ASSERT failure in QList<T>::operator[]: "index out of range"

Jerwinprabu

I am still relatively new to Qt and I have recently been working on a large project. When I attempt to run the project I get this error:

/ASSERT failure in QList<T>::operator[]: "index out of range", file /usr/include/arm-linux-gnueabihf/qt5/QtCore/qlist.h, line 487
The program has unexpectedly finished.

How I might go about tracking down the source of the problem?

I believe that the addition of this code is causing the error

startvaluexy = Client::straightxy;
qDebug() << "start value Received from server :" << startvaluexy;
QStringList xy = startvaluexy.split("|");

x = xy[2];

QString num1 = x;
int x = num1.toInt();

qDebug() << "start x value :" << x;

y = xy[3];

QString num2 = y;
int y = num2.toInt();

qDebug() << "start y value :" << y;

When this x = xy[2]; y = xy[3]; is taken out, then runs fine.

JohanSolo

What is the real size of your xy list?

jsulm

@Jerwinprabu You should never assume that you get what you expect!
You expect that this line

QStringList xy = startvaluexy.split("|");

will generate a list with at least 4 elements. But whether this is the case depends on the content of startvaluexy.
So, you should check the length of xy before accessing any elements of it!
"index out of range" - this message actually tells you what is wrong: you're accessing an element in the list which does not exist.

joeQ

@Jerwinprabu Hi,friend,welcome.

I modified your code snippet.

startvaluexy = Client::straightxy;
qDebug() << "start value Received from server :" << startvaluexy;
QStringList xy = startvaluexy.split("|");
int xySize = sy.size(); ///< get the xy list size;

if(xySize < 4){
    return false;
}

bool ok;
int x = xy[2].toInt(&ok); ///< you should check whether convert success
if(!ok){
    return false;
}

int y = xy[3].toInt(&ok);
if(!ok){
    return false;
}

// when run here, you will get the correct x,y value
return true;

Jerwinprabu

If I sent start command from server, output displayed like this on client side

WebSocket connected
message received : "straight|xy|3|0" 

start xy value Received from server : "straight|xy|3|0"
start x value emitted to Robot : 3
start y value emitted to Robot: 0
int of start x value emitted : 3
int start y value emitted : 0

If I click start button on client side that time only I got the error.

WebSocket connected

start xy value Received from server : ""
ASSERT failure in QList<T>::operator[]: "index out of range", file /usr/include/arm-linux-gnueabihf/qt5/QtCore/qlist.h, line 487

jsulm

@Jerwinprabu You see: you're trying to split an empty string, this is not going to work. So please add some check to your code as @joeQ suggested.
And you should check why you don't get what server sent, but this is another issue.

Jerwinprabu

@joeQ When I was try to run the program I got the error

error: return-statement with a value, in function returning 'void' [-fpermissive]
         return false;

joeQ

@Jerwinprabu (⊙o⊙)…

bool myFunctionName() ///< Notes； function return value type must bool
{
      /* ... */
      return true;
}

jsulm

@Jerwinprabu @joeQ Or just do

return;

if you do not use the return value...

Jerwinprabu

@jsulm @joeQ Thanks for your help. Ya @joeQ given proper sample. I have tried that. Here I have posted the solution now client side that is working fine, one time I want to check from server side. Kindly check, if anything wrong awaiting for your further response

startvaluexy = Client::straightxy;
qDebug() << "start xy value Received from server :" << startvaluexy;
QStringList xy = startvaluexy.split("|");
int xySize = xy.size(); 
qDebug() << "start xy size :" << xySize;

if(xySize > 4){
     x = xy[2];
     y = xy[3];
  }

     QString num1 = x;
     int x = num1.toInt();

     qDebug() << "start x value  :" << x;

     QString num2 = y;
     int y = num2.toInt();

     qDebug() << "start y value :" << y;

    xstart = x;
    ystart = y;

After checking the size will modify this xySize > 4

jsulm

@Jerwinprabu said in ASSERT failure in QList<T>::operator[]: "index out of range":

if(xySize > 4){

shouldn't it be

if(xySize >= 4){

?
And if it is not >= 4 what would be the values of x and y? Shouldn't you just skip processing in this case (do return as @joeQ suggested)?

What are you trying to do with this

QString num1 = x;
int x = num1.toInt();

?! Why do you assign an integer to a string and then convert this string to integer again? Same for

QString num2 = y;
int y = num2.toInt();

Jerwinprabu

@jsulm from server I will receive the string then I want to convert that string into int. Again I want to convert for sending to server(for map).

jsulm

@Jerwinprabu Then why do you convert it back from string to int? x is already int, what's the point to convert from string to int?
Also

QString num1 = x;

is not a conversion to string from int! It will not even compile. If you want to convert int to string then do it like shown here http://doc.qt.io/qt-5/qstring.html#number

Jerwinprabu

@jsulm Because I want to take that value to int only xstart, ystart. I want to convert from string to int only. x, y is not int.

QString x;
QString y;

jsulm

@Jerwinprabu Again

QString num1 = x;
int x = num1.toInt();

x already contains the integer value! Now you convert it to string then again to int - it will be same as before.

int x = 1;
QString num1 = QString::number(x);
x = num1.toInt();
qDebug() << x;

What do you think the last line will print out?
Shouldn't it be just

QString num1 = QString::number(x);
xstart = x;

?

jsulm

@Jerwinprabu Also here you redefine the variable x

QString num1 = x;
int x = num1.toInt();

is there a reason why you're doing this?

jsulm

After chatting with @Jerwinprabu I realised that I misunderstood his code: x and y are actually QString not int.

mrjj

@jsulm
Well x is a pretty confusing name for a string in most cases.
I was sure also it was int.