In the example above, if the discount is 10%, then the first if statement will be evaluated as true and "you have to pay $30" will be printed out. If discount < 11 % then print ( you have to pay $ 30 ) elseif discount < 21 % then print ( you have to pay $ 20 ) elseif discount < 31 % then print ( you have to pay $ 10 ) end if Structured if–then–else statements like the example above are one of the key elements of structured programming, and they are present in most popular high-level programming languages such as C, Java, JavaScript and Visual Basic.
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While it is possible while using only GOTO statements in if–then statements to write programs that are not spaghetti code and are just as well structured and readable as programs written in a structured programming language, structured programming makes this easier and enforces it. Such mechanisms and principles were based on the older but more advanced ALGOL family of languages, and ALGOL-like languages such as Pascal and Modula-2 influenced modern BASIC variants for many years. As a result, structured programming, which allows (virtually) arbitrary statements to be put in statement blocks inside an if statement, gained in popularity, until it became the norm even in most BASIC programming circles.
This led to a hard-to-read style of programming known as spaghetti programming, with programs in this style called spaghetti code. In early programming languages, especially some dialects of BASIC in the 1980s home computers, an if–then statement could only contain GOTO statements (equivalent to a branch instruction). Otherwise, the execution continues in the following branch – either in the else block (which is usually optional), or if there is no else branch, then after the end If.Īfter either branch has been executed, control returns to the point after the end If.
If the condition is true, the statements following the then are executed. When an interpreter finds an If, it expects a boolean condition – for example, x > 0, which means "the variable x contains a number that is greater than zero" – and evaluates that condition. In contrast, the combination of this expression, the If and Then surrounding it, and the consequent that follows afterward constitute a conditional statement, having intrinsic meaning (e.g., expressing a coherent logical rule) but no intrinsic value. In the example code above, the part represented by (boolean condition) constitutes a conditional expression, having intrinsic value (e.g., it may be substituted by either of the values True or False) but having no intrinsic meaning. If stock = 0 Then message = order new stock Else message = there is stock End If
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