Chapter 3: Loops, loops, and more loops¶

What this lecture will cover...¶

Loops¶

• What they are
• Different parts of a loop
• Different types of loops
  • while loop
  • for loop
  • do-while loop
• Nested loops
• How to use loops to process user input

Background¶

With most programs we write, we will want to repeat a specific
action/task/chunk of code more than once.

Examples:¶

- Processing user input.  
- ATMs: The menu loops until the user selects the option to quit.

• Loops in programming allow us to repeat a chunk of code as many times
  as we want without having to retype the code over and over again.
• C++ has the following loops available:
  • while
  • for
  • do-while

General Format for Loops¶

• Regardless of the loop type, there are important “parts” of a loop that you should keep in mind
  to make sure it works properly.
• Initialization Statement
  • The initialization statement will initialize the variable that will control the loop, or the loop control variable (LCV). This variable will be the main determinant of how many times the loop will execute and will typically be part of the loop condition.
• Loop Condition
  • For while and for loops, this condition must evaluate to TRUE in order for the loop to begin and continue. (do-while is a little different).
  • The LCV value is typically checked here.
  • Once this condition evaluates to FALSE, the program breaks out of the loop and proceeds to the statement directly after the end of the loop.
  • The condition can be made up of a Boolean expression, Boolean variables, integers, cin statements, etc.
• Body
  • Contains the statements/code that you wish to repeat while the loop runs.
  • The loop body is able to use variables that are declared outside of the loop, and you can declare new variables inside the loop. Loop variables will only exist while the loop is running.
• Update Statement
  • At some point in your loop, you need to make sure to update the LCV so that eventually your loop will end.
  • Without this, you risk having an infinite loop.

while Loop¶

The while loop executes a block of code repeatedly WHILE its condition
continues to evaluate to TRUE.

C++ Syntax¶

initialization statement;
while(condition){
    //Body of loop
    //Update statement;
}

Example: Write a program to display “Hello” 100 times¶

/*
 * The following program will use a while loop to display 
 * "Hello" 100 times
 */
#include <iostream>
using namespace std;

int main()
{
    //initialization statement
    int count = 0; // 1) Count set to 0
    //the beginning of the while loop
    while(count < 100) // 2) The condition is checked, if true, loop starts
    {
        //the body of the loop
        cout << "\tHello\n";
        count++; // 3) This code runs, count increased by 1
    } // 4) go back to 2)
    cout << "Goodbye!\n"; // 5) as soon as condition is false, will go to line directly after loop
    return 0;
}

Example: Write a program that prompts the user for 5 positive integers, and prints the sum.¶

/*
 * This program prompts the user for 5 positive integers and prints 
 * their sum
 */
#include <iostream>
using namespace std;

int main()
{
    //initialize variables
    int sum = 0, num = 0, count = 0;
    //start of while loop
    while(count < 5){
        //body of the loop that will repeat
        cout << "Enter a positive number:";
        cin >> num;
        //check that it is positive
        if(num<=0)
            cout << "The integer must be greater than zero!\n";
        else{
            //positive int
            sum += num;//same as sum = sum + num
            count++; //increase count by 1
        }
    }//end of while
    cout << "The sum of the 5 integers is: " << sum << "\n";
    return 0;
}

Using cin.fail(), cin.clear() & cin.ignore()¶

The reason our loop goes into an infinite loop is because when we enter a character or
string when cin expects an integer, it goes into a “failed state”.
- What is cin>> ? - cin is actually an input stream OBJECT.
- In Chapter 5 will learn that objects have different values associated with them, and in this case cin has a value that represents its “state”. - When it is in a “good” state, it allows for user input. - When it is in a “bad” or “failed” state, it does not allow input. - Our loop runs infinitely because cin remains in a failed state, and does not allow for a new value to be input, which means the LCV “count” is never updated. - We can fix this with: - Check if cin is in a failed state with cin.fail(), if it returns “true”, we know it’s no bueno.
- cin.clear(); // resets the state of cin back to “good” - cin.ignore(1000,’\n’); // will flush any unwanted data from the input stream

