Why use an assignment in state?

Question

In many languages, assignments are legal under conditions. I never understood the reason for this. Why should you write:

if (var1 = var2) { ... } 

instead:

 var1 = var2; if (var1) { ... } 

Answer 1

This is more useful for loops than for statements.

 while( var = GetNext() ) { ...do something with var } 

What else would have to be written down

 var = GetNext(); while( var ) { ...do something var = GetNext(); } 

Answer 2

This is more useful if you call a function:

 if (n = foo()) { /* foo returned a non-zero value, do something with the return value */ } else { /* foo returned zero, do something else */ } 

Of course you can just put n = foo (); on a separate statement, then if (n), but I think this is higher, this is a pretty readable idiom.

Answer 3

I find it most useful in action chains, which often include error detection, etc.

 if ((rc = first_check(arg1, arg2)) != 0) { report error based on rc } else if ((rc = second_check(arg2, arg3)) != 0) { report error based on new rc } else if ((rc = third_check(arg3, arg4)) != 0) { report error based on new rc } else { do what you really wanted to do } 

An alternative (without using the assignment in the condition) is:

 rc = first_check(arg1, arg2); if (rc != 0) { report error based on rc } else { rc = second_check(arg2, arg3); if (rc != 0) { report error based on new rc } else { rc = third_check(arg3, arg4); if (rc != 0) { report error based on new rc } else { do what you really wanted to do } } } 

With long-term error checking, the alternative may exit the RHS page, while the conditional version does not.

Answer 4

This can be useful if you call a function that returns data for work or a flag to indicate an error (or what you did).

Something like:

 while ((c = getchar()) != EOF) { // process the character } // end of file reached... 

Personally, this is an idiom that I don’t really like, but sometimes the alternative is uglier.

Answer 5

GCC can help you discover (with -Wall) if you inadvertently try to use a destination as a true value, if it recommends writing

 if ((n = foo())) { ... } 

those. use extra parentheses to indicate that this is really what you want.

Answer 6

The idiom is more useful when you write a while instead of an if . For the if you can break it down as you described. But without this design, you will either have to repeat:

 c = getchar(); while (c != EOF) { // ... c = getchar(); } 

or use a loop and half structure:

 while (true) { c = getchar(); if (c == EOF) break; // ... } 

I would prefer a loop-and-half shape.

Answer 7

In PHP, for example, it is useful for cyclically using the results of an SQL database:

 while ($row = mysql_fetch_assoc($result)) { // Display row } 

This looks a lot better than:

 $row = mysql_fetch_assoc($result); while ($row) { // Display row $row = mysql_fetch_assoc($result); } 

Answer 8

The short answer is that Expression-oriented programming languages ​​allow you to use shorter code. Do not force you to separate commands from requests .

Answer 9

Another advantage is the use of gdb. In the following code, the error code is not known if we must take one step.

 while (checkstatus() != -1) { // process } 

Instead

 while (true) { int error = checkstatus(); if (error != -1) // process else //fail } 

Now, in one step, we can find out what the return error code from checkstatus () was.

Answer 10

 if((var = someFunction())){ //Enclosed in additional pair of brackets to show its intentional ... 

The above can be used instead of ...

 var = someFunction(); if(var){ ... 

Answer 11

The reason is this:

• Performance improvement (sometimes)

• Small code (always)

Take an example: there is someMethod() , and in if you want to check if the return value of the method is null . If not, you will use the return value again.

 If(null != someMethod()){ String s = someMethod(); ...... //Use s } 

This will interfere with performance as you call the same method twice. Use instead:

 String s; If(null != (s = someMethod())) { ...... //Use s }