- Concepts of Programming Languages

Statements and Expressions

Instructor:

Learning Objectives

What should be the basic building blocks of computations?

  • Identify different ways of expressing computations
  • Identify the difference between statement sequences and compound expressions

Does it Translate?

Scala

  1. def f (x: Int) : Int =
  2.   val y: Int = if x!=0 then 1 else 2
  3.   y
  4. end f
  • Conditionally set a variable

Is this Java/C?

  1. int f (int x) {
  2.   int y = if (x!=0) 1 else 2;
  3.   return y;
  4. }
  • In Java/C, if ... else is statement language, not expression language

Does it Translate?

Java/C

  1. int f (int x) {
  2.   int y = x!=0 ? 1 : 2;
  3.   return y;
  4. }
  • Conditionally set a variable

Is this Scala?

  1. def f (x: Int) : Int =
  2.   var y: Int = (x!=0 ? 1 : 2)
  3.   y
  4. end f
  • Ternary operator not in Scala expression language
  • if ... in Scala expression language!

Does it Translate?

Scala

  1. def f (x: Int) : Int =
  2.   val y: Int = {
  3.     var i = x
  4.     var z=0
  5.     while i>0 do {i=i-1; z=z+1}
  6.     z
  7.   }
  8.   y
  9. end f
  • Set value of y with a compound expression

Is this Java/C?

  1. int f (int x) {
  2.   int y = {
  3.     int i=x;
  4.     int z=0;
  5.     while (i>0) {i--; z++;}
  6.     return z;
  7.   }
  8.   return y;
  9. }
  • Loops and ; are not in the Java/C expression language
  • How can we make it Java/C?

Does it Translate?

Scala

  1. def g (x: Int) : Int =
  2.   var i = x
  3.   var z = 0
  4.   while i>0 do { i=i-1; z=z+1 }
  5.   z
  6. end g
  8. def f (x: Int) : Int =
  9.   val y = g(x)
  10.   y
  11. end f

Java/C

  1. int g (int x) {
  2.   int i=x;
  3.   int z=0;
  4.   while (i>0) { i--; z++; }
  5.   return z;
  6. }
  8. int f (int x) {
  9.   int y = g(x);
  10.   return y;
  11. }
  • Functions are in the expression language of Scala, Java, and C

Statements and Expressions

  • Assembly language consists of statements
    1. mov eax, 5
    2. add eax, 6
    3. mov ebx, eax
  • Expressions are a more abstract way of expressing computations
    1. 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
  • Pure expression does not modify store
  • Anything else is a side effect
    • Assignment to a variable
    • Change of control (goto)
    • I/O (console, network)
    • etc.
  • Side-effecting expression modifies store

Expressions

  • Literals (boolean, character, integer, string)
  • Operators (arithmetic, bitwise, logical)
  • Function calls
    1. f(1 + 2 * "hello".length)

  • Assignment

    1. var y = 0
    2. val x = y = 5
    • Type and value of x?
    1. int y = 0;
    2. int x = y = 5;

In Scala, everything is an expression!

Statements in C

  • Expression statements (including assignment)
    1. printf("hello");
    2. ^^^^^^^^^^^^^^^    expression
    3. ^^^^^^^^^^^^^^^^   statement
  • Return statements
    1. return 1+x;
    2.        ^^^    expression
    3. ^^^^^^^^^^^   statement

Statements in C

  • Selection statements (if-then-else; switch-case)
  • Iteration statements (while; do-while; for)
  1. int count = 0;
  2. while (1) {
  3.   int ch = getchar();
  4.   switch (ch) {
  5.     case -1: return count;
  6.     case 'a': count = count + 1;
  7.     default: continue;
  8.   }
  9. }

Cannot use statements verbatim as part of expressions
Use functions to turn statements into expressions

Side-Effecting Expressions in C

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

  1. int x = 1;
  2. printf ("%d\n", x++);   //
  3. //

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

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

Side-Effecting Expressions in C

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

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

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

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

Side-Effecting Expressions in C

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

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

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

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

Side-Effecting Expressions in C

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

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

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

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

Sequence point: A point in the execution of a C program at which all previous side effects are guaranteed to be complete.

Side-Effects in OOP

  • Side effects are common in object-oriented programming
  1. class C {
  2.   private int x = 0;
  3.   private int y = 0;
  4.   public int f(int z) {
  5.     x = x-z;
  6.     return x;
  7.   }
  8.   public int g() {
  9.     y = y+5;
  10.     return y;
  11.   }
  12. }
  13. C c = new C();
  14. c.f(c.g()); // same as x -= (y += 5)
  • Results often depend on object state potentially on entire execution history

Sequencing in C Expressions

  • Operator , creates a sequence of expressions: last expression provides value

Statement sequence

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

Expression sequence

  1. int main () {
  2.   int x = 5;
  3.   printf ("%d\n", (x *= 2, x));
  4. }

Summary

Statements

  • Change memory
  • Are executed in sequence

Expressions

  • Pure vs. side-effecting
  • Sequencing by operator in expression language, e.g., C e1, e2, ... en
  • Conditional by operator in expression language, e.g., C e1 ? e2 : e3

# <span class="fa-stack"><i class="fa-solid fa-circle fa-stack-2x"></i><i class="fa-solid fa-code fa-stack-1x fa-inverse"></i></span> Is this Accepted Syntax? <div class="grid grid-cols-2 gap-4"> <div> **Is this Scala?** ```scala def f (x: Int) : Int = var y: Int = 0 if x!=0 then y=1 else y=2 y end f ``` - Enter in REPL to find out </div> <div> **Is this C?** ```c int f (int x) { int y; if (x!=0) y=1; else y=2; return y; } ``` - Compile to find out </div> </div> ---

# <span class="fa-stack"><i class="fa-solid fa-circle fa-stack-2x"></i><i class="fa-solid fa-code fa-stack-1x fa-inverse"></i></span> Side-Effecting Expressions in C <div class="grid grid-cols-2 gap-4"> <div> - :fa fa-question-circle: Name some side-effecting expressions in C * Post-increment `x++` * Add and assign `x += 2` * Assignment `x = (y = 5)` * Combined `x -= (y += 5)` </div> <div data-marpit-fragment> - Side effects common in OOP ```java class C { private int x = 0; private int y = 0; public int f(int z) { x = x-z; return x; } public int g() { y = y+5; return y; } } C c = new C(); c.f(c.g()); // same as x -= (y += 5) ``` - Results often depend on object state :fa fa-arrow-right: potentially on entire execution history </div> </div> ---

# <span class="fa-stack"><i class="fa-solid fa-circle fa-stack-2x"></i><i class="fa-solid fa-list-ol fa-stack-1x fa-inverse"></i></span> Sequencing in Expressions <div class="grid grid-cols-2 gap-4"> <div> - Scala `{ e1; e2; ...; en }` <div class="text-sm"> ```scala { e1 e2 ... en } ``` </div> </div> <div> - C `( e1, e2, ..., en )` <div class="text-sm"> ```c ( e1, e2, ... en ) ``` </div> </div> </div> <div class="grid grid-cols-2 span-4"> <div> - `e1` ... `en-1` executed for side effect - Result is the value of `en` </div> <div> * :fa fa-question-circle: Why have statement and expression sequences? * A C [example](https://stackoverflow.com/questions/52550/what-does-the-comma-operator-do-in-c): ```c string s; while(read_string(s), s.len() > 5) { // do something } ``` </div> </div> ---