String data type

Introduction

Maybe you already know what is a string variable from previous program examples that you have tried, but you don't know what operations and what functions one can apply to them and how can they be manipulated in order to obtain another form of string whatsoever.

A data type used in CREATE TABLE and ALTER TABLE statements.

Syntax:

In the column definition of a CREATE TABLE statement:

column_name STRING

Length: Maximum of 32,767 bytes. Do not use any length constraint when declaring STRING columns, as you might be familiar with from VARCHAR, CHAR, or similar column types from relational database systems. If you do need to manipulate string values with precise or maximum lengths, in Impala 2.0 and higher you can declare columns as VARCHAR(max_length) or CHAR(length), but for best performance use STRING where practical.

Character sets: For full support in all Impala subsystems, restrict string values to the ASCII character set. Although some UTF-8 character data can be stored in Impala and retrieved through queries, UTF-8 strings containing non-ASCII characters are not guaranteed to work properly in combination with many SQL aspects, including but not limited to:

  • String manipulation functions.
  • Comparison operators.
  • The ORDER BY clause.
  • Values in partition key columns.

For any national language aspects such as collation order or interpreting extended ASCII variants such as ISO-8859-1 or ISO-8859-2 encodings, Impala does not include such metadata with the table definition. If you need to sort, manipulate, or display data depending on those national language characteristics of string data, use logic on the application side.

Conversions:

  • Impala does not automatically convert STRING to any numeric type. Impala does automatically convert STRING to TIMESTAMP if the value matches one of the accepted TIMESTAMP formats; see TIMESTAMP Data Type for details.

  • You can use CAST() to convert STRING values to TINYINT, SMALLINT, INT, BIGINT, FLOAT, DOUBLE, or TIMESTAMP.

  • You cannot directly cast a STRING value to BOOLEAN. You can use a CASE expression to evaluate string values such as 'T', 'true', and so on and return Boolean true and false values as appropriate.

  • You can cast a BOOLEAN value to STRING, returning '1' for true values and '0' for false values.

Partitioning:

Although it might be convenient to use STRING columns for partition keys, even when those columns contain numbers, for performance and scalability it is much better to use numeric columns as partition keys whenever practical. Although the underlying HDFS directory name might be the same in either case, the in-memory storage for the partition key columns is more compact, and computations are faster, if partition key columns such as YEAR, MONTH, DAY and so on are declared as INT, SMALLINT, and so on.

Zero-length strings: For purposes of clauses such as DISTINCT and GROUP BY, Impala considers zero-length strings (""), NULL, and space to all be different values.

Text table considerations: Values of this type are potentially larger in text tables than in tables using Parquet or other binary formats.

Task

1. What assigning values are wrong and that there are mistakes observed?

Var b,s: string; k,n,m: integer; c,t: char;

begin

     Readln(s);   t:=length(s);  m:=pos(K,s);   c:=copy(s,k-7,4);  c:=copy(s,k,4);   

  c:=s[2];   c:=s+s;  s:=c+c;  insert(c,s);   b:=delete(s,4,3);  Val(m,c,cod);

end.

 

2.What effect has the following sequence of instructions? Indicate the types of variables contained in this sequence of instructions.

b1:=true; b2:=true; suma:=0.1; readln(s); val(s,n,cod);  

if cod =0 then suma :=n else b1:=false; readln(s); val(s,n,cod);

if cod =0 then suma :=suma+n else b2:=false;

if b1 and b2 then writeln(suma) else writeln('Nu este posibila calcularea sumei');

 

3. For more information: https://www.tutorialspoint.com/pascal/pascal_strings.htm

 

Conclusion

In computer programming, a string is traditionally a sequence of characters, either as a literal constant or as some kind of variable. The latter may allow its elements to be mutated and the length changed, or it may be fixed (after creation). A string is generally understood as a data type and is often implemented as an array data structure of bytes (or words) that stores a sequence of elements, typically characters, using some character encoding. A string may also denote more general arrays or other sequence (or list) data types and structures.

Depending on programming language and precise data type used, a variable declared to be a string may either cause storage in memory to be statically allocated for a predetermined maximum length or employ dynamic allocation to allow it to hold a variable number of elements.

When a string appears literally in source code, it is known as a string literal or an anonymous string.

In formal languages, which are used in mathematical logic and theoretical computer science, a string is a finite sequence of symbols that are chosen from a set called an alphabet.