Fixed Example!¶

/*
 * This program prompts the user for 5 positive integers and prints 
 * their sum. It fixes the problem of an infitine loop when a non-int is entered
 */
#include <iostream>
using namespace std;

int main()
{
    //initialize variables
    int sum = 0, num = 0, count = 0;
    //start of while loop
    while(count < 5){
        //body of the loop that will repeat
        cout << "Enter a positive number:";
        cin >> num;
        //check if cin has gone into a failed state
        //cin.fail() will return TRUE if it is
        if(cin.fail()){
            cout << "Invalid entry\n";
            cin.clear();//reset state to "good"
            cin.ignore(1000,'\n');//clear the stream
        }
        //if cin is NOT in a failed state, then a number was entered
        //and we can proceed as usual
        else{
            //check that it is positive
            if(num<=0)
                cout << "The integer must be greater than zero!\n";
            else{
                //we will only get here if it is a valid, positive int
                sum += num;//same as sum = sum + num
                count++; //increase count by 1
            }//end of inner else
        }//end of outer else
    }//end of while
    cout << "The sum of the 5 integers is: " << sum << "\n";
    return 0;
}

Processing Inputs with while Loops¶

• At this point when taking user input, there is a set number of inputs we
  are accepting from the user.
• There are cases when we want the user to enter as many values as they
  want and let us know when they are done.
• We need a way to know the end of input has been reached.
• Some methods:
  • Use a specific value to represent end of input aka a sentinel, and break statements
  • Check for valid input/state of cin
  • Use a Boolean variable in the condition
  • Advanced Topic (I will go over this Monday if we have time): Taking all in as string, converting to decimal, checking for valid value, catching exceptions (exception handling).

Using a sentinel and break¶

• A sentinel is a value used to determine termination of input.
• The program explicitly tells the user what value to enter if
  they are done with input and the program checks for this
  value.
• The program then uses a break statement to stop the loop

Sentinel Example¶

You can check for a sentinel in the body of the loop, and then use a break statement to end the loop.
break: - When a break statement is encountered inside a loop, the loop immediately ends and the program continues to the statement directly after the loop.

/*
 * This program uses a sentinel to process user input
 * Will allow user to enter any amount of positive numbers
 */
#include <iostream>
using namespace std;

int main()
{
    //variable declarations
    double sum = 0, num = 0;
    int count = 0;
    //the sentinel here is -1
    cout << "Enter positive numbers to sum, enter -1 to quit.\n";
    //while loop that will run as long as the user doesn't enter -1
    while(1){
        cout << "Enter a positive number:";
        cin >> num;
        //check if cin has gone into a failed state
        if(cin.fail()){
            cout << "Invalid entry. Enter a positive number.\n";
            cin.clear();
            cin.ignore(1000,'\n');
        }
        else if(num == -1)
            break;
        else if(num <= 0)
            cout << "The integer must be greater than zero!\n";
        else{
            sum+=num;
            count++;//just used to keep track of the number of inputs
        }
    }//end of while loop
    cout << "The sum of the " << count << " number(s) you entered is: " << sum << "\n";
    return 0;
}

Checking for valid input with cin¶

• Recall that cin is an input stream and has a state associated with it.
• Let’s see how we can use this to our advantage when we are taking in numerical values.

Using cin>> in the condition¶

• We can check the state of cin in the condition of the while
  loop.
• We can tell the user to enter a string if they are done
  entering values.
• Pros: You can sum ANY numbers, not limited to just a
  specific range.
• Cons: Would end input if they
  enter any string.
  • We’ll see how to fix this on Monday

/*
 * This program checks cin as it processes user input
 */
#include <iostream>
using namespace std;

int main()
{
    //variable declarations
    double sum = 0, num = 0;
    cout << "Enter numbers to sum, Enter \"Q\" to quit.\n";
    cout << "Enter #:";
    //while loop that will run as long as the user doesn't enter Q
    //as soon as a non-numerical value is entered, cin goes into a failed state
    while(cin>>num){
        //the body of the loop will only run while cin is in a good state
        sum+= num;
        cout << "Enter #:";
    }//end of while loop

    cout << "The sum of the numbers is: " << sum << "\n";
    return 0;
}

Using a Boolean Variable in the Condition¶

• Boolean data type:
  • A data type whose value can only be equal to TRUE (1 or anything other than zero), or FALSE (0)

