In the real-world, it would be nice to execute something when the condition fails. To do so, If else statement used. Here, Else statement will execute the statements when the condition fails. Let us see the syntax of If Else in C: If Else statement in C Syntax: The basic syntax of If Else Statement in C language is as follows. C if-else Statements - If else statements in C is also used to control the program flow based on some condition, only the difference is: it's used to execute some statement code block if the expression is evaluated to true, otherwise executes else statement code block.
If else statements in C is also used to control the program flow based on some condition, only the difference is: it's used to execute some statement code block if the expression is evaluated to true, otherwise executes else statement code block.
The basic format of if else statement is:
Figure - Flowchart of if-else Statement:
Example of a C Program to Demonstrate if-else Statement
if-else Statements in C - Video Tutorial
To understand 'C if-else Statements' in more depth, please watch this video tutorial.
Resolving the General Dangling Else/If-Else AmbiguityA troublesome feature of grammars for many programming languages, including C and C++, is a recurring problem known variously as dangling else, if-else ambiguity, or if-else conflict. We show a way to overcome this problem by syntax alone, yielding a conflict-free syntax without the need for 'disambiguation rules'. In addition, we provide an overview of the algebraic process used to develop the conflict-free syntax. In the following discussion, we use the notation of the AnaGram LALR parser generator to describe the syntax.
Description of the AmbiguityCommonly, the syntax for if-else statements is written thus: where simple statement subsumes expression statements, block statements, break and continue statements, and even the do/while statement; in short, any statement which is neither left nor right recursive. loop statement
If Statements In C++ Pdfsubsumes while statements and for statements, i.e., right recursive statements of the form:This syntax is ambiguous, as can be illustrated by the following example: Using the syntax given above, this statement can be interpreted either as: or as: Both interpretations are consistent with the syntax definition given above, but they have very different outcomes. Conventionally, parsers are rigged using some sort of trick to select the first interpretation. This trick is often called a 'disambiguation rule'. In AnaGram, you could use the sticky attribute statement: However, such tricks are not necessary, since the statement syntax can be made conflict-free as described below. A simpler conflict-free syntax for statements is provided as an example in section 4.3 of 'Compilers: Principles, Techniques and Tools' by Aho, Sethi and Ullman, but their syntax is incomplete in that it does not, in fact, provide for right-recursive statements such as
If Else Statement In C Language Ppt Formatwhile and for.
An Unambiguous Syntax for StatementsThe problem with the conventional syntax for statements is that it represents a simple taxonomy of statements rather than an appropriate description of syntax. Although it is true that there are
If Else Statement In C Language Ppt Formatif statements, loop statements and simple statements, this is not the most useful way to classify statements from a syntactic point of view.
The following syntax requires a following else to be paired with the most recent unpaired if, thus disallowing the if-else ambiguity.
We define an open statement as one which has at least one if that is not paired with a following else within the statement. A closed statement either does not contain an if at all, or if it does, the if is paired with a following else. We can then write the statement syntax as follows: In this syntax, we allow only closed statements between an if and its matching else. In other words, between an if and an else an if is allowed only if it is paired with a matching else clause. The net effect is that each else is associated with the nearest preceding if.
In the next section we will show how this syntax can be developed by suitably rewriting our original syntax.
Developing the Conflict-Free SyntaxTo see how to resolve the ambiguity and develop the above syntax, let us begin with the customary ambiguous syntax described in the first section and try rewriting statement as where, as described above, open statement is any statement that has a 'dangling' if, that is, an
C# If Else Exampleif which could be paired with a following else. A closed statement is one which does not have a dangling if.
Substituting the above into the second rule for if statement yields: Clearly, the last of these three rules for if statement is always ambiguous, since open statement, by definition, contains one or more ifs not paired with elses, and thus it is not clear which if should be associated with the else.
Therefore, let us eliminate the last rule, leaving our rules for if statement as follows: One could ask whether it is legitimate to discard a rule so cavalierly. In fact it can be shown, by means of algebraic manipulations too tedious to include here, that all sentences produced by the discarded rule can also be produced by the remaining rules. Thus the discarded rule adds nothing to the grammar but ambiguity.
Now, it remains to determine just which rules for statement belong to open statement and which to closed statement.
To this end, we expand the original rules for statement using the two rules above: The first rule is clearly closed. The second rule is clearly open since it contains at least one unpaired if. The last two rules cannot be classified as they stand, since they would be open or closed depending on whether the final statement were open or closed. Therefore, we expand the final token of the last two rules to yield: Now, we can reorder the above rules: By our definitions, the first three rules are open statements and the last three rules are closed statements. So we can finally rewrite the syntax: yielding the conflict-free syntax presented in the previous section.