CSCI 235

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Do…While Loop and For Loops ​

Chapter 5

Do…While Loops ​

Do…while loops work just like a while loop but the body of the loop will be run at least once.

Syntax:

cpp
do
{
    //These things
}
while (guard statement); //<- notice the semicolon

INFO

To use INT_MAX as described in this video, include the climits hearder.

For Loops ​

This type of loop runs for a specific number of times. Use this type for all counter-controlled loops.

Syntax:

cpp
for (initial statement; loop condition; update statement)
{
      statement;
}

  • The initial statement, loop condition, and update statement are called for loop control statements

  • It is like a counter-controlled while loop, but you initialize, compare, and update the counter all on the first line.

Example: Count from 0 to 9:

cpp
for (int counter = 0; counter < 10; ++counter)
{
      cout << counter << " ";
}
cout << endl;

Big no-no: Don’t put a semicolon after the for loop:

cpp
for (int iter = 0; iter < 100; iter++); // <- no-no

This will compile, but it will do nothing 100 times and then go on to your intended looped instruction (running them only once).

Choosing the Right Looping Structure ​

All three loops have their place in C++

  1. If the loop must run at least once, use a do…while loop.
  2. Otherwise, if you know or can determine in advance the number of repetitions needed, the for loop is the correct choice (i.e., a counter-controlled loop).
  3. Otherwise, use a while loop.

break and continue Statements ​

The break and continue alter the flow of control of loops.

The break statement:

  • The break statement is used for two purposes:
    1. To exit early from a loop
      • Can eliminate the use of certain (flag) variables.
      • Generally, using a break in this way is bad design.
    2. To skip the remainder of a switch structure
  • After a break executes, the program continues with the first statement after the structure.

The continue statement:

  • continue is used in while, for, and do…while structures
  • When executed in a loop
  • It skips the remaining statements and proceeds with the next iteration of the loop.

Avoid using break and continue statements with loops. Generally, exiting interrupting the normal flow of execution is considered poor programming design. Usually, you can avoid break statements by updating the loop condition. You can avoid continue statements by using a condition (if or switch statement) inside the loop.

Nested Control Structures ​

  • To create the following pattern:

    *
**
***
****
*****

  • We can use the following code:

    cpp
    const int SIZE = 5;
for (row = 1; row <= SIZE; ++row)
{
    for (col = 1; col <= row; ++col)
        cout << "*";
    cout << endl;
}

What is the result if we replace the first for statement with this?

cpp
for (row = SIZE; row >= 1; --row)

Self-Check Questions ​

  1. What is the difference between a while and a do…while?
  2. What is the syntax for a while statement?
  3. What it is an infinite loop?
  4. Compare and contrast while and for loops.
  5. Know the syntax of a for loop.