Syntax:¶

- bool variableName = true, false or 0, 1;

Using a Boolean in the condition¶

• While the boolean variable is true, the loop will run.
• You change the value of the boolean variable somewhere inside the loop.
• Similarly to using cin>>, this program would stop for ANY non-numerical value.

/*
 * Program that uses a Boolean variable in the condition 
 * to control the loop
 */ 

#include <iostream>
using namespace std;

int main()
{
    //declare variables
    double sum = 0, num = 0;
    bool more = true;
    //prompt the user for values
    cout << "Enter #s to sum, enter \"Q\" to quit.\n";
    //the value of more will be either true or false
    //As long as it is true, the loop will run
    while(more){
        cout << "#: ";
        cin >> num;
        if(cin.fail())
            more = false;
        else
            sum+=num;
    }//end of while
    cout << "The sum of those numbers is: " << sum << "\n";
    return 0;
}

The while loop can be considered the most versatile and used loop, but there’s more!¶

for loop¶

A for loop is ideal to use when you are dealing with a value that runs from a starting point to and ending point with a constant increment or decrement.
Use a for loop when your program has enough information to know how many times the loop needs to run.

C++ syntax of for loops¶

    for(initialization statement; condition; update statement) {
        //Body of loop
    }

Works in the following way:
1. Initialization statement is executed a single time when the loop is first encountered.
2. The condition is checked
a. If it is TRUE, continue to step 3
b. If it is FALSE, continue to step 5
3. The body of the loop runs. As usual it can be a single statement or a block{}
4. At the end of each loop, the update statement runs. Return to Step 2.
5. The loop ends, execution of the program continues to the statements directly after the loop.

Example: Write a program to display “Hello” 100 times while vs for¶

#include <iostream>
using namespace std;

int main()
{
    //initialization statement
    int count = 0;
    //the beginning of the while loop
    while(count < 100)
    {
        //the body of the loop
        cout << "This is Hello #" << count + 1 << "\n";
        count++;
    }
    cout << "Goodbye!\n";
    return 0;
}

#include <iostream>
using namespace std;

int main()
{
    //the beginning of the for loop
    for(int count = 0; count < 100; count++)
    {
        //the body of the loop
        cout << "This is Hello #" << count + 1 << "\n";
    }
    cout << "Goodbye!\n";
    return 0;
}

Notes:
- The update statement of a for loop can increment the LCV by any value, but increasing it by 1 is very common so the ++ operator can do that for us.
- For the while loop, count exists outside the scope of the loop, but for the for, count only exists within the scope of the loop.

for loops - ranges (different from range based for loops)¶

When you have a known range of numbers that you want a loop to iterate through (a known start and end value with a constant change between each iteration), you can use the following format:

for(int i = start; i < end;  
i++){  
}

/*
 * Program that uses a for loop to display odd numbers within a range
 */ 
#include <iostream>
using namespace std;

int main()
{
    //prompt user for start and end point
    int start = 0, end = 0;
    cout << "Enter a starting point:";
    cin >> start;
    cout << "Enter an ending point:";
    cin >> end;
    //display the odd numbers within the range
    cout << "The odd numbers between " << start << " and " << end << " are:";
    //veginning of for loop that will display all the odd numbers
    for(int i = start; i <= end; i++){
        if(i % 2 == 1)
            cout << i << " ";
    }

    cout << "\n";
    return 0;
}

Output:¶

Enter a starting point:20 Enter an ending point:30 The odd numbers between 20 and 30 are:_21 23 25 27 29

Range Based for loops¶

• Execute a for loop over a certain range

Syntax:¶

for(range_declaration:range_expression) {
    //body
}  

• range_declaration:
  • Declaration of a variable whose type is the same as the sequential elements in range_expression.
• range_expression:
  • Any expression that represents a sequence of values or a brace initialized list. The datatype of the values should be the same as range_declaration.

