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  Go to Statement

Why the Go to Statement is Considered Harmful?

After placing  structured programming  in its  historical  perspective the question still remains; why  is the go to  considered harmful? There are many  reasons and arguments  against go to  and are discussed next.

1. Go to  are unnecessary

2. Clarity and Expressive  Power

3. Readability ( static and Dynamic Structures )

4. Proving Program correct

1. Go to  are Unnecessary : There  is a mathematical theorem which  guarantees that may  program written  using  go to  statement can be  transformed into  an equivalent  program  that uses only  the structured constructs. This converted program will need  some  more data items that are used as flag s to control  the actions  of the  program. Therefore, data  items  that are used programs with go to  in them.

(a) Clarity and Expressive  power :  Use of go to  statement  reduces  the clarity and expressive power of a  program. It can be better understood with the help of an example. Consider and compare the following  two equivalent program fragments as shown in fig 6.22.

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In the program  fragment . i we are  not immediately  sure  about  the roles of label and go to  until we take  some  time  to read  and study the program in order to discover that they are being used to create the repetition of a piece of code.  Whereas in the case of program  fragment II it is  immediately seen that while statement  is used here for  the repetition purpose.

There is  a remaining doubt in  the program  fragment I that there  may be  another  go to  aimed  at this  same label form other point  in the  program. Even  more a backward go to  is like  a loop statement and  readability is considerably  enhanced by  coding  a loop using  the appropriate  languages  construct, rather than  using  a go to  statement. 

The facilities of a programming languages  should allow  people  to describe what  they want to do in a meaningful  way. If we  typical  program written suing  go to  we see that the go to are  used  for a variety of purpose for example:

  • To avoid a piece of code , which is to be executed in different circumstances.
  • To perform repetition.
  • To exit from the middle of a loop.
  • To invoke a shared piece of code.

As the go to  has a variety of purposes, it  is not  clear immediately that which   purpose is  used for go to  in the  program.

In summary  the go to  lacks expressive  power and it is  therefore  difficult to understand the  logic of a program that is written using a lot of go to  when we look  at a piece  of coding words like while and if give  us a strong clue as to what  is intended go to  do not.

(c) Readability  ( Static and Dynamic Structures ) :A go to  can decrease readability and hence, maintainability. Even  worse is  a conditional go to ( if -go to ) , because  the flow of control depends on the  values of the  variables at execution time.

But the use of structured programming shows that it always  increases readability. A  structured program is said static as it can always be read from top to bottom, if  it has  no procedures. The exception to this rule arises in comprehending a while loop,  during  which  repeated references back  to the terminating condition at the start of the loop are necessary.

Program are essentially dynamic beings that exhibit a flow  of control while  program  listing is a static piece of text. In  a structured program, the  flow  of control  is always down  the page which exactly corresponds to the  way that text is normally  read.

(d) Proving  Program Correct: To formally prove, all  programs correct is not a practical  proposition with  present day techniques so there are some lessons  that can be learned from  proving.

In one technique  of program proving, assertions are made at  strategic  point  in the program. An assertion is a statement  of what things are true at that point in the  program. More exactly as assertion describes the relationships that hold between  data items that the program acts upon. An assertion at the start and end of a piece  of program code are called the input and output assertions respectively. Proving demonstrates  that if the input assertion is true the  action of the program will lead to the output assertion being true.

A structured  program  consists  solely of  components  that have a single  entry and a single exit point. This  considerably  aids the process of reasoning about  the effect  of the program. In contrast it  is usually impossible to isolate single entry single exit structures within  a program with  go to  init and  reduces checking  and understand ability. 

These were  some reasons why go to  statements should be avoided and should  not be used.

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