Section 2.1 Control Structures.

A program is usually not limited to a linear sequence of instructions. During its process it may bifurcate, repeat code or take decisions. For that C++ provides to us control structures that serve to specify what and how has to perform our program.

With the introduction of control sequences we are going to have to introduce a new concept: the block of instructions. It is a group of instructions separated by semicolons (;) but grouped in a block delimited by key bracket signs: { and }.

Most of the control structures that we will see in this section allow a generic statement as parameter, this refers to either a single instruction or a block of instructions, as we want. If we want the statement to be a single instruction we do not need to enclose it between key-brackets ({}). If we want the statement to be more than a single instruction we must enclose them between key-brackets ({}) forming a block of instructions.

Conditional structure: if and else

if (condition) statement

For example, the following code fragment prints out x is 100 only if the value stored in variable x is indeed 100:

if (x == 100)
  cout << "x is 100";

if (x == 100)
 {
  cout << "x is ";
  cout << x;
 }

We can additionally specify what we want that happens if the condition is not fulfilled by using the keyword else. Its form used in conjunction with if is:

if (condition) statement1 else statement2

if (x == 100)
  cout << "x is 100";
else
  cout << "x is not 100";

The if + else structures can be concatenated with the intention of verifying a range of values. The following example shows its use telling if the present value stored in x is positive, negative or none of the previous, that is to say, equal to zero.

if (x > 0)
  cout << "x is positive";
else if (x < 0)
  cout << "x is negative";
else
  cout << "x is 0";

Repetitive structures or loops

The while loop.

Its format is:

while (expression) statement

and its function is simply to repeat statement while expression is true.

For example, we are going to make a program to count down using a while loop:

// custom countdown using while #include <iostream.h> int main () { int n; cout << "Enter the starting number > "; cin >> n; while (n>0) { cout << n << ", "; --n; } cout << "FIRE!"; return 0; }

Enter the starting number > 8 8, 7, 6, 5, 4, 3, 2, 1, FIRE!

When the program starts the user is prompted to insert a starting number for the countdown. Then the while loop begins, if the value entered by the user fulfills the condition n>0 (that n be greater than 0 ), the block of instructions that follows will execute indefinite times while the condition (n>0) remains being true.

All the process in the program above can be interpreted according to the following script: beginning in main:

• 1. User assigns a value to n.
• 2. The while instruction checks if (n>0). At this point there are two possibilities:
  • true: execute statement (step 3,)
  • false: jump statement. The program follows in step 5..
• 3. Execute statement:
  cout << n << ", ";
  --n;
  (prints out n on screen and decreases n by 1).
• 4. End of block. Return Automatically to step 2.
• 5. Continue the program after the block: print out FIRE! and end of program.

We must consider that the loop has to end at some point, therefore, within the block of instructions (loop's statement) we must provide some method that forces condition to become false in some moment, otherwise the loop will continue looping forever. In this case we have included --n; that causes the condition to become false after some loop repetitions: when n becomes 0, that is where our countdown ends.

Of course this is a so simple action for our computer that the whole countdown is instantly performed without practical delay between numbers.

The do-while loop.

Format:

do statement while (condition);

Its functionality is exactly the same as the while loop except that condition in the do-while is evaluated after the execution of statement instead of before, granting at least one execution of statement even if condition is never fulfilled. For example, the following program echoes any number you enter until you enter 0.

// number echoer #include <iostream.h> int main () { unsigned long n; do { cout << "Enter number (0 to end): "; cin >> n; cout << "You entered: " << n << "\n"; } while (n != 0); return 0; }

Enter number (0 to end): 12345 You entered: 12345 Enter number (0 to end): 160277 You entered: 160277 Enter number (0 to end): 0 You entered: 0

The do-while loop is usually used when the condition that has to determine its end is determined within the loop statement, like in the previous case, where the user input within the block of intructions is what determines the end of the loop. If you never enter the 0 value in the previous example the loop will never end.

The for loop.

Its format is:

for (initialization; condition; increase) statement;

and its main function is to repeat statement while condition remains true, like in the while loop. But in addition, for provides places to specify an initialization instrauction and an increase instruction. So this loop is specially designed to perform a repetitive action with a counter.

It works in the following way:

1, initialization is executed. Generally it is a initial value setting for a counter varible. This is executed only once.
2, condition is checked, if it is true the loop continues, otherwise the loop finishes and statement is skipped.
3, statement is executed. It can be a single instruction or a block of instructions enclosed within key brackets { }.
4, finally, whatever is specified in the increase field is executed and the loop gets back to step 2.

