Question about while vs if statements

arjones85

Quick question regarding how the = sign is used for those particular statements. In a while loop, comparing two variables is:

while ($var1 = $var2)

But in an if statement, it is:

if ($var1 == $var2)

Why does the while loop use only 1 equal sign?

NogDog

No, while($var1 = $var2) assigns the value of $var2 to $var1, it does not compare them. If you want to do a comparison, always use "==" (or "===" if you want to compare that the values are both the same type and value).

Piranha

I guess that you might refer to

while ($row = mysql_fetch_whatever($result))

That is a special case. It assigns $row with the next result from the database, and you use that variable in the loop. When there is no more results it will return false and it will exit the loop.

arjones85

Oh okay, that is exactly what I was referring to Piranha.

Thanks for clearing that up!

Piranha

Next time please be more specific. If you mean a specific case (like above) post that exactly as it is. Otherwise it could be hard to know what you catually mean, and it takes extra time and effort to solve your problems.

arjones85

All of the cases I had seen regarding while statements, when what appeared to be comparing, only used one = sign.

I was not aware that the statement when actually comparing used two = sign's, as I had yet to see a while statement that used two, so I thought it was a normal thing and was just curious as to why it was different than an if statement when comparing variables, which I now know it was not different.

My apologies, I thought I was being clear enough

Piranha

No problem, I didn't mean that you didn't supply enough information. I only ment to give you a tip for next time.

NogDog

arjones85 wrote:
All of the cases I had seen regarding while statements, when what appeared to be comparing, only used one = sign.

I was not aware that the statement when actually comparing used two = sign's, as I had yet to see a while statement that used two, so I thought it was a normal thing and was just curious as to why it was different than an if statement when comparing variables, which I now know it was not different.

My apologies, I thought I was being clear enough

A little clarification: when you do an "if(<condition>) {" or a "while(<condition>) {", the "<condition>" expression does not have to be a comparison. Whatever it is, it is evaluated and, if not already so, cast into a boolean true or false. Therefore, each of the following would all evaluate at true:

if(true) {
if(1) {
if(1 == 1) {
if($var = true) {  // assignment here
if($var = 'non-emtpy string') {  // another assignment

The following would all evaluate as false:

if(false) {
if(0) {
if(1 == 2) {
if($var = false) {  // assignment here
if($var = '') {  // another assignment

Weedpacket

And just to add another couple of examples to NogDog's list:

if($var==false) { // comparsion: true if $var is false - doesn't assign anything to $var
if(1 = 2) { // compiler error

And of course the same for all examples with "if" replaced by "while".