How to resolve error:label and prediction size not matching while implementing XGBoost in R

@hackers wrote:

Hello,

I am trying to use xgboost for a binary classification problem.

# xgboost parameters
param <- list("objective" = "binary:logistic",    # binary classification 
              "num_class" = 2,    # number of classes 
              "eval_metric" = "error",    # evaluation metric 
              "nthread" = 8,   # number of threads to be used 
              "max_depth" = 16,    # maximum depth of tree 
              "eta" = 0.3,    # step size shrinkage 
              "gamma" = 0,    # minimum loss reduction 
              "subsample" = 1,    # part of data instances to grow tree 
              "colsample_bytree" = 1,  # subsample ratio of columns when constructing each tree 
              "min_child_weight" = 12)  # minimum sum of instance weight needed in a child

Here since the classification is binary I am using error as the eval_metric as per:

However when I am running the xgboost model:

system.time( bst.cv <- xgb.cv(param=param, data=train.matrix, label=train$Survived, 
                              nfold=10, nrounds=nround.cv, prediction=TRUE, verbose=FALSE) )

I am getting an error:

Error in xgb.iter.eval(fd$booster, fd$watchlist, i - 1, feval) : 
  label and prediction size not matchhint: use merror or mlogloss for multi-class classification

However if I change the eval_metric to "merror" this works fine.

Why this discrepancy and how to resolve it??

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