Chapter 3: Control Flow: if-else, relational operators, boolean operators¶

Background¶

At this point in the course, the programs we have written have a very linear flow. There is a set of instructions we give the program for it to make it from point a to point b, and it does not veer off or go in different directions.

But in many cases, we nee d to control what the program does based on user input, or the state of a variable. We want to control the flow of our program as it runs to give us multiple options for routes it can take.

The if Statement¶

• The if statement is used to implement a decision in the program. It is made up of a condition and a body.
  • The condition of an if statement is made up of an expression that evaluates to TRUE or FALSE. It typically compares values using relational operators.
  • The body of an if statement is made up of either one statement or multiple statements. These statements will run ONLY if the condition evaluates to TRUE.

testing markdown file

C++ Syntax for if statements¶

if(condition)
    single statement body

if(condition) {
//multiple statements body
}

• If there is more than one statement in the body, you MUST use curly braces.

if.cpp

//Program Description: Example program for a simple if statement
#include <iostream>  
using namespace std;  
int main()  
{
    //initialize variables
    int num = 0;
    //prompt user for a value
    cout << "Please enter a number greater than zero:";
    cin >> num;//store user input into variable
    cout << "You entered: " << num << "\n";
    //this if statement will check if the number
    //entered is less than or equal to zero
    //if, and only if, it is true, the body of the if statement will run
    if(num <= 0)
    cout << "You did not listen to my instructions!\n";
    return 0;
}

Relational Operators¶

The condition of if statements typically compares values with one another.
To do this comparison, we use relational operators. |Operator Name|Operator|Example Expression| |-|:-:|:-:| |Equal To|==|a == b| |Not Equal To|!=|a != b| |Less Than|<|a < b| |Greater Than|>|a > b| |Less Than or Equal To|<=|a <= b| |Greater Than or Equal To|>=|a >= b|

These operators are used to compare the values between two expressions. The expression demonstrated can be
either TRUE or FALSE.

Example¶

int a = 10, b = 12, c = 5; |Expression|True or False?| |-|-| |a == b|False| |a+5 > b|True| |c % 2 == 0|False| |b+2<=14|True|

The if-else statement¶

• You can also pair an if with an else.
  • If the condition following the if is TRUE, the body of the if will execute.
    • If the condition following the if is FALSE, the body of the else will execute.
  • Only ONE body will run.
  • C++ Syntax for if-else statements

if(condition)
    //single statement body
else
    //single statement body
    ```
```c++
if(condition) {
    //multiple statements body
}
else{
    //multiple statement body
}```
- Note: an `else` does not have a condition associated with it. Its performance is based on  
whether the previous condition(s) evaluates to TRUE or FALSE
###### Example

```mermaid
graph TD;
    A([Start]):::Red-->B;
    B[Take user Input]:::Green-->C;
    C{{Is input greater than 0?}}:::Yellow--No-->D;
    C--Yes-->E;
    E{{Is Input divisible by 2}}:::Yellow--No-->F;
        E--Yes-->G;
    F[Output 'Odd..']:::Green-->H([End]):::Red;
    D[Output 'You did not listen...']:::Green-->H;
    G[Output 'Even..']:::Green-->H;
    classDef Red fill:#d05e56;
    classDef Yellow fill:#d8a027, color:#000;
    classDef Green fill:#448d7a;
    linkStyle default stroke-width:2px,fill:none, stroke:white

if_else.cpp

//Program Description: Example program for a simple if-else statement
#include <iostream>
using namespace std;

int main()
{
    //initialize variables
    int num = 0;
    cout << "Please enter a number greater than zero:";
    cin >> num;
    cout << "You entered: " << num << "\n";

    //this if statement will check if the number
    //entered is less than or equal to zero.
    if(num <= 0)
        cout << "You did not listed to my instructions!\n";
    //this else will only run if the above if evaluates to false
    else{
        //if the number is greater than zero, the following
        //if statement will check if it is divisible by 2
        if(num % 2 == 0) // check if num is even
            cout << num << " is even.\n";
        else
            cout << num << " is odd.\n";
    }
    //regardless of whether the if or the else above runs, the program will always end
    return 0;
}

The if(),else if()...else statement¶

• When writing a program that has more
  than 2 possible outcomes, we can use a
  sequence of if/else if/else if/else
  statements.
• When your program encounters an if-else
  branch like this, the conditions will be
  tested from top to bottom.
• As soon as one condition is true, it runs
  the statements within that if, and does not
  check any other conditions.
• If none of the conditions are true, it will
  run the body of the else.

