Programming Language Concepts Control Flow · Programming Language Concepts Control Flow Janyl...
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Programming Language ConceptsControl Flow
Janyl Jumadinova
13-15 October, 2020
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 1 / 25
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Operators
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A Very Unusual Operator: ?
Most operators are either binary (+,−, ∗,
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A Very Unusual Operator: ?
Most operators are either binary (+,−, ∗,
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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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 4 / 25
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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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 4 / 25
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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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 4 / 25
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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):
&, ∗,→
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 5 / 25
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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):
&, ∗,→
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 5 / 25
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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):
&, ∗,→
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 5 / 25
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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.
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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 8 / 25
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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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 8 / 25
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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 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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 10 / 25
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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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 10 / 25
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Declaring Data-Items in COBOL
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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 11 / 25
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Declaring Data-Items in COBOL
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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 11 / 25
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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.or 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,
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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");
}
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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–wealready know that the “&&” will be false.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 16 / 25
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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–wealready know that the “&&” will be false.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 16 / 25
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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–wealready know that the “&&” will be false.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 16 / 25
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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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 17 / 25
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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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 17 / 25
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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 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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 18 / 25
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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 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.
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 18 / 25
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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 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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 19 / 25
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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 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
Janyl Jumadinova Programming Language Concepts Control Flow 13-15 October, 2020 19 / 25
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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. “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 ...
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The “go to” Statement
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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