Welcome to Lesson 14!

Learning Objectives
By the end of today's class, you should know...
  • What is a do-while loop?
  • How does the syntax of a do-while loop compare to that of a while loop?
  • When do you use a do-while loop instead of a while loop?
  • When a user enters a string rather than a number, how do you handle the user error?
  • What is the difference between cin.good() and cin.fail()?
  • What are cin.clear() and cin.ignore(1000,'\n');?
  • What is a string index?
  • How to iterate through a string using indexing.
  • How to use getline() to take in string input.
  • What is cin >> ws and when is it needed?


  • De Anza Women in Computer Science Club Meeting today at 12:30pm in ATC 203 Lab!
  • Last day to drop with a "W" is this Friday
    • Check your grade on Canvas
  • Midterm 2 one week from today
    • I will provide a practice quiz with answer key on Monday to help you to study.
    • Midterm will cover Lesson 8 - Lesson 15
  • Don't forget about Lab 7 for Friday - Login under "CodeLab3" to make sure you can see the solutions.

Do-While Loops

  • In some cases, it would be preferable to execute the body of a loop at least once before performing the loop test.
  • In these cases, we can use a do-while loop.
  • The structure of a do-while loop looks like this
    do {
    } while (test); // loop test
  • Where:
    • statements: the statements to execute the first time and repeatedly while the test remains true
    • test: the test condition to evaluate
  • The loop test is placed after the body and executes at the end of the loop
  • The statements in the body of the loop always execute at least once

Flowchart of do-while loop:

Flowchart of working of do...while loop in C programming.

Image source.

  • One common use of a do-while loop is to validate user input (i.e. check for errors in the user input)
  • The following code shows an example where we force the user to enter a positive value

Example do-while Loop Used to Validate Input


#include <iostream>
using namespace std;

int main() {
    double input;
    do {
        cout << "Enter a positive number: ";
        cin >> input;
    } while (input <= 0.0); // test condition at end
    cout << "You entered: " << input << endl;

    return 0;

When to Use do-while Statements

  • Use the do-while loop when you want to force a minimum of one iteration
  • Note that you can accomplish the same control flow with a while loop
  • However, in some cases we can save a statement by using a do-while loop

Group Activity
With a partner, answer the following questions:
  • How is the syntax of a do-while loop different from a while loop?
  • When would you use choose a do-while loop over a while loop?
  • Change the following while loop into a do-while loop:

int sum = 0;
int score = 0;

cout << "Enter a score or 0 to quit: ";

cin >> score;

while (score != 0)
    sum += score;
    cout << "Enter a score or 0 to quit: ";
    cin >> score;
cout << "The sum is: " << sum << endl;

Using Loops for Error Checking

Using Loops to Check for cin Failure

  • A problem arises if the user of a our programs enters a string when we expect a number
  • For instance, in the following, we get incorrect results if the user enters "seven"
    double input = 0.0;
    cin >> input;
  • The problem is that cin cannot convert the word "seven" into the number 7
  • When this happens cin fails, sets an error flag and skips the rest of the input operation.
  • You have probably seen this when you have mistakenly entered the wrong content in your program.
  • We can detect the failure condition using code like:
    while (cin.fail()) { 
        // the stream failed... code to handle failure here
//Keep prompting the user to enter valid input
        cout << "Please enter a number not a string: ";
        cin >> input;
  • As we saw when discussing uses of loops, It is good practice to use a loop in cases like these as the user might enter invalid input more than once.
  • To clear the failed state you need to both clear the flags and the input stream, like:
    cin.ignore(1000, '\n');
  • Using cin.clear() will reset the error flag but leaves the bad input in the input stream
  • Adding cin.ignore(1000, '\n'); will clear all the input a user enters until the user presses an enter key
  • For example:
while (cin.fail()) { 
        //clear the invalid input
        cin.clear(); //clear the error flag
        cin.ignore(1000, '\n'); //ignore the last 1000 characters or until \n

