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1. Top results related to what is the purpose of coalesce?

   • What is the purpose of the byte size of the type of a variable if I know the address of the variable?

     Top Answer

     Answered Sep 25, 2012 · 4 votes

     Michael Krelin gave a very compact answer but I think I can expand on it a bit more.

     In any language, not just C, you need to know the size for a variety of reasons:

     • This determines the maximum value that can be stored
     • The memory space an array of those values will take (1000 bytes will get you 250 ints or 125 longs).
     • When you want to copy one array of values into another, you need to know how many bytes are used to allocate enough space.
     • While you may dereference a pointer and get the value, you could dereference the pointer at a different portion of that value, but only if you know how many bytes it is composed of. You could get the high-value of an int by grabbing just the first two bytes, and the low value by getting the last two bytes.
     • Different architectures may have different sizes for different variables, which would impact all the above points.

     Edit:

     Also, there are certainly reasons where you may need to know the number of bits that a given variables is made of. If you want 32 booleans, what not a better variable to use than a single int, which is made of 32 bits? Then you can use some constants to create pointers to each bit and now you have an "array" of booleans. These are usually called bit-fields (correct me if I am wrong). In programming, every detail can matter, just not all the time for every application. Just figured that might be an interesting thought exercise.

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     1/5

   • What is the purpose of the assembler and symbol table? What is at a symbol's address?

     Top Answer

     Answered Aug 06, 2021 · 3 votes

     A label IS an address, it is a way for programmers to provide an address to the assembler but not have to know the physical address. Let the toolchain do that work for you.

     I don't remember my MIPS off hand so here is some pseudo code.

     loop_top: nop nop sub r0,1 cmp r0,0 bne loop_top

     Depending on the instruction set, but in general the conditional branch will be pc-relative. Tables in general used during assembly with one or more passes on the table will resolve the distance between the branch and the destination so that the branch can be encoded completely. Most instruction sets the above can be resolved in one pass. loop_top is a label that will have an address, but for the branch here it is pc-relative and you don't need to know the physical address.

     But

     call my_fun-

     once making a pass on the code, the assembler finds that my_fun is not defined in this file and/or the assembly language has some syntax to mark it as external before used. Either way it is external. Cannot be resolved at the time this file is assembled. So tables are required indicating the label name, and where in this object that instruction lives, depending on the assembler it may fill in the temporary offset or full address as zero for now or encode it as an infinite loop.

     The linker later determines the actual address for things in the processors memory space, the linker will ultimately have a table of all (relevant labels at this phase of the toolchain) labels and their addresses while linking, then the linker will go back into the code and repair/create the machine code for this call instruction now that it knows what the actual address is for that label.

     j hello-

     the object:

     Disassembly of section .text:-00000000 <.text>: 0: 08000000 j 0x0 4: 00000000 nop

     another object:

     .globl hellohello: j hello.word hello

     link them

     Disassembly of section .text:-00001000 <_ftext>: 1000: 08000402 j 1008 <hello> 1004: 00000000 nop00001008 <hello>: 1008: 08000402 j 1008 <hello> 100c: 00000000 nop 1010: 00001008 0x1008

     As objects all the toolchain has to go on is the label hello being used as an address to be resolved later. In this case at link time, the linker works through the objects, counting bytes making a table of labels and their addresses. During the first or some other pass it will change the instructions or data as needed to resolve these labels.

     Now old school assemblers that did the job of assembling and linking from the same source file, the statement "assembler must determine the addresses corresponding to all labels". It is not the assembler in general with commonly used toolchains that does the linker work. So that quoted statement could use some improvement. But hopefully this demonstrates that labels are addresses, they represent a yet to be determined address so the code is easier to write than something like this

     nop nop j pc-2

     then if you add another instruction

     nop add r0,r1 nop j pc-3

     Or

     j 0x1008-

     then have to spend a significant amount of time re-writing the program to get each and every address hard-coded into the program. Add/remove a single line and a lot of other code has to be changed. Labels representing addresses make that all significantly easier and the toolchain determines addresses, then goes back and replaces the labels with addresses basically...