Here is an example of countdown using a for loop.

// countdown using a for loop #include <iostream.h> int main () { for (int n=10; n>0; n--) { cout << n << ", "; } cout << "FIRE!"; return 0; }

10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!

The initialization and increase fields are optional. They can be avoided but not the semicolon signs among them. Thus, for example we could write: for (;n<10;) if we want to specify no initialization neither increase; or for (;n<10;n++) if we want to include an increase field but not an initialization.

Optionally, using the comma operator (,) we can specify more than one instruction in any of the fields included in a for loop, like in initialization, for example. The comma operator (,) is an instruction separator, it serves to separate more than one instruction where only one instruction is generally expected. For example, suppose that we wanted to intialize more than one variable in our loop:

for ( n=0, i=100 ; n!=i ; n++, i-- )
{
  // whatever here...
}

This loop will execute 50 times if neither n nor i are modified within the loop:

n starts with 0 and i with 100, the condition is (n!=i) (that n be not equal to i). Beacuse n is increased by one and i decreased, the loop's condition will become false after the 50th loop, when both n and i will be equal to 50.

Bifurcation of control and jumps.

The break instruction.

Using break we can leave a loop even if the condition for its end is not fulfilled. It can be used to end an infinite loop, or to force it to end before its natural end. For example, we are going to stop the count down before it naturally finishes (an engine failure maybe):

// break loop example #include <iostream.h> int main () { int n; for (n=10; n>0; n--) { cout << n << ", "; if (n==3) { cout << "countdown aborted!"; break; } } return 0; }

10, 9, 8, 7, 6, 5, 4, countdown aborted!

The continue instruction.

The continue instruction causes the program to skip the rest of the loop in the present iteration as if the end of the statement block would have been reached, causing it to jump to the following iteration. For example, we are going to skip the number 5 in our countdown:

// break loop example #include <iostream.h> int main () { for (int n=10; n>0; n--) { if (n==5) continue; cout << n << ", "; } cout << "FIRE!"; return 0; }

10, 9, 8, 7, 6, 4, 3, 2, 1, FIRE!

The goto instruction.

It allows to make an absolute jump to another point in the program. You must use this carefully since its execution ignores any type of nesting limitation.

The destination point is identified by a label, which is then used as argument for the goto instruction. A label is made of a valid identifier followed by a colon (:)

This instruction does not have any concrete utility in structured or object oriented programming aside from those that low-level programming fans may find to it. For example, here is our countdown loop using goto:

// goto loop example #include <iostream.h> int main () { int n=10; loop: cout << n << ", "; n--; if (n>0) goto loop; cout << "FIRE!"; return 0; }

10, 9, 8, 7, 6, 5, 4, 3, 2, 1, FIRE!

The exit function.

exit is a function defined in cstdlib (stdlib.h) library.

The purpose of exit is to terminate the program in course with an specific exit code. Its prototype is:

void exit (int exit code);

The exit code is used by some operating systems and may be used by calling programs. By convention, an exit code of 0 means that the program finished normally and any other value means error.

The selective Structure: switch.

switch (expression) {
  case constant1:
    block of instructions 1
    break;
  case constant2:
    block of instructions 2
    break;
  .
  .
  .
  default:
    default block of instructions
  }

Both following code fragments are equivalent:

switch (x) {
  case 1:
    cout << "x is 1";
    break;
  case 2:
    cout << "x is 2";
    break;
  default:
    cout << "value of x unknown";
  }

if (x == 1) {
  cout << "x is 1";
  }
else if (x == 2) {
  cout << "x is 2";
  }
else {
  cout << "value of x unknown";
  }

I have commented before that the syntax of the switch instruction is a bit peculiar. Notice the inclusion of the break instructions at the end of each block. This is necessary because if for example we did not include it after block of instructions 1 the program would not jump to the end of the switch selective block (}) and it would follow executing the rest of blocks of instructions until the first appearing of the break instruction or the end of the switch selective block. This makes unnecessary to include key brackets { } in each of the cases, and can also be useful to execute one same block of instructions for different possible values for the expression evaluated, for example:

switch (x) { case 1: case 2: case 3: cout << "x is 1, 2 or 3"; break; default: cout << "x is not 1, 2 nor 3"; }

If you need to check ranges or values that are not constants use a concatenation of if and else if sentences.

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