Loop Design with While Loops

Chapter 5

Why Is Repetition Needed?

• Repetition allows efficient use of variables.
• We can input, add, and average multiple numbers using a limited number of variables and lines of code.
• For example, to add five numbers:
  • Declare a variable for each number, input the numbers and add the variables together.
  • Create a loop that reads a number into a variable and adds it to a variable that contains the sum of the numbers.

While Loops

Note: One example in this video is summing the numbers from 1 to 10, in which the program outputs the result of 45. However, the actual sum of the numbers from 1 to 10 is 55! See if you can find the bug in the example code.

• The syntax of the while statement is very similar to the if statement:

  while (expression)
  {
      statement;
  }

• The expression acts as a decision-maker and is evaluated as a logical expression.

• The statement can be simple or compound (i.e., multiple statements surrounded by {}).

• The statement is called the body of the loop.

• The parentheses are part of the syntax.

There are a few ways to use a while loop, which we will discuss below.

Loop Design

There are counter-, sentinel-, and flag-controlled loops.

Counter-Controlled While Loop

Here is an example where you do stuff 15 times.

int counter = 0; // set a counter to 0
while (counter < 15) // repeat the while counter is less than 15.
{
    //  Do stuff here...
    counter++; // update by one each time the loop iterates
}

In this example, counter is called the loop-control variable.

Common mistakes that cause infinite loops (loops that continue to execute endlessly):

• Make sure to include a statement in the loop body that will eventually make the test condition false.

  • Make sure to update your loop-control variable inside the loop, so the loop will exit.
  • If you have an infinite loop, press Ctrl+C to exit your program.

• Make sure you initialize the counter before the while loop or the value may be incorrect.

• Don’t put a semicolon after the test condition.

  while (counter < 15); // Infinite loop, because of the ; placement

Sentinel-Controlled While Loop

A sentinel value is used to determine if the loop should end.

Usually, this is given to you by the user (from a file).

int num = 0;
cout << "Enter numbers to add until you are finished"
    << ", them input -1" << endl;
cin >> num;

while (num != -1) // -1 is the sentinel value causing the program to exit.
{
    num += num;
    cin >> num;
}

Flag–Controlled While Loop (very similar to sentinel)

If you are looking for something and can quit once you find it, you can set a Boolean value to false and when you find what you are looking for, you set the value to true, and the loop exits.

Here is some sample code:

bool done = false;

while (!done) // done is the flag variable. (!done is preferred over done == false)
{
    //Do something...;
    if (found what you wanted to)
    {
        done = true;
    }
    //Do something else...;
}

EOF-Controlled While Loop

This is the data file from the video, data.txt

43	64	28	81	71	57	52	36	72	24
76	11	86	89	50	54	63	1	15	71
42	88	33	26	99	29	37	97	73	19
26	52	49	50	53	76	49	62	26	94
1	55	29	98	1	39	6	80	37	4
10	39	10	94	49	13	44	36	28	50
1	10	84	22	78	87	76	41	15	2
54	43	94	45	8	46	24	72	22	82
20	55	80	49	73	72	33	45	16	34
27	96	41	37	33	66	79	67	82	25

Often, we want to read a file until we are at the end of a file (EOF). If you are importing from a file, there is a function called eof(), which returns true if you have reached the end of the file, and false otherwise. Alternatively, the input stream variable itself will return true if (1) you are not at end of the file and (2) you do not have an input error.

ifstream inFile; // declare the input stream variable
string word; // variable to hold input

inFile.open("test.txt"); // open the file

inFile >> word; // read first value (if there is a value to read)
while (inFile) // make sure the stream is good (no errors, not at EOF).
{
    ... // do something with word.
    inFile >> word; // update the loop control variable
}
inFile.close();// close the file

If you use eof(), make sure your input file does not have a blank line of space after the last value.

Other Expressions

The expression in a while statement can be complex. For example,

while ((noOfGuesses < 5) && (!isGuessed))
{
    . . .
}