Sample output


Please enter your midterm and final score: 89.5 99.2
Your grade is A.

A better approach

Break down the problem into subtasks:

calculate the final score
calculate the grade

Example

#include<iostream>
using namespace std;

double finalScore(double midterm, double exam);
char grade(double score);

void main() {
    double midterm, exam; // score for midterm and final exam
    double final; // final result
    cout << "Please enter your midterm and final score: ";
    cin >> midterm >> exam;
    
    final = finalScore(midterm, exam);

    
    cout << "Your grade is " << grade(final) << ".\n";

    char line[100];
    cin.getline(line,100); 
    cin.getline(line,100); 
}


// input: scores for midterm, exam
// return: final score
double finalScore(double midterm, double exam) {
    double result1 = 0.5*midterm + 0.5*exam;
    double result2 = 0.4*midterm + 0.6*exam;
    // the final score is the max of result1 and result2
    if(result1 >= result2) 
        return result1;
    else 
        return result2;
}

// input: score
// output: grade
char grade(double score) {
    if (score >= 90) {
        return 'A';
    } else if (score >= 80) {
        return 'B';
    } else if (score >= 65) {
        return 'C';
    } else if(score >= 50) {
        return 'D';
    } else {
        return 'F';
    }
}

Sample output


Please enter your midterm and final score: 75.3 88.1
Your grade is B.

The program will be explained in more details during the class.

Predefined functions

Question

1) Find the square root of 2.

Example

#include<iostream>
#include<cmath>
using namespace std;

void  main() {
     double x=2;
     cout << "The square root of " << x << " is " << sqrt(x) << endl;
        
    char line[100];
    cin.getline(line,100);

}

Output

The square root of 2 is 1.41421

2) Find 2.25 to the power 3. (i.e. the cube of 2.25)

Example

#include<iostream>
#include<cmath>

using namespace std;

void main() {
    cout << "2.25 to the power 3 is " << pow(2.25,3)<< endl;

    char line[100];
    cin.getline(line,100);
}

Output

2.25 to the power 3 is 11.3906

Pay attention to sqrt(2) and pow(2.25,3). They are all functions. sqrt(2)returns the square root of 2. pow(2.25,3) returns the cube of 2.25.

ring

Parts of a function

In order to use a function, you should know

Name
Arguments: what gets plugged in
Value returned
Where does it locate: which library(cmath? iostream?).
Definition: algorithm to determine value returned.

Example :`pow`

Name: pow
Arguments: two double
Value returned: double
library : <cmath>. That is why we have #include<cmath> at the beginning of the program.
Definition: pow(x,y) return x to the power y. The order of the arguments are important!

How to use the function?

Use the directive #include to include the header, e.g. #include<cmath>
Know the number and types of argument
Know what to expect as value returned
Function call.

List of math functions

sqrt, pow, log, log10, abs, exp, sin, cos, tan, , acos, asin, atan, cosh, sinh, tanh, labs, fabs, ceil, floor
labs is the absolute value function of a long
fabs is the absolute value function of a float
floor: the largest integer integer smaller than or equal to the parameter.
ceil: the smallest integer bigger than or equal to the parameter.
Check cmath (math.h) - C++ Reference

Example

#include<iostream>
#include<cmath>

using namespace std;

void main() {
    const double PI=3.14159265358;
    cout << "PI is " << PI << ".\n";
    cout << "log(7.2) is " << log(7.2) << ".\n";
    cout << "cos(PI/6) is " << cos(PI/6) << ".\n";
    cout << "sin(10.5) is " << sin(10.5) << ".\n";
    cout << "e to the power 5 " << exp(double(5)) << ".\n"; // don't forget type casting
    cout << "Cubic root of 10 is " << pow(10, 1./3.) << ".\n";
    cout << "The integer part of 3.2 is " << floor(3.2) << ".\n";
    cout << "The integer part of -3.2 is " << floor(-3.2) << ".\n";
    cout << "The absolute value of -3.8 is " << abs(-3.8) << ".\n";
    
    char line[100];
    cin.getline(line,100);
}

Output


PI is 3.14159.
log(7.2) is 1.97408.
cos(PI/6) is 0.866025.
sin(10.5) is -0.879696.
e to the power 5 148.413.
Cubic root of 10 is 2.15443.
The integer part of 3.2 is 3.
The integer part of -3.2 is -4.
The absolute value of -3.8 is 3.8.

Math2220 Lecture 7

Type Casting

Explicit type casting

Inplicit type casting

Function

Question

A better approach

Predefined functions

Question

Parts of a function

Example :pow

How to use the function?

List of math functions

Example :`pow`