Programming Language Concepts Control Flow · Programming Language Concepts Control Flow Janyl Jumadinova 13-15 October, 2020 Janyl Jumadinova Programming Language Concepts Control

Programming Language ConceptsControl Flow

Janyl Jumadinova

13-15 October, 2020

Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 1 / 25

Operators

A Very Unusual Operator: ?

Most operators are either binary (+,−, ∗,

Conditional operator “?”

boolean-expression ? expression1 : expression2

The boolean-expression is evaluated.

If it is true, the value is expression1, otherwise it is expression2.

For example:5 < 10 ? 70 : −3 is 70, while 5 > 10 ? 70 : −3 is −3

For example:5 < 10 ? 70 : −3 is 70,

while 5 > 10 ? 70 : −3 is −3

For example:5 < 10 ? 70 : −3 is 70, while 5 > 10 ? 70 : −3 is −3

Many Other Operators

Bitwise Operators

10|7 = 15 (bitwise “or”)10&7 = 2 (bitwise “and”)

10 > 1 = 5 (right shift)

String operators:

“Hello” + “world”

Referencing/dereferencing operators (C):

&, ∗,→

Bitwise Operators

String operators:

&, ∗,→

Bitwise Operators

String operators:

&, ∗,→

Operators in Other Languages

COBOL Operators:

Arithmetic

Logical (AND, OR, NOT)

Relational (IS [NOT] LIKE, IS LESS THAN ..)

COBOL Basics

Every variable must be described in the DATA DIVISION.

Data Categories:

Variables

Literals: String/Alphanumeric Literals and Numeric Literals

Figurative Constants

SPACE or SPACES - Acts like one or more spaces

ZERO or ZEROS or ZEROES - Acts like one or more zeros

QUOTE or QUOTES - Used instead of a quotation mark

HIGH-VALUE or HIGH-VALUES - Uses the maximum value possible

LOW-VALUE or LOW-VALUES - Uses the minimum value possible

ALL literal - Allows a ordinary literal to act as

Figurative Constant

COBOL Basics

Every variable must be described in the DATA DIVISION.

Data Categories:

Variables

Literals: String/Alphanumeric Literals and Numeric Literals

Figurative Constants

SPACE or SPACES - Acts like one or more spaces

ZERO or ZEROS or ZEROES - Acts like one or more zeros

QUOTE or QUOTES - Used instead of a quotation mark

HIGH-VALUE or HIGH-VALUES - Uses the maximum value possible

LOW-VALUE or LOW-VALUES - Uses the minimum value possible

ALL literal - Allows a ordinary literal to act as

Figurative Constant

COBOL Basics

Data Types:

Numeric

Alphanumeric (text/string)

Alphabetic

Declaring Data-Items in COBOL

A variable (elementary item) declaration consists of a line in the DATADIVISION that contains the following:

A level number

A data-name or identifier.

A Picture clause.

9 Indicates the occurrence of a digit.X Indicates the occurrence of any character from the character set.A Indicates the occurrence of any alphabetic character (A to Z plus blank).V Indicates the position of the decimal point in a numeric value.S Indicates the presence of a sign and can only appear at the beginning of PIC.

A variable (elementary item) declaration consists of a line in the DATADIVISION that contains the following:

A level number

A data-name or identifier.

A Picture clause.

9 Indicates the occurrence of a digit.X Indicates the occurrence of any character from the character set.A Indicates the occurrence of any alphabetic character (A to Z plus blank).V Indicates the position of the decimal point in a numeric value.S Indicates the presence of a sign and can only appear at the beginning of PIC.

It maybe convenient to treat a collection of elementary items as a singlegroup– (e.g., group YearofBirth, MonthofBirth, DayOfBirth under thegroup name - DateOfBirth).

Group items are declared using a level number and a data name only.

Hierarchical relationship between the various subordinate items of thegroup is expressed using level numbers.

The higher the level number, the lower the item is in the hierarchy.

It maybe convenient to treat a collection of elementary items as a singlegroup– (e.g., group YearofBirth, MonthofBirth, DayOfBirth under thegroup name - DateOfBirth).