     Added a nop:

     Disassembly of section .text:-00001000 <_ftext>: 1000: 08000403 j 100c <hello> 1004: 00000000 nop 1008: 00000000 nop0000100c <hello>: 100c: 08000403 j 100c <hello> 1010: 00000000 nop 1014: 0000100c

     If we didn't have labels and had to hard-code the address instead then you would have to change those three places as a result of the nop. One line. If you added dozens of lines, hundreds. How would you keep track of it all? By putting labels in comments? Assemble and disassemble and patch up the source over and over again until it looked somewhat right and hope for no bugs.

     mips-elf-readelf -s so.elf-Symbol table '.symtab' contains 14 entries: Num: Value Size Type Bind Vis Ndx Name 0: 00000000 0 NOTYPE LOCAL DEFAULT UND 1: 00001000 0 SECTION LOCAL DEFAULT 1 2: 00400000 0 SECTION LOCAL DEFAULT 2 3: 00400018 0 SECTION LOCAL DEFAULT 3 4: 00000000 0 SECTION LOCAL DEFAULT 4 5: 0000a010 0 NOTYPE LOCAL DEFAULT 2 _gp 6: 00002018 0 NOTYPE GLOBAL DEFAULT 4 _fdata 7: 0000100c 0 OBJECT GLOBAL DEFAULT 1 hello 8: 00001000 0 NOTYPE GLOBAL DEFAULT 1 _ftext 9: 00000000 0 NOTYPE GLOBAL DEFAULT UND _start 10: 00002018 0 NOTYPE GLOBAL DEFAULT 2 __bss_start 11: 00002018 0 NOTYPE GLOBAL DEFAULT 2 _edata 12: 00002018 0 NOTYPE GLOBAL DEFAULT 2 _end 13: 00002018 0 NOTYPE GLOBAL DEFAULT 2 _fbss

     and here is the one of interest:

     7: 0000100c 0 OBJECT GLOBAL DEFAULT 1 hello-

     the label hello once assembled and linked into a final binary is equal to address 0x100C.

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     2/5

   • What is the purpose of using unclass() function? and why the error ""the column indexes must be at most 2 if postive,not 3,4,5,6,7,8,9,10" appears

     Top Answer

     Answered Mar 06, 2019 · 16 votes

     Let me try explaining the usage of 'unclass'

     Let's suppose you have this vector of colors:

     cores = c('blue','green','red')-

     And a group of strings stored as factors, for example:

     val = c('setosa','setosa','virginica','versicolor','virginica','setosa')val_fac = factor(val)

     If you apply unclass to this group of factors, unclass will convert the factors to their numbers, like:

     unclass(val_fac)[1] 1 1 3 2 3 1attr(,"levels")[1] "setosa" "versicolor" "virginica"

     With these numbers you can convert the factors to colors, by doing:

     cores[unclass(val_fac)][1] "blue" "blue" "red" "green" "red" "blue"

     Hope this helps you,

     Best regards,

     Gustavo,

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     3/5

   • What is the purpose of the reverse loop and where is the element being injected to the array

     Top Answer

     Answered Jan 18, 2022 · 1 votes

     Very short:

     The loop copies each item from index i - 1 to index i. If the loop was not in reverse order, it would copy first item to every item.

     Then it overwrites item at index 0 (now also copied to index 1) with my_item.

     A more readable way to write the loop would be to use array indexing instead of plain pointer arithmetic. This is exactly equal code:

     for (i = arr->nelts - 1; i > 0; --i) { (some_pointer *)arr->elts[i] = (some_pointer *)arr->elts[i-1];}(some_pointer *)arr->elts[0] = my_item;

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     4/5

   • What is the purpose of the condition "if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {" in the Firebase Cloud Messaging sample project?

     Top Answer

     Answered Nov 24, 2017 · 6 votes

     What is the purpose of using the condition

     To avoid executing that block of code on devices older than Android 8.0. Notification channels were added in Android 8.0. Attempting to call createNotificationChannel() on older devices would result in a crash, as that method will not exist.

     This is a standard backwards-compatibility recipe. Frequently, utility classes hide this stuff (e.g., most of the classes named ...Compat in the SDK), but sometimes, as is the case here, we get to do it ourselves.

     Is the code asking to not execute the code that creates the channel to show notifications, if the user has a device with an API level that is lower than the compileSdkVersion that I am using to define which SDK version I am compiling against?

     Yes.

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     5/5

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2. People also ask

   What is a COALESCE function?

   • The COALESCE function accepts a number of arguments and returns the first non-NULL argument. The following illustrates the syntax of the COALESCE function. COALESCE(argument1, argument2,...); Code language: SQL (Structured Query Language) (sql) The COALESCE function evaluates its arguments from left to right.

   SQL COALESCE Function: Handling NULL Values Effectively

   www.sqltutorial.org/sql-comparison-functions/sql-coalesce/

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   What does select coalesce() do?

   • Evaluates the arguments in order and returns the current value of the first expression that initially doesn't evaluate to NULL. For example, SELECT COALESCE(NULL, NULL, 'third_value', 'fourth_value'); returns the third value because the third value is the first value that isn't null. Transact-SQL syntax conventions Is an expression of any type.

