2.4 Repetition statements, arrays and structured programming  (Page 4/8)

Example

#include<iostream.h>

int main()

{

const int MAXI = 5;

const int MAXJ = 4;

int i, j;

for(i = 1; i<= MAXI; i++) // start of outer loop

{

cout<<"\ni is now "<<i<<endl;

for(j = 1; j<= MAXJ; j++) // start of inner loop

cout<<" j = "<<j; // end of inner loop

} // end of outer loop

cout<<endl;

return 0;

}

The output of the above program:

i is now 1

j = 1 j = 2 j = 3 j = 4

i is now 2

j = 1 j = 2 j = 3 j = 4

i is now 3

j = 1 j = 2 j = 3 j = 4

i is now 4

j = 1 j = 2 j = 3 j = 4

i is now 5

j = 1 j = 2 j = 3 j = 4

Do-while loops

The do..while statement executes a statement or statements once, then repeats the execution as long as a given conditional expression evaluates to true.

The do..while statement is used to create post-test loops.

The syntax for the do..while statement:

do {

statements;

} while (conditional expression);

Example:

do {

cout<<“\nEnter an identification number:”;

cin>>idNum;

} while (idNum<1000|| idNum>1999);

Here, a request for a new id-number is repeated until a valid number is entered.

do {

cout<<“\nEnter an identification number:”;

cin>>idNum;

if (idNum<1000 || idNum>1999)

{

cout<<“An invalid number was just entered\n”;

cout<<“Please reenter an ID number /n”;

}

else break;

} while (true);

Structured programming with c++

The goto statement

In C++, goto statement – an unconditional branch, is just a legacy code from C language. The result of the goto statement is a change in the flow of control of the program to the first statement after the label specified in the goto statement.

Example:

start: // label

if (cout>10) go to end;

…

…

go to start;

end: cout<<endl;

The goto statement can lead to programs that are more difficult to debug, maintain, and modify.

Structured programming

During the 1960s, it became clear that the indiscriminate use of transfers of control through goto statements was the root of much difficulty experienced by programmer groups. The notion of so-called structured programming became almost synonymous with “ goto elimination .”

Bohm and Jacopini’s work demonstrated that all programs could be written in terms of only three control structures:

• sequence structure
• selection structure
• repetition structure

The sequence structure is built into C++. Unless directed otherwise, the computer executes C++ statements one after the other in the order in which they are written. Below is a sequence structure.

C++ provides three types of selection structures:

- if statement (single-selection structure)

- if-else statement (double-selection structure)

- switch statement. (multiple-selection structure)

C++ provides three types of repetition structures:

- while statement

- do-while statement

- for statement

So C++ has only seven control structures: sequence, three types of selection and three types of repetition. Each C++ program is formed by combining as many of each type of control structures as is appropriate for the algorithm the program implements.

We will see that each control structure has only one entry point and one exit point . These single-entry/single-exit control structures make it easy to build programs.