Group items are declared using a level number and a data name only.

Hierarchical relationship between the various subordinate items of thegroup is expressed using level numbers.

The higher the level number, the lower the item is in the hierarchy.

Assignment Statements in COBOL

1 MOVE statement (MOVE 25 TO NUM1 NUM3.)

2 Computation written out as COMPUTE var = var operator var.or by using ADD, DIVIDE, MULTIPLY, SUBTRACT ... GIVING.

Conditional Branches

Familiar to most novice programmers:

“if” and “if-else” statements “switch” statements

Basic idea: if (condition) then ... else ...

It wasn’t always quite this easy, though

Conditional branches–switch statements

In C and Java:

switch(i) {

case 0:

case 2:

case 4: System.out.println(i+": even,

Conditional branches–switch statements

Without break statements?

i=0;

switch(i) {

case 0:

case 2:

case 4: System.out.println(i+": even, 4");

}

Short Circuit Evaluation

According to the laws of logic, order doesn’t matter in “and”:

“p AND q” is the same as “q AND p”.Similarly, for OR.

But in Java and C, order of evaluation is important:

int i = 10, j = 0, k = 0;

if (i > 10 && 5/j < 3) {

k = 5;

}

Since i > 10 is false, there is no need to look at the second condition–wealready know that the “&&” will be false.

int i = 10, j = 0, k = 0;

if (i > 10 && 5/j < 3) {

k = 5;

}

int i = 10, j = 0, k = 0;

if (i > 10 && 5/j < 3) {

k = 5;

}

If we switch the ordering:

int i = 10, j = 0, k = 0;

if ( 5/j < 3 && i > 10 ) {

k = 5;

}

If we start with 5/j < 3, we”ll get a “division by zero” error.

If we switch the ordering:

int i = 10, j = 0, k = 0;

if ( 5/j < 3 && i > 10 ) {

k = 5;

}

If we start with 5/j < 3, we”ll get a “division by zero” error.

Short circuit evaluation is used often in situations like this:

if (i >= 0 && sqrt(i) > 5.0) ...

By checking i >= 0 first, we guarantee that we won’t try taking squareroot of a negative value.

More generally,

if (valid(data) && meets criteria(data)) ... It is more efficientthan evaluating both operands and then performing an “and” or an “or”on them.

Short circuit evaluation is used often in situations like this:

if (i >= 0 && sqrt(i) > 5.0) ...

By checking i >= 0 first, we guarantee that we won’t try taking squareroot of a negative value.

More generally,

if (valid(data) && meets criteria(data)) ... It is more efficientthan evaluating both operands and then performing an “and” or an “or”on them.

What if, for some reason, we WANT both operands to be evaluated?

Languages like Ada provide for both full evaluation of all operandsand also short-circuit operations:

if (a and b) : full evaluation of both a and b

if (a and then b) : short-circuit--quit if a is false

What if, for some reason, we WANT both operands to be evaluated?

Languages like Ada provide for both full evaluation of all operandsand also short-circuit operations:

if (a and b) : full evaluation of both a and b

if (a and then b) : short-circuit--quit if a is false

Old FORTRAN Days

The “go to” Statement

“go to” is an UNCONDITIONAL branch.

Most early programming languages had “go to” statements.

Later languages like C also adopted them.

But, they were easy to misuse.

HUGE response. Letter is now famous; many imitations. “Consideredharmful” essays appear about almost every topic in computer science:

XMLHttpRequest Considered Harmful

Csh Programming Considered Harmful

Turing Test Considered Harmful

Considered Harmful Essays Considered Harmful

... etc ...

But why?

We can “break out of scope” with a goto (the for-loop block mighthave its own local variables)

We can write incomprehensible code (“spaghetti code”)

IN-CLASS EXERCISE (Oct. 15th): C and goto

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Programming Language Concepts Control Flow · Programming Language Concepts Control Flow Janyl Jumadinova 13-15 October, 2020 Janyl Jumadinova Programming Language Concepts Control