CSC 347 - Concepts of Programming Languages

Statements and Expressions

Instructor: Stefan Mitsch

Learning Objectives

What should be the basic building blocks of computations?

• Understand different ways of expressing computations
• Understand the importance of sequencing in computations

PL Development

• Assignment: x := 5, comparison x >= y, conditional if (x<4) then 2 else 3

• Develop syntax for picking a random value from an interval
• x := * from [0,10]
• Develop syntax for picking a value that satisfies some property
• x := * s.t. 0 <= x
• Express ordinary assignment in terms of the property-satisfying assignment above
• x := 5 is shortcut for x := * s.t. x=5
• What else do popular programming languages provide?

Is this Accepted Syntax?

def f (x: Int) : Int =
  var y: Int = 0
  if x!=0 then y=1 else y=2
  y

int f (int x) {
    int y;
    if (x) y=1; else y=2;
    return y;
}

Is this Accepted Syntax?

def f (x: Int) : Int = {
  var y: Int = 0
  y = if x!=0 then 1 else 2
  y
}

int f (int x) {
  int y;
  y = if (x) 1 else 2;
  return y;
}

Is this Accepted Syntax?

def f (x: Int) : Int = {
  var y: Int = 0
  y = (x!=0 ? 1 : 2)
  y
}

int f (int x) {
  int y;
  y = x ? 1 : 2;
  return y;
}

Is this Accepted Syntax?

def f (x: Int) : Int = {
  var xx = x
  var y: Int = 0
  y = { var z=0 
        while (xx>0) do {xx=xx-1; z=z+1}
        z }
  y
}

int f (int x) {
  int y;
  y = { int z=0; 
        while (x>0) {x--; z++;} 
        return z; }
  return y;
}

Is this Accepted Syntax?

def g (x: Int) : Int = {
  var xx = x
  var z = 0
  while xx>0 do { xx=xx-1; z=z+1 }
  z
}
def f (x: Int) : Int = g(x)

int g (int x) {
  int z=0; 
  while (x>0) { x--; z++; } 
  return z;
}
int f (int x) {
  return g(x);
}

• Functions are in the expression language of Scala and C

Statements and Expressions

• Assembly language consists of statements

  mov eax, 5
  add eax, 6
  mov ebx, eax
  

• Expressions are a more abstract way of expressing computations

  5+6
  

• Many imperative PL distinguish statement language from expression language
• Functional languages tend to emphasize expressions (Scala has only expressions)

Pure vs Side-Effecting Expressions

• A mathematical function takes arguments and gives results

• An expression is pure if that is all it does
• Anything else is a side effect
  • Assignment to a variable
  • Change of control (goto)
  • I/O (console, network)
  • etc.

Expressions

• Literals (boolean, character, integer, string)
• Operators (arithmetic, bitwise, logical)
• Function calls

f (1 + 2 * "hello".length)

Scala does not have statements, everything is an expression!

Statements in C

• Expression statements (including assignment)

  printf("hello");
  ^^^^^^^^^^^^^^^    expression
  ^^^^^^^^^^^^^^^^   statement
  

• Return statements

  return 1+x;
         ^^^    expression
  ^^^^^^^^^^^   statement
  

Statements in C

• Selection statements (if-then-else; switch-case)
• Iteration statements (while; do-while; for)

int count = 0;
while (1) {
  int ch = getchar();
  switch (ch) {
    case -1: return count;
    case 'a': count = count + 1;
    default: continue;
  }
}

Cannot use statements verbatim as part of expressions
How can we turn statements into an expression?

Side-Effecting Expressions in C

• Post-increment

  x++
  

• Add and assign

  x += 2
  

• Assignment

  x = (y = 5)
  

• Combined

  x -= (y += 5)
  

Side-Effecting Expressions in C

int x = 1;
printf ("%d\n", ++x);   //
//

int x = 1;
printf ("%d\n", x++);   //
//

x = 1 + (y = 5);   //

int x = 1;
printf ("%d\n", (x = x + 1) + x);   //

Side-Effecting Expressions in C

int x = 1;
printf ("%d\n", ++x);   // pre increment, prints 2
// value of x is now 2

int x = 1;
printf ("%d\n", x++);   //
//

x = 1 + (y = 5);   //

int x = 1;
printf ("%d\n", (x = x + 1) + x);   //

Side-Effecting Expressions in C

int x = 1;
printf ("%d\n", ++x);   // pre increment, prints 2
// value of x is now 2

int x = 1;
printf ("%d\n", x++);   // post increment, prints 1
// value of x is now 2

x = 1 + (y = 5);   //

int x = 1;
printf ("%d\n", (x = x + 1) + x);   //

Side-Effecting Expressions in C

int x = 1;
printf ("%d\n", ++x);   // pre increment, prints 2
// value of x is now 2

int x = 1;
printf ("%d\n", x++);   // post increment, prints 1
// value of x is now 2

x = 1 + (y = 5);  // assigns 5 to y and 6 to x

int x = 1;
printf ("%d\n", (x = x + 1) + x);   //

Side-Effecting Expressions in C

int x = 1;
printf ("%d\n", ++x);   // pre increment, prints 2
// value of x is now 2

int x = 1;
printf ("%d\n", x++);   // post increment, prints 1
// value of x is now 2

x = 1 + (y = 5);  // assigns 5 to y and 6 to x

int x = 1;
printf ("%d\n", (x = x + 1) + x);   // no "sequence point", undefined!

Sequencing in Expressions

• e1 ... en-1 executed for side effect
• Result is the value of en

• A C example:

  string s; 
  while(read_string(s), s.len() > 5) { 
    // do something 
  } 
  

Sequencing in C Expressions

int main () {
  int x = 5;
  x *= 2;
  printf ("%d\n", x);
}

int main () {
  int x = 5;
  printf ("%d\n", (x *= 2, x));
  // behavior defined because comma operator introduces sequence point
}

Summary

• Should variable declarations be statements or expressions? What are they in C?
• Should loops be statements or expressions? What are they in C?