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Conditional statements in C++

C++ has if-else and switch to determine whether statements would execute based on a given condition.

if statement

Use if to specify a block of code to be executed, if a specified condition is true. The syntax is as follows.

if (condition) {
    // statements
}

For example,

test-if.cpp
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#include <iostream>

using namespace std;

int main()
{
    int a, b;

    cout << "a is ";
    cin >> a;
    cout << "b is ";
    cin >> b;

    if (a > b)
    {
        cout << "a is greater than b" << endl;
    }

    return 0;
}

else statement

Use else to specify a block of code to be executed, if the same condition is false. The syntax is as follows.

if (condition)
{
    // statements
}
else
{
    // other statements
}

For example,

test-if-else.cpp
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#include <iostream>

using namespace std;

int main()
{
    int a, b;

    cout << "a is ";
    cin >> a;
    cout << "b is ";
    cin >> b;

    if (a > b)
    {
        cout << "a is greater than b" << endl;
    }
    else
    {
        cout << "a is no greater than b" << endl;
    }

    return 0;
}

else if statement

Use the else if statement to specify a new condition if the first condition is false. The syntax is as follows.

if (condition1)
{
    // statements for condition1
}
else if (condition2)
{
    // statements for condition2
}
else
{
    // other statements
}

For example,

test-if-elif-else.cpp
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#include <iostream>

using namespace std;

int main()
{
    int a, b;

    cout << "a is ";
    cin >> a;
    cout << "b is ";
    cin >> b;

    if (a > b)
    {
        cout << "a is greater than b" << endl;
    }
    else if (a == b)
    {
        cout << "a equals b" << endl;
    }
    else
    {
        cout << "a is less than b" << endl;
    }

    return 0;
}

Takeaway

Only if is required, else and else if are both optional.

Ternary Operator

variable = (condition) ? valueIfTrue: valueIfFalse

This is equivalent to

if (condition)
{
    variable = valueIfTrue;
}
else
{
    variable = valueIfFalse;
}

For example,

show-num-records.cpp
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#include <iostream>

using namespace std;

int main()
{
    int num;

    cout << "num is ";
    cin >> num;

    string be = (num > 1) ? "are " : "is ";
    char s = (num > 1) ? 's' : '\0';

    cout << "There " << be << num << " record" << s << "." << endl;

    return 0;
}

Assignment 4

Create a C++ program named bmi-calculator.cpp that prompts for weight in pounds and height in inches. It does not only calculates and displays the body mass index (BMI), but will also prompt the BMI classification.

FYI, BMI = (Weight in Pounds / (Height in inches x Height in inches)) x 703

The BMI classification is as follows.

BMI BMI Classification
18.5 or less Underweight
18.5 to 24.99 Normal Weight
25 to 29.99 Overweight
30 to 34.99 Obesity (Class 1)
35 to 39.99 Obesity (Class 2)
40 or greater Morbid Obesity
Sample Solution
bmi-calculator.cpp
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#include <iostream>

using namespace std;

int main()
{
    float weight, height;

    cout << "weight (in pound) is ";
    cin >> weight;
    cout << "height (in inch) is ";
    cin >> height;

    float bmi = weight / (height * height) * 703;

    string classification;

    if (bmi <= 18.5)
    {
        classification = "Underweight";
    }
    else if (bmi <= 25)
    {
        classification = "Normal Weight";
    }
    else if (bmi <= 30)
    {
        classification = "Overweight";
    }
    else if (bmi <= 35)
    {
        classification = "Obesity (Class 1)";
    }
    else if (bmi <= 40)
    {
        classification = "Obesity (Class 2)";
    }
    else
    {
        classification = "Morbid Obesity";
    }

    cout << "BMI is " << bmi << endl;
    cout << "BMI classification is " << classification << endl;

    return 0;
}

switch statement

In C++ we can use the switch statement to select one of many code blocks to be executed. The syntax is as follows.

switch(expression) {
    case value1:
        // code block 1
        break;
    case value2:
        // code block 2
        break;
    case value3:
        // code block 3
    default:
        // code block default
}
  • The expression is evaluated once.
  • The value of expression is then compared with values in each of case. When there's a match, the according code bock will execute.
  • The break allows us to break out of switch block and to stop further case testing. Usually, if there's a match, we can break afterwards. But there are exceptions.
  • The default specifies what to do if there's no match.

Here's a regular use case of switch.

month-num-to-name.cpp
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#include <iostream>

using namespace std;

int main()
{
    int month;

    cout << "month is ";
    cin >> month;

    switch (month)
    {
    case 1:
        cout << "Jan" << endl;
        break;
    case 2:
        cout << "Feb" << endl;
        break;
    case 3:
        cout << "Mar" << endl;
        break;
    case 4:
        cout << "Apr" << endl;
        break;
    case 5:
        cout << "May" << endl;
        break;
    case 6:
        cout << "Jun" << endl;
        break;
    case 7:
        cout << "Jul" << endl;
        break;
    case 8:
        cout << "Aug" << endl;
        break;
    case 9:
        cout << "Sept" << endl;
        break;
    case 10:
        cout << "Oct" << endl;
        break;
    case 11:
        cout << "Nov" << endl;
        break;
    case 12:
        cout << "Dec" << endl;
        break;

    default:
        cout << "Invalid" << endl;
        break;
    }

    return 0;
}

Here's a special usage of not haveing to break

month-num-to-season.cpp
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#include <iostream>

using namespace std;

int main()
{
    int month;

    cout << "month is ";
    cin >> month;

    switch (month)
    {
    case 12:
    case 1:
    case 2:
        cout << "Winter" << endl;
        break;
    case 3:
    case 4:
    case 5:
        cout << "Spring" << endl;
        break;
    case 6:
    case 7:
    case 8:
        cout << "Summer" << endl;
        break;
    case 9:
    case 10:
    case 11:
        cout << "Fall" << endl;
        break;

    default:
        cout << "Invalid" << endl;
        break;
    }

    return 0;
}

Assignment 5

Create a C++ program named grades.cpp that prompts for a grade in percentage and translates it to a letter grade.

By the way the letter grade assignment is as follow.

Percentage Letter Grade
90-100 A
80-89 B
70-79 C
60-69 D
< 60 F
Sample Solution
grades.cpp
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#include <iostream>

using namespace std;

int main()
{
    float grade;

    cout << "percentage grade is ";
    cin >> grade;

    if ((grade > 100) || (grade < 0))
    {
        cout << "Invalid" << endl;
    }
    else
    {
        int level = grade / 10;
        switch (level)
        {
        case 10:
        case 9:
            cout << "A" << endl;
            break;
        case 8:
            cout << "B" << endl;
            break;
        case 7:
            cout << "C" << endl;
            break;
        case 6:
            cout << "D" << endl;
            break;
        case 5:
        case 4:
        case 3:
        case 2:
        case 1:
        case 0:
            cout << "F" << endl;
            break;
        }
    }

    return 0;
}