CSC447 - Concepts of Programming Languages

Variables

Consider
```
x = x + 1;
```
Right-hand x denotes
- value read from storage location
Left-hand x denotes
- the storage location (address)
Goes back to Strachey in the 1960s
- Fundamental Concepts in Programming Languages, Christopher Strachey (1967) (local)

In C, take the address of l-values

 int x = 5;
int y = 6;
int *p = &x;
int *q = &y;
int **r = &p;
r = &q;
q = p;
**r = 7;

In C

 int x = 5;
int y = 6;
int *p = &(x + y); /* not allowed */
(x + y) = 7;       /* not allowed */

L-values may require r-mode evaluation
Array access in C, Java, etc.
```
arr[n + 2] = 7;
```
Field access in Java, Scala, etc.
```
obj.f1 = 7
```
Combinations (including method calls) in Java, etc.
```
obj.m1 ().f1[n + 2].m2 ().f2 = 7
```