Examples of range-based for loops¶

#including <iostream>
using namespace std;

int main()
{
    //range based for loops with a
    //brace initialized list
    for (int n: {2,8,10,15,25}) {
        //n will be set equal to each
        //of the values in the list
        cout << n << " ";
    }
    cout << "\n";
    return 0;
}

Output¶

2 8 10 15 25

#include <iostream>
#include <cctype> //toupper()
using namespace std;

int main()
{
    //declare a string
    string cat = "Obi-Wan Katnobi";
    cout << "My cat's name is |";
    //range based for loop that will traverse a string
    //recall that strings are arrays of characters
    for (char c: cat){
        cout << (char)toupper(c) << "|";
    }
    cout << "\n";
    return 0;
}

Output¶

My cat's name is |O|B|I|-|W|A|N| |K|A|T|N|O|B|I|

do-while loop¶

A do-while loop is a loop whose body will run once BEFORE the condition is checked. The condition must be TRUE for the loop to continue.

C++ Syntax¶

initialization statement;
do{
    //body of loop
    //update statement
}
while(condition);

Works in the following ways: 1. Initialization statement runs 2. Run the body of the loop 3. AFTER the first iteration, the condition is checked 4. If the condition is true, return to the top of the loop and repeat.

When would we use such a thing? When you want to ensure the body of the loop runs AT LEAST once. Commonly used for menu-driven programs (ATM).

Example¶

Program that repeatedly displays a menu to the user and asks for their input until they enter Q to quit.

/*
 * Program that uses a do-while loop to display a menu
 */
#include <iostream>
using namespace std;

int main()
{
    //variable for user input
    int input = 0;
    //the body of the loop will run no matter what
    do{
        cout << "Enter 1 for Message 1\n"
             << "Enter 2 for Message 2\n"
             << "Enter 3 for Message 3\n"
             << "Enter 0 to quit\n";
        cout << "Input:";
        cin >> input;
        //check user input
        if(input == 1)
            cout << "\tMessage 1: Greetings!\n";
        else if(input == 2)
            cout << "\tMessage 2: I love cats!\n";
        else if (input == 3)
            cout << "\tMessage 3: I also love dogs!\n";
        else if (input == 0)
            cout << "\tGoodbye!\n";
        else
            cout << "\tInvalid Input. Try again.\n";
        //we now check the condition
    }while(input !=0);

    return 0;
}

Nested Loops¶

You can have a loop inside of a loop (any combo of loops)

Example¶

1 | int loopCount = 0;
2 | for(int i = 0; i < 2; i++) {
3 |     cout << "Hello from the first loop";
4 |     for(int j = 10; j < 16; j+=2){
5 |         cout << "\tHello from the second loop\n";
6 |         loopCount++;
7 |     }
8 |     loopCount++;
9 |     cout << "\n";
10| }

1) Lets call the loop at lines (1-3,8-10), Loop 1. And the loop at lines (4-7), Loop 2. Notice that Loop 2 is inside of Loop 1. 2) Loop 1's statement runs, then the condition is checked, if it is true, we go into the body of the loop. 3) The cout statement inside Loop 1. is executed. 4) We now reach Loop 2. Loop 2's initialization statement runs, then the condition is checked. If it is true, we go into the body of Loop 2. 5) The cout statement inside of Loop 2 is executed. LoopCount is incremented by 1. 6) At the end of Loop 2, the j+=2 update statement is executed. 7) Loop 2's condition is checked again, if it is true, we go back to step 5. Repeat until Loop 2's condition is false. 8) Once Loop 2's condition is false, the second cout statement of Loop 1 is executed and loopCount is incremented by 1 9) At the end of Loop 1, the i++ statement Is executed. 10) Loop 1's condition is checked again. If it is true, we go back to step 3. 11) Repeat steps 3-10 until Loop condition is false.

So when should you use one loop over another?¶