Concepts of Programming Languages

C: Statements versus Expressions

Instructor: Stefan Mitsch

Is this C?

int f (int x) {
    int y;
    if (x) y=1; else y=2;
    return y;
}
int main() { printf ("%d\n", f(5)); return 0; }
          

Is this C?

int f (int x) {
    int y;
    y = if (x) 1 else 2;
    return y;
}
int main() { printf ("%d\n", f(5)); return 0; }
          

Is this C?

int f (int x) {
    int y;
    y = x ? 1 : 2;
    return y;
}
int main() { printf ("%d\n", f(5)); return 0; }
          

Is this C?

int f (int x) {
    int y;
    y = {int z=0; while (x>0) {x--; z++;} z}
    return y;
}
int main() { printf ("%d\n", f(5)); return 0; }
          

Statements and Expressions

• Assembly language consists of statements
• Expressions were added later
• Roughly:
  • Statements: should change memory
  • Expressions: should not

Pure / Side Effecting

• A mathematical function takes arguments and gives results
• An expression is pure if that's 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 * strlen ("hello")))
            

Statements

• Expression statements (including assignment)

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

• Return statements

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

Statements

• 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;
  }
}
            

Side-Effecting Expressions

x++
          

x += 2
          

x = (y = 5)
          

x -= (y += 5)
          

Side-Effecting Expressions

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

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

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

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

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!
          

Side-Effecting Expressions

int global = 0;

int post_inc () {
  return global++;
}

int main () {
  printf ("%d\n", post_inc () + post_inc ());
}
          

C'S Comma Operator

• (e1, e2, ..., en)
• e1 ... en-1 executed for side effect
• Result is the value of en
• Sequential execution of expressions!
• An example:

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

Sequencing

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
}
          

C Expressions

• Sequencing: e1, e2
• Conditional: e1 ? e2 : e3
• Variable declarations?
• Loops?