        // keep prompting user to enter a valid input until they get it right
        cout << "Please enter a number not a string: ";
        cin >> input;
  • Note that we could also use a do-while loop as in the next example.
  • Why would a do-while loop be useful here?
Example Program with Input Validation
#include <iostream>
using namespace std;

int main() {
    double input = 0.0;
    do {
        cout << "Enter a positive number: ";
        cin >> input;
        if (cin.fail()) {
            cout << "You must enter digits, not words\n";
            cin.ignore(1000, '\n');
            input = -1; // set loop test to fail
        } else if (input <= 0.0) {
            cout << "You must enter a positive number\n";
    } while (input <= 0.0);
    cout << "You entered: " << input << endl;

    return 0;
More information on cin functions

Activity 14.1: Scores (10 pts)

  • In this exercise we use indefinite loops to process user input and to ensure correct user input.
  • Find a partner for pair programming.
  • Copy the following program into an Eclipse project, save it as scores.cpp, and then compile and run the starter program to make sure you copied it correctly.
    #include <iostream>
    #include <climits>
    using namespace std;
    int main() {
        int sumScores = 0, nextScore = 0, count = 1;
        cout << "Enter a series of scores or -1 to quit\n\n";
  • Make sure the code compiles and runs.
  • Now we want to use a loop to enter a series of scores. Since we do not know how many scores to enter, we use an indefinite loop like the following:

  • In addition, add a statement to display sumScores after the loop.

    Compile your code to make sure you added the loop correctly. To exit the loop you will need to enter a negative number.

  • The loop includes statements to collect the sum of the scores in the variable named sumScores. Add a statement after the loop to print sumScores to the console. When you run the program after adding this code, the output should look like:
    Enter Score#1: 38
    Enter Score#2: 39
    Enter Score#3: -1
    Sum of scores: 77

    The loop uses the sumScores variable to accumulate scores during each repetition of the loop. 

  • We could write our indefinite loop using a do-while loop instead. Replace your current loop with the following:

  • Note that the statements inside the loop did not change, only the loop statement itself. To make sure you made the changes correctly, compile and run your code and check to see if it works the same. The difference between a while and do-while loop is that a do-while ensures the body of the loop is executed at least once. 

  • One problem with our program is the user can still enter letters instead of digits. We can prevent this error by checking cin.fail() and looping until the user enters a correct value. Replace the current if statement with the following:
  • When finished, upload your scores.cpp file to Canvas.

Using Loops with Strings

Review: Strings Versus Characters

  • Remember that a string is a series of characters enclosed in double quotes such as:
    "Hello"  "b"  "3.14159"  "$3.95"  "My address is 378 Eastbrook Dr"
  • We can store text in a variable of type string, like:
    string firstName;             // declaration
    firstName = "Jennifer";       // assignment
    string lastName = "Parrish";  // declaration + assignment
    string fullName = firstName + " " + lastName; // concatenation (+) of 2 stings
  • On the other hand, a character is a single letter, number or special symbol
  • We enclose characters in a single quote, rather than a double quote, like:
    'a'   'b'   'Z'   '3'   'q'   '$'   '*'
  • Also, we can store a a single character using a variable of type char, such as:
    char letterA = 'A';
    char letterB = 'B';
  • Each character is stored as a number, using its ASCII Table value
  • By declaring a char variable or using single quotes, C++ knows to treat the number as a character
  • Thus, when we print a character, we see a letter rather than a number:
    char letter = 'A';
    cout << letter << 'B' << endl;
  • As we can see, a string is made up of characters and characters are numerical codes
  • We can use this information to work with characters and strings

Indexing a String

  • Strings have a built-in indexing system with each stored in a character sequence starting at 0 (zero)

  • We can access any individual character of a string variable using square brackets [ ]
  • The general syntax is:
  • Where:
    • stringVariable: the name of your string variable
    • index: the number of the character position
  • For example:
    string str = "abcdef";
    char firstLetter = str[0];
    cout << firstLetter << str[1] << endl;
  • The above code displays:
  • Notice that the square bracket notation returns a char data type
  • What will the following print?
#include <iostream>
using namespace std;

int main() {
      string sport = "Basketball";
      cout << "The league: " << sport[1] << sport[7] << endl;

      cout << "Abbreviation for this sport: " << sport[0] << sport[6] << endl;

     return 0;


