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assignment statements

• one of the central constructs in imperative languages
• allows user to dynamically change the bindings of values to variables

simple assignments

• nearly all use = but all of them must use a different symbol for equalty relational operator to avoid confusion
• = for FORTRAN, BASIC, PL/I, C, C++, Java
• := for ALGOLs, Pascal, Modula-2, Ada

In PL/1: A = B = C;

• first is the assignment operator
• second is the comparison operator
• The value '1'B is assigned to A if B is equal to C; otherwise, the value '0'B is assigned. '1'B stands for true and '0'B for false.

int main()
{
   int a = 3;
   int b = 2;
   int c = 1;
   a = b = c;
   printf("%d %d %d",a,b,c);
}
// output 1 1 1

• since the value of c which is 1 is assigned to b, and the value of b which is now 1 is assigned to a. Note that the assignment operator is right associative.

multiple assignments

• perl, ruby, lua, and python allow multiple-target, multiple-source assignment statements like a, b = 100, 200 so swapping can be done by doing

x =2  
y =3  
x,y = y,x  
print(x,y)

conditional targets

• perl, c, c++, and java allow conditional targets on assignment statements

($flag ? $count1 : $count2) = 0;

this is equivalent to

if ($flag) {  
   $count1 = 0;  
} else {  
   $count2 = 0;  
}

compound assignment operators

• shorthand method of specifying a commonly needed form of assignment
• part of c-based languages, perl, javascript, python, and ruby
• syntax: concat the binary operator to the = operator like sum += value

unary assignment operators

• c-based languages, perl, and javascript have 2 pecial unary arithmetic operators ++ and --
• combine increment and decrement operations with assignment
• appear as postfix or prefix

sum = ++ count;
// is equivalent to
count = count + 1;  
sum = count;

//POSTFIX
sum = count ++;
//is equivalent to
sum = count;  
count = count + 1;

assignment as expression

• in c, c++, java, perl, and javascript the assignment statement produces a result so they can be used as operands in expressions

c: while ((ch = getchar()) != EOF) { ... }

• Note that the assignment must be parenthesized because the assignment operator has a lower precedence than the relational operators.
  c also allows multiple-target assignments like sum = count = 0
• the disadvantage is that it can also lead to side effects. this side effect leads to poor readability

a = b + (c = d / b) - 1

• cannot be read as an expression like in math
• we need to list the instructions

Assign d / b to c  
Assign b + c to temp  
Assign temp - 1 to a

if (x=y) instead of if (x==y)

• loss of error detection that leads to errors

assignment in functional programming languages

• in pure functional languages, identifiers are just names so their values never change.
• ML: names are bound to values with the val declaration like val cost = quantity * price
  • if cost appears on the left side of a subsequent val declaration, that declaration creates a new version of the name cost which has no relationship with the previous version (which is then hidden)

in haskell, if an identifier x is declared by let x = 3 then the value of x will always be 3. it can't be changed by assignment.

• haskell declarations as introducing constants not variables

mixed mode assignment

• assignment statements are frequently mixed-mode (refer to coercion in previous modules)
• design question: does the type of the expression have to be the same as the type of the variable being assigned, or can coercion be used in some cases of type mismatch?
• in c, c++, and perl: many possible type mixes are legal so coercion is freely applied. has the same rules as mixed-mode expressions
• java and c# allow mixed-mode assignment if the coercion is widening. (int -> float but not vice versa)
  • functional languages have no mixed-mode assignment