   COALESCE (Transact-SQL) - SQL Server | Microsoft Learn

   learn.microsoft.com/en-us/sql/t-sql/language-elements/coalesce-transact-sql?view=sql-server-ver16

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   Which database systems support COALESCE function?

   • Almost all relational database systems support the COALESCE function e.g., MySQL, PostgreSQL, Oracle, Microsoft SQL Server, Sybase. Note that the COALESCE function is the most generic function of the NVL function and can be used instead of the NVL function. Assuming that we have a products table with the following structure and data:

   SQL COALESCE Function: Handling NULL Values Effectively

   www.sqltutorial.org/sql-comparison-functions/sql-coalesce/

   See all results for this question

   Why does the COALESCE function stop evaluating the remaining arguments?

   • This is because the COALESCE function is short-circuited. It stops evaluating the remaining arguments after it finds the first non-NULL arguments. Almost all relational database systems support the COALESCE function e.g., MySQL, PostgreSQL, Oracle, Microsoft SQL Server, Sybase.

   SQL COALESCE Function: Handling NULL Values Effectively

   www.sqltutorial.org/sql-comparison-functions/sql-coalesce/

   See all results for this question

3. learnsql.com › blog › coalesce-function-sqlHow to Use the COALESCE() Function in SQL | LearnSQL.com

   learnsql.com › blog › coalesce-function-sql

   • Cached

   May 24, 2022 · In the above query, the COALESCE() function is used to return the value ‘Unknown’ only when marital_status is NULL. When marital_status is not NULL, COALESCE() returns the value of the column marital_status. In other words, COALESCE() returns the first non-NULL argument. Get to Know the Example Data

4. www.w3schools.com › SQL › func_sqlserver_coalesceSQL Server COALESCE() Function - W3Schools

   www.w3schools.com › SQL › func_sqlserver_coalesce

   • Cached

   Definition and Usage. The COALESCE () function returns the first non-null value in a list. Syntax. COALESCE ( val1, val2, ...., val_n) Parameter Values. Technical Details. More Examples. Example. Return the first non-null value in a list: SELECT COALESCE(NULL, 1, 2, 'W3Schools.com'); Try it Yourself » Previous SQL Server Functions Next .

   Code sample

   -- Return the first non-null value in a list

   SELECT COALESCE(NULL, NULL, NULL, 'W3Schools.com', NULL, 'Example.com');

   See more on w3schools

5. www.sqltutorial.org › sql-coalesceSQL COALESCE Function: Handling NULL Values Effectively

   www.sqltutorial.org › sql-coalesce

   • Cached

   The COALESCE function evaluates its arguments from left to right. It stops evaluating until it finds the first non-NULL argument. It means that all the remaining arguments are not evaluated at all. The COALESCE function returns NULL if all arguments are NULL.

6. Videos

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     What is Coalesce? | Demo for beginners

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     Aug 31, 2021

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     4:28

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     Aug 19, 2012

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7. learn.microsoft.com › en-us › sqlCOALESCE (Transact-SQL) - SQL Server | Microsoft Learn

   learn.microsoft.com › en-us › sql

   • Cached

   Evaluates the arguments in order and returns the current value of the first expression that initially doesn't evaluate to NULL. For example, SELECT COALESCE(NULL, NULL, 'third_value', 'fourth_value'); returns the third value because the third value is the first value that isn't null. Transact-SQL syntax conventions.

8. www.datacamp.com › tutorial › coalesce-sql-functionCOALESCE() SQL Function | DataCamp

   www.datacamp.com › tutorial › coalesce-sql-function

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   What is the COALESCE() function? Coalesce returns the first non-null value in a list. If all the values in the list are NULL, then the function returns null. When to use COALESCE()? This function is useful when combining the values from several columns into one. For example, a table called users contains values of users' work_email and personal ...

9. www.dbvis.com › thetable › the-complete-guide-to-theThe Complete Guide to the SQL COALESCE Function

   www.dbvis.com › thetable › the-complete-guide-to-the

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   Jan 2, 2024 · COALESCE is an SQL function that accepts a list of parameters and returns the first non-NULL value from the list. Here is what the syntax looks like: COALESCE(expression1, expression2, expression, ...) expression1, expression2, expression3, and so on, are the parameters (values or expressions) to be evaluated.

10. www.mssqltips.com › sqlservertip › 6788COALESCE SQL Function - SQL Server Tips

    www.mssqltips.com › sqlservertip › 6788

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    Mar 22, 2021 · What Is the COALESCE Function. The SQL function COALESCE () is a basic SQL function that can be used to evaluate a list of values and return the first non-NULL value. For example, the following list would return 'A'. (NULL, NULL, 'A', 'B', '1', '2') Why use the COALESCE Function.