Rohit:

You are right of course and here is the way it should look

In[250]:= data[[ 1]]
Out[250]= "Total Number of Cards =  6 "

(* BEGIN PROLOGUE to do a bunch of cards
  The variable data has been set up above. data is a list of 101strings *)

numCards = StringExtract [ data[[1]], 6 ]
"6"

In[295]:= Head[numCards]
Out[295]= String

In[296]:= numCards = ToExpression [numCards ]
Out[296]= 6

In[297]:= Head [ numCards ]
Out[297]= Integer

Because of the way the metaData is set up I know where the integers are to be found in each metaData string. Otherwise ToExpression wouldn't work.

Thanks again.

LR

StringExtract requires 2 arguments. This fails to evaluate

StringExtract ["Total Number of Cards = 6 "]

Assuming

blix = "Total Number of Cards = 6 "

This

ToExpression[blix]

fails with "Set::write: Tag Times in Cards Number of Total is Protected." Because it is equivalent to evaluating

Total Number of Cards = 6

Total and Number are built-in functions, space between symbols is interpreted as Times. It also has the side effect of creating symbols named of and Cards.

Lesson: