CSC 347 - Concepts of Programming Languages

Strict and Nonstrict Evaluation

Instructor: Stefan Mitsch

Learning Objectives

How much of an expression should be evaluated when computing a result?

• Understand expression evaluation
• Understand function call evaluation
• Understand macro evaluation

Strict and Nonstrict Operators

• A strict construct evaluates all of its operands before it runs
• Name some strict constructs in C
  • Arithmetic operators: +, -, *, /, ...
  • Comparison operators: <, <=, ==, !=, ...
  • Bitwise operators: |, &, ~, ^
  • Function calls: f(x,y)

Strict and Nonstrict Operators

• A strict construct evaluates all of its operands before it runs
• Name some non-strict constructs in C
  • e1 && e2 is strict in e1, but not e2, useful e.g., p && p->m()
  • e1 || e2 is strict in e1, but not e2, useful e.g., p==NULL || p->m()
  • e1 ? e2 : e3 is strict in e1, but not e2 or e3
  • Macro expansion

Ternary Operator

Conditional expression (e1 ? e2 : e3)

• Evaluate e1; if true then evaluate e2, else evaluate e3

int main () {
  for (int i=0; i<10; i++) {
    int x = 0;
    int y = 0;
    int z = (rand()%2) ? (x=1,111) : (y=2,222);
    printf ("x=%d, y=%d, z=%d\n", x, y, z);
  }
}

$ gcc statements-01.c && ./a.out 
x=1, y=0, z=111
x=0, y=2, z=222
...

Conditionals

• Conditional statement vs conditional expression

int fact (int n) {
  if (n <= 1) {
    return 1;
  } else {
    return n * fact (n - 1);
  }
}

int fact (int n) {
  return (n <= 1) ? 1 : n * fact (n - 1);
}

What Happens?

• Function calls are strict

int fcond (int b, int t, int f) { return b ? t : f; }
int main () {
  for (int i=0; i<10; i++) {
    int x = 0;
    int y = 0;
    int z = fcond (rand()%2,  (x=1,111), (y=2,222));
    printf ("x=%d, y=%d, z=%d\n", x, y, z);
  }
}

$ gcc statements-02.c && ./a.out 
x=1, y=2, z=111
x=1, y=2, z=222
...

What Happens?

• Macro calls are non-strict

#define mcond(b, t, f) (b)?(t):(f)
int main () {
  for (int i=0; i<10; i++) {
    int x = 0;
    int y = 0;
    int z = mcond (rand()%2,  (x=1,111), (y=2,222));
    printf ("x=%d, y=%d, z=%d\n", x, y, z);
  }
}

$ gcc statements-03.c && ./a.out 
x=1, y=0, z=111
x=0, y=2, z=222
...

What Happens?

• Macro calls are evaluated in the compiler, by textual substitution

int ftriple (int i) { return i + i + i; }
#define mtriple(i) (i)+(i)+(i)
int main () {
  int x = 10;  
  int rx = ftriple(x=x+1);
  int y = 10;
  int ry = mtriple(y=y+1);
  printf ("x=%d, rx=%d, y=%d, ry=%d\n", x, rx, y, ry);
}

$ gcc statements-04.c && ./a.out 
x=11, rx=33, y=13, ry=36

Summary

• Strict evaluation: evaluates all operands when determining the value of an expression
• Nonstrict evaluation: evaluates only necessary operands when determining the value of an expression