Control Structures

what is a command in programming languages?

a construct that performs and action does not return a value

give examples of commands

assignment statements, if statements, while statements, void function/procedure calls

can commands be written where expressions normally appear?

depends on the language. some languages treat certain commands as expressions with values

basic form of an assignment?

l value = r-value

what is the difference between l-values and r-values?

• l-value: identifies a storage location

• r-value: provides a value to be stored there

what common error occurs due to = being allowed in expressions?

using = instead of == in conditionals

copy semantics?

assignment copies the value of the right side into the left side. subsequent changes do not affect each other

reference semantics?

assignment makes both variables refer to the same underlying object. changing one affects the other

languages that use copy semantics?

Pascal, C, Ada, most built-in types in Java, default behavior in C++

languages that use reference semantics?

LISP and Java for all objects

in copy semantics, what does x = y; do?

copies the value of y into x. they become independent

what can serve as an l-value?

x, A[i], rec.field

what can’t serve as an l-value?

expressions that only yield values, such as x+y, 23, foo(x)

what is a shallow copy?

copy the pointer value; the new object refers to the same inner objects.

what is a deep copy?

copy the entire object graph; internal referenced objects are duplicated

which languages normally perform shallow copies?

C, Pascal, Ada, Java

why may shallow copy be insufficient?

mutating a shared referenced object changes both copies — unexpected aliasing.

why does C++ allow custom assignment operators and copy instructors?

to allow the programmer to implement deep copy when needed

what distinguishes a command from an expression?

commands are statements with effects but no return value; expressions compute and return a value

in C/Java, is = an opeator?

yes — = returns the assigned value

what enables code like a = b = c = 0; ?

assignment being an expression that associates right-to-left

what is a danger of assignment-as-expression?

using = instead of == inside conditionals

what two features define imperative PLs?

assignment and control flow

what is control flow?

the specific statement currently being executed

what is a basic block?

a consecutive sequence of statements with single entry and single exit

what does GOTO do?

unconditional branch to labeled statement

what is an arithmetic IF?

IF (expr) s, t, u → go to label based on negative/zero/positive

why was early FORTRAN known for “spaghetti code”?

arbitrary jumps created tangled, nonstructured flow.

what is structured programming?

programs composed only of single-entry/single-exit constructs: sequencing, selection, iteration

what is sequencing?

grouping statements into ordered blocks with one entry and exit

what problem does structured programming solve in selection statements?

ensures nested structure matches control-flow intent

what is a dangling else?

ambiguity of which if an else belongs to

what is a definite loop?

number of iterations known at loop entry

why is the C for loop flexible?

initialization, condition, and increment are arbitrary statements/expressions

what are the three major syntactic forms of expressions?

infix, prefix, postfix

write x * (y+1) - z in postfix

x y 1 + * z -

write x * (y+1) - z in prefix

- * x + y 1 z

why can all three forms be generated from an AST?

because the AST captures structure independent of notation

what tree traversal produces postfix notation?

post-order traversal

why is postfix easy to evaluate?

because operators come after operands, enabling stack-based evaluation

what is the general algorithm for evaluating postfix?

1. read tokens left to right

2. push operands

3. on operator: pop two operands, apply operator, push result

why does short-circuiting change observable behavior?

because operands may have side effects; skipping evaluation means skipping side effects

why is short-circuiting important?

• prevents invalid operations

• prevents unwanted side effects

when does the second operand of expr1 || expr2 get evaluated?

only when expr1 is false

when does the second operand of expr1 && expr2 get evaluated?

only when expr1 is true

what does short-circuiting mean?

the second operand is evaluated only if needed

what must conditional operator code generation include?

branching logic (Jump to L1/L2)

what is the idea behind postfix code generation?

generate the code that pushes operands, then pops them wehn applying operators

what does the non-stack method use instead of stack pushes/pops?

temporary variables

why is this approach often faster?

accessing temporaries is quicker than stack operations

why is memory cost not as bad as it seems?

optimizers often merge temporary variables

what is a side effect?

any effect of evaluating an expression beyond producing a value