Programming Language Concepts Control Flow · Janyl Jumadinova Programming Language Concepts...

Programming Language ConceptsControl Flow

Janyl Jumadinova

8 October, 2020

Janyl Jumadinova Programming Language Concepts Control Flow 8 October, 2020 1 / 13

Expression Evaluation

Answer Poll atitempool.com/jjumadinova/live


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What is the value of the variable i after executing the following code (ineither Java or C)?

i = 10;

i = i++;

Same Question:

i = 10;

i = ++i;



Sequential execution–expression evaluation:



int i=6, j=3, k=2;

int m = i/j*k;

What is the value of m?

So, since 6/6 = 1, m = 1, right?

Problem: / and * have equal precedence and they are left-associative:i/j*k = (i/j)*k = (6/3)*2 = 4.



int i=6, j=3, k=2;

int m = i/j*k;

What is the value of m?So, since 6/6 = 1, m = 1, right?

Problem: / and * have equal precedence and they are left-associative:i/j*k = (i/j)*k = (6/3)*2 = 4.


Assignment Operators in Java and C

The assignment operator “=” produces a value, just like otheroperators.

The value of the expression “i = 10” is 10.

This is a right-associative operator:

“i = j = k = 10” means “i = (j = (k = 10))” and has the effect ofsetting all three variables to the same value, 10.In C, this can cause serious program bugs (e.g., a = b = c to beinterpreted as a = (b = c))!


Assignment Operator in C

The following is legal in C:i = 0;

if (i = 10) printf("i is 10");

The effect is to assign 10 to the variable i , then see if the resulting value(namely 10) is non-zero (in C, non-zero values represent “true”). This willalways evaluate to true!

The programmer probably meant to write: if (i == 10) printf(‘‘iis 10’’);


Assignments and Expressions

Some programming languages use the idea of l-values and r-values.

In an assignment statement such as:i = i+1

the variable i plays two different roles.

On the left, it stands for a memory location, or reference, called an“l-value”.

An l-value refers to an object that persists beyond a single expression.

On the right, it represents a value (“r-value”).

An r-value is a temporary value that does not persist beyond theexpression that uses it.


Assignment Statements in Functional Languages

In a functional language, there are no variables, hence no assignmentstatements.

A program in a functional language is just a collection of expressionsto be evaluated.

More on functional languages in a few weeks.


Initialization

Some languages allow a variable to be initialized at the same time itis declared:int i = 10

Not all languages check to see if a variable is initialized:

int main() { /* C example */

int i,j,k;

printf("i,j,k = %d %d %d \n",i,j,k);

}

Output: i,j,k = 32767 1740734558 32767


Initialization

Compilers can often determine whether a variable is initialized at thepoint where it is used as an r-value, e.g., Java

...

int i;

int j = i;

...

javac Init.java

Init.java:12: error: variable i might not have been initialized


Automatic Initialization

In some languages, (some) variables are automatically initialized to adefault value.

Example: Java initializes all instance variables (e.g., ints and doublesare zero, objects are null, etc.), but not local variables


COBOL Activity

Common Business Oriented Language.Divisions - Identification, Environment, Data, Procedure.

Identification division

Entries → ClausesEnvironment division

Sections → Paragraphs → Entries → Clauses, PhrasesData division

Sections → Entries → Clauses → PhrasesProcedure division

Sections → Paragraphs → Sentences → Statements → Phrases


Programming Language Concepts Control Flow · Janyl Jumadinova Programming Language Concepts...

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