Using a For Loop to Iterate Strings
  • Recall that member function length() returns the number of characters in a string variable:
    string s = "abcdef";
    int n = s.length();
  • After we know the length, it is easy to iterate through the individual characters of a string using a counting loop:
    cout << "Enter a message: ";
    string msg;
    cin >> msg;
    for (int i = 0; i < msg.length(); i++) {
        cout << "Char[" << i << "]: " << msg[i] << endl;

Group Activity: Iterating Strings

  • Copy the following program into Eclipse, save it as test.cpp, and then compile and run the

  • string msg = "Hello, world!";
    for (int i = 0; i < msg.length(); i++) {
        cout << i << ": " << msg[i] << endl;
  • What do you see when you compile?

String Input With Spaces

  • We have been using the >> operator to enter data into a string variable:
    string something;
    cout << "Enter something: ";
    cin >> something;
    cout << "You entered: " << something << "END OF OUTPUT\n";
  • However, there are some complications
  • >> skips whitespace and stops on encountering more whitespace
  • Thus, we only get a single word for each input variable
  • If a user types in "Hello Mom!", we would only read "Hello" and not " Mom!"
  • This is because cin >> s1 works as follows:
    1. Skips whitespace
    2. Reads non-whitespace characters into the variable
    3. Stops reading when whitespace is found

Input Using getline()

  • To read an entire line we use function getline()
  • Syntax:
    getline(cin, stringVariable);
  • Where:
    • stringVariable: the name of the string variable
  • For example:
    string line;
    cout << "Enter a line of input:\n";
    getline(cin, line);
    cout << line << "END OF OUTPUT\n";
  • Note that getline() stops reading when it encounters a '\n'

Activity 14.2: How many words in your sentence? (10 pts)

  • Let's write a program that counts the number of words in a sentence input by the user.
  • Find a partner and open up a new C++ file called numWords.cpp.
  • Copy and paste the following starter code into your file:
#include <iostream>
using namespace std;

int main() {
      //Your statements go here

      return 0;
  • Add the appropriate block comments with your name and section information.
  • Next declare a string variable at the top of your program named sentence:
string sentence;
  • Welcome the user to the program with the following message:
Give me a sentence, and I will count the number of words.
  • Prompt the user to input a sentence and store the user input as the sentence variable.
Please enter your sentence:_
  • Don't forget to use getline() here.
  • How can we determine how many words are in a sentence?
  • We need to look at the whitespace.
  • Next, we will use a for loop to scroll through the sentence looking for blank spaces. 
  • Each time we encounter a new blank space, we will add one to our total for the number of words in the sentence.
  • Create a new variable at the top of main to store our counter for the number of words in the sentence. Assign it a value of 1. Why do we want to give it an initial value of 1 not 0?
int numWords = 1;
  • Now, create a for loop to iterate through the sentence. Don't forget to use the length() function here.
for (int i = 0; i < sentence.length(); i++) {
      cout << sentence[i] << endl;
  • Now, run your program and verify that you get the following output.

  • However, this is not the purpose of our program.
  • We want to count the number of words.
  • Remove the cout statement from your for loop.
  • Repalce the cout with an if statement to check the value of the the s[i] variable to determine if it is a blank space.

if (sentence[i] == ' ') 

  • When we encounter a blank space, we need to add one to the numWords variable.
  • Your for loop should now look like this:

for (int i = 0; i < sentence.length(); i++) {
    if (sentence[i] == ' ') {
  • Finally, outside of your for loop, add a statement to print out the number of words.
cout << "There are " << numWords << " words in \"" << sentence << "\"" << endl;

  • Run your program to verify it works correctly. Then, upload to Canvas.
  • Your program should now look like this:

Wrap Up

  • With your partner, answer the questions from today's learning objectives

Upcoming Assignments
  • Assignment 14 due Monday at 9:20am on Canvas
  • Lab 7 due Friday at midnight

~Have a Great Weekend!~