Programming Language Concepts Control Flow Janyl Jumadinova 13-15 - - PowerPoint PPT Presentation

Programming Language Concepts Control Flow

Janyl Jumadinova 13-15 October, 2020

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Operators

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A Very Unusual Operator: ?

Most operators are either binary (+, −, ∗, <, ==, &&, etc.) or unary (“plus sign” +, “minus sign” -, ++, !, etc.). However, C and Java also have a ternary operator (takes 3 arguments).

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A Very Unusual Operator: ?

Most operators are either binary (+, −, ∗, <, ==, &&, etc.) or unary (“plus sign” +, “minus sign” -, ++, !, etc.). However, C and Java also have a ternary operator (takes 3 arguments). Conditional operator “?” boolean-expression ? expression1 : expression2

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A Very Unusual Operator: ?

Conditional operator “?” boolean-expression ? expression1 : expression2 The boolean-expression is evaluated. If it is true, the value is expression1, otherwise it is expression2.

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A Very Unusual Operator: ?

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,

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A Very Unusual Operator: ?

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

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Many Other Operators

Bitwise Operators 10|7 = 15 (bitwise “or”) 10&7 = 2 (bitwise “and”) 10 << 3 = 80 (left shift) 10 >> 1 = 5 (right shift)

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Many Other Operators

Bitwise Operators 10|7 = 15 (bitwise “or”) 10&7 = 2 (bitwise “and”) 10 << 3 = 80 (left shift) 10 >> 1 = 5 (right shift) String operators: “Hello” + “world”

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Many Other Operators

Bitwise Operators 10|7 = 15 (bitwise “or”) 10&7 = 2 (bitwise “and”) 10 << 3 = 80 (left shift) 10 >> 1 = 5 (right shift) String operators: “Hello” + “world” Referencing/dereferencing operators (C): &, ∗, →

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Operators in Other Languages

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Operators in Other Languages

COBOL Operators: Arithmetic Logical (AND, OR, NOT) Relational (IS [NOT] LIKE, IS LESS THAN ..)

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COBOL Basics

Every variable must be described in the DATA DIVISION.

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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

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COBOL Basics

Data Types: Numeric Alphanumeric (text/string) Alphabetic

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Declaring Data-Items in COBOL

A variable (elementary item) declaration consists of a line in the DATA DIVISION that contains the following: A level number A data-name or identifier. A Picture clause.

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Declaring Data-Items in COBOL

A variable (elementary item) declaration consists of a line in the DATA DIVISION 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.

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Declaring Data-Items in COBOL

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

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Declaring Data-Items in COBOL

It maybe convenient to treat a collection of elementary items as a single group – (e.g., group YearofBirth, MonthofBirth, DayOfBirth under the group name - DateOfBirth). Group items are declared using a level number and a data name only. Hierarchical relationship between the various subordinate items of the group is expressed using level numbers. The higher the level number, the lower the item is in the hierarchy.

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Assignment Statements in COBOL

1 MOVE statement (MOVE 25 TO NUM1 NUM3.) 2 Computation written out as COMPUTE var = var operator var.

• r by using ADD, DIVIDE, MULTIPLY, SUBTRACT ...

GIVING.

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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

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Conditional branches–switch statements

In C and Java: switch(i) { case 0: case 2: case 4: System.out.println(i+": even, <= 4"); break; case 1: System.out.println(i+" is one"); break; default: }

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Conditional branches–switch statements

Without break statements? i=0; switch(i) { case 0: case 2: case 4: System.out.println(i+": even, <= 4"); case 1: System.out.println(i+" is one"); default: System.out.println(i+": odd or > 4"); }

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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.

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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:

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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–we already know that the “&&” will be false.

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Short Circuit Evaluation

If we switch the ordering:

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Short Circuit Evaluation

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.

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Short Circuit Evaluation

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 square root of a negative value.

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Short Circuit Evaluation

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 square root of a negative value. More generally, if (valid(data) && meets criteria(data)) ... It is more efficient than evaluating both operands and then performing an “and” or an “or”

• n them.

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Short Circuit Evaluation

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

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Short Circuit Evaluation

What if, for some reason, we WANT both operands to be evaluated? Languages like Ada provide for both full evaluation of all operands and 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

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Old FORTRAN Days

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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.

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The “go to” Statement

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The “go to” Statement

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The “go to” Statement

HUGE response. Letter is now famous; many imitations. “Considered harmful” 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 ...

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The “go to” Statement

But why? We can “break out of scope” with a goto (the for-loop block might have its own local variables) We can write incomprehensible code (“spaghetti code”) IN-CLASS EXERCISE (Oct. 15th): C and goto

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