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.
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:
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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