Example¶

if(condition 1){
    //Body
}
else if(condition 2){
    //Body
}
else if(condition 3){
    //Body
}
...
else if(condition n){
    //Body
}
else{
    //Body will run if all else false
}

Example¶

if_else_if.cpp

//Program Description: Example program for a simple if-else statement
//that prints whether a number is negative, zero, or positive
#include <iostream>
using namespace std;

int main()
{
    //initialize variables
    int num = 0;
    //prompt the user for an integer
    cout << "Enter an integer:";
    cin >> num;

    //the program will check the following conditions from top to bottom
    //Once one evaluates to true, it will execute its body and ignore the remaining
    //ones. If none are true, the final else will run
    if (num == 0)
        cout << "You entered zero.\n";
    else if(num > 0)
        cout << "You entered a positive integer.\n";
    else 
        cout << "You entered a negative integer.\n";

    return 0;
}

Boolean/Logical Operators¶

• There may be times in our programs when running certain code is dependent on multiple conditions.
  • Sometimes all conditions need to be true.
  • Sometimes only one needs to be true.
• We can use Boolean operators to combine our conditions!
• Boolean operators combine conditions where the overall expression will result in either TRUE or FALSE.
  • && - AND operator
  • || - OR operator

&& - AND Operator: ALL must be TRUE¶

• Combines two or more conditions where the overall result is true if and only if ALL conditions are true.
• You can combine as many conditions as you want.
• C++ Syntax for using the AND operator
  • Can user either && or and

if(condition 1 && condition 2 &&...)
    ...

if(condition 1 and condition 2 and...)
    ...
}```

###### Logic Table for AND
|A|B|A && B|
|-|-|:-:|
|True|True|True && True = True|
|True|False|True && False = False|
|False|True|False && True = False|
|False|False|False && False = False|

# `||` - OR Operator: Just ONE needs to be TRUE
- Combines two or more conditions where the overall results is TRUE if AT LEAST ONE condition is true.
- You can combine as many conditions as you want.  
- C++ Syntax for using the OR operator  
    - Can user either `||` or `or`

```c++
if(condition 1 || condition 2 ||...)
    ...

if(condition 1 or condition 2 or...)
    ...
}```

###### Logic Table for AND
|A|B|A && B|
|-|-|:-:|
|True|True|True && True = True|
|True|False|True && False = True|
|False|True|False && True = True|
|False|False|False && False = False|

# Representing ranges with Boolean Operators
- You can use a combination of Boolean operators and conditions to make sure a value is within a range.
- Consider the following number line. Where the red arrow show the values that we want to represent. An enclosed box includes the value, and open box excludes the value.
###### Number Line 1:
```mermaid
flowchart LR  
    C[... -4]:::Yes---D
    D[-3]:::Yes---E
    E[-2]:::Yes---F
    F[-1]:::Num---G
    G[0]:::Num---H
    H[1]:::Yes---I
    I[2]:::Yes---J
    J[3]:::Yes---K
    K[4 ...]:::Yes
    classDef Num fill:#fff,color:#000,stroke-width:0px;
    classDef Yes fill:#f00,stroke-width:0px;
    classDef No fill:#0000,stroke-width:2px,stroke:#f00;
    linkStyle 0 stroke-width:37px,fill:none,stroke:red;
    linkStyle 1 stroke-width:37px,fill:none,stroke:red;
    linkStyle 5 stroke-width:37px,fill:none,stroke:red;
    linkStyle 6 stroke-width:37px,fill:none,stroke:red;
    linkStyle 7 stroke-width:37px,fill:none,stroke:red;
    linkStyle default stroke-width:37px,fill:none,stroke:white;

• Condition 1: if(x <= -2 || x >= 1)

Number Line 2:¶

• Condition 2: if(x < -2 || x >= 1)

Number Line 3:¶

• Condition 3: if(x > -2 || x <= 1)

Combination of Boolean Operators¶

You can use any combination of && and || in a condition¶

• The && operator takes precedence over ||

  • Examples:¶

    • if( TRUE || TRUE ) = TRUE
    • if (TRUE || FALSE && TRUE) = FALSE
    • if((TRUE || FALSE) && TRUE) = TRUE

Shipping Program Example¶

Write a program that calculates a shipping rate for a company based on the destination and weight of the package.
- If the package is sent outside the U.S, shipping is $70.00. - If the package is sent within the U.S and the weight is more than 10lbs, shipping is $25.00 - If the package is sent within the U.S and the weight is between 1-10lbs, shipping is $7.50 - If the package is sent within the U.S and the weight is less than 1 lb, shipping = $2.50. - If the package is sent within the U.S to Alaska (AK) or Hawaii (HI), the shipping rate is $40.00, regardless of weight.

/*
* Name: James Wibowo
* File Name: Shipping.cpp
* Program: This program will calculate shipping rates based on destination and weight.
* Date: 09/07/22
*/

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

int main()
{
    //init
    double packWeig;
    string packDest,packDestState;
    AllDest = 
    //input weight and destination
    cout << "Package Weight?\nLbs: ";
    cin >> packWeig;
    cout << "Package Country Destination? (Enter Two Letter Code): ";
    cin >> packDest;
    if (packDest == "US")
    {
        cout << "Package State Destination? (Enter Two Letter Code): ";
        cin >> packDestState;
        if (packDestState == "AK")
        {
            cout << "\n\nShipping Cost: $40.00";
            return 0
        }
        else if (packDestState == "HI")
        {
            cout << "\n\nShipping Cost: $40.00";
            return 0
        }
        else
        {
            if (packWeig < 1)
            {
                cout << "\n\nShipping Cost: $2.50";
                return 0;
            }
            else if (packWeig <= 10)
            {
                cout << "\n\nShipping Cost: $7.50";
                return 0
            }
            else
            {
                cout << "\n\nShipping Cost: $25.00";
                return 0
            }
        }
    }
    else
    {
        cout << "\n\nShipping Cost: $70.00";
        return 0
    }
    return 0;
}