Topic 4 - Expressions

Question 1

What are expressions built from?

Answer 1

Variables, constants & operators.

Question 2

What does the unary operator i = +1 do

j = -i?

Answer 2

increase by one, decrease by i

Question 3

Do all binary arithmetic operators allow integer and floating point operand mixing?

Answer 3

Yes except modulo.

Question 4

What is the type of there result when you mix float and int with binary operators?

Answer 4

It always has type float

Question 5

What happens when you use / and % operators with integers and true answer is has a decimal point?

Answer 5

The answer gets truncated (ie. all numbers after decimal and the decimal itself are dropped ie. truncated)

Question 6

The behaviour when / and % are used with negative operands is what? (In C89 and C99)?

Answer 6

C89: implementation-defined
C99: 
10%3 > 1
10%-3 > 1
-10 % 3 > -1
-10 % -3 > -1

Question 7

What is the order precedence?

Answer 7

postfix
prefix
unary, * ? %, + -

Question 8

What is the binary operator associativity?

Answer 8

Left associative

Question 9

In many programming languages, an assignment is a statement, however, in C, what is it called?

Answer 9

An assignment is an operator, just like +, * etc.

Question 10

What is another name for an assignment statement?

Answer 10

A side effect - where the right side is computed and a side-effect is the computed value is assigned to the variable to the left

Question 11

What is the = operator associativity?

Answer 11

Right-associative
ie. i = j = k = 0; is equal to:
i = (j = (j = 0));

Question 12

What is an advantage of side-effects of assignment statements?

Answer 12

They can be chained together:

i = j = k = 0;

Question 13

What’s an Lvalue?

Answer 13

a variable (object stood in computer memory, not a constant or the result of a computation)

Question 14

Give examples of compound assignment operators:

Answer 14

+=, -=, *=, /=, %=

Question 15

Operator precedence in compound assignments are like the usual or different?

Answer 15

Different:
i = j+k is same as:
i = i(j+k)

Question 16

What is the order of subexpression evaluations?

Answer 16

Undefined. Doesn’t matter

Question 17

Comment on the order of the expression:

c = (b = a + 2) - (a = 1);

Answer 17

The effect of executing the second statement is undefined

Question 18

What is the cure to undefined precedence for subexpression evaluation?

Answer 18

Separating assignments into series of assignments

Question 19

What is the value of:
i = 2;
j = i * i++;

Answer 19

Undefined behaviour; avoid when you can

Question 20

What is happening here:
++i;
Is it useful at all to do this?

Answer 20

i is first incremented, then the new value of i is fetched but then discarded.
No, only expressions with side effects are useful.