What is a sub-expression in C?

Question

What is sub expression in C? I thought combination of smaller expression is subexpression eg: a*(b+C/d)/20

b+c/d is subexpression is it correct? or alone c/d is subexpression ?

Anora

Answer 1

A sub-expression is not just any part of a larger expression.

Consider:

2 * 3 + 4 * 5

Here 3+4*5 is not a sub-expression.

The full expression parses as

(2 * 3) + (4 * 5)

and so the direct sub-expressions are 2*3 and 4*5.

Each of those again parse as compositions of smaller things, with 2*3 composed of the sub-expressions 2 and 3, and with 4*5 composed of the sub-expressions 4 and 5.

These sub-expressions of sub-expressions are indirect sub-expressions of the original full expression, so that in total it has these sub-expressions: 2*3, 4*5, 2, 3, 4 and 5.

While e.g. 3+4*5 is not a sub-expression.

In summary, a sub-expression is an argument to an operator or function, and such an argument expression can itself have sub-expressions.

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Regarding your example

a*(b+C/d)/20

and your concrete questions

b+c/d is subexpression is it correct? or alone c/d is subexpression ?

Yes, and yes (modulo uppercase/lowercase typo).

However, for example, here b+C is not a sub-expression.

Campy

Trying to understand why b+C is not a sub-expression. Is it not an argument that is passed to the division operator? – Art

@codingManiac: the expression b+C/d is parsed as b+(C/d), due to the grammar of C++. A simplified way to view that is that / has higher precedence than +, and you can find operator precedence tables everywhere. However, it's worth noting that the C++ standard only specifies the grammar, from which precedence is deduced, and that the simpler precedence view breaks down in certain situations, e.g. for the ?: choice operator. The only common programming language I know that doesn't effectively have precedence for arithmetic operators, is old Smalltalk. – Campy

Answer 2

what is sub expression in c.i thought combination of smaller expression is subexpression

Yes. You thought correct.
A sub-expression is a part of an expression that is by itself a correct expression.

Sometimes a sub-expression is a constant, like 20 here. /20 is not a correct expression itself hence it can't be a sub-expression.
In general:

a subexpression is a part of an expression that corresponds to a subtree in the parse tree -- that is, some node in the parse tree plus all of its descendants. The subexpression is a proper subexpression if it is not the entire expression.

To answer your specific questions:

b+c/d is sub-expression is it correct?

Yes.

or alone c/d is subexpression ?

Yes. a, b, C, d, 20, b+C/d and a*(b+C/d) are all sub-expressions.

Suborn

In a well-known quirk of the C grammar, in the constant -20, 20 is a subexpression. – Dipteran

@PascalCuoq; Added a new, more general, definition for you :0. BTW, I hope - sign is unary - here. – Suborn

Answer 3

A sub expression is any smaller unit in an expression, so C/d and b+C/d both are sub-expressions,

Whereas a unit here is a combination of two operands and an operator in between.
Example,

C/d is a unit, and so is (b+C/d)
note that in (b+C/d), C/d will be executed first after that (b+C/d) will be a unit.

Also, a*(b+C/d) is a sub-expression.

and adding /20 to the rest of the expression will be complete expression, so that will not be a sub-expression.

In other words we can say that in an expression, all smaller expressions contained in the complete expression are sub-expressions except the expression executed in the last(order of execution depends upon the priorities of operators).

Wilda

I think you mean “smaller”. Using “smallest” makes it appear as you are saying the opposite of what you are trying to say. – Dipteran

Note: /d)/20 is not a subexpression. – Veliger

@GrijeshChauhan Yes because that will be evaluated in the last and that will also be in the last part of the expression, and if added will be the whole expression. – Wilda

"any smaller unit" might make sense, but in order for it to make sense the term "unit" must be explained (otherwise it's full circularity) – Campy

@Cheersandhth.-Alf I have edited my answer please suggest any more editions if think they are necessary. – Wilda

@AnkurKumawat: Well the word "here" in the new text means that it's an explanation by example, and that's fine, as I see it. It probably won't hurt to add a "more in general", about e.g. unary operators and function calls. But it may not necessarily be more clear either. – Campy

@Cheersandhth.-Alf thanks for your suggestion, i have improved my answer accordingly. – Wilda

Answer 4

An expression is a sequence of operators and operands that specifies a computation. An expression can result in a value and can cause side effects. [§5.1]

So, b+c/d and c/d both are expressions. Even, a,b,c,d and 20 are expressions.

Mobley