Why would you use an assignment in a condition?

Question

In many languages assignments are legal in conditions. I never understood the reason behind this. Why would you write:

if (var1 = var2) {   ... } 

instead of:

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

Answer 1

It's more useful for loops than if statements.

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

Which would otherwise have to be written

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

Answer 2

I find it most useful in chains of actions which often involve 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 } 

The alternative (not 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 protracted error checking, the alternative can run off the RHS of the page whereas the assignment-in-conditional version does not do that.

The error checks could also be 'actions' — first_action(), second_action(), third_action() — of course, rather than just checks. That is, they could be checked steps in the process that the function is managing. (Most often in the code I work with, the functions are along the lines of pre-condition checks, or memory allocations needed for the function to work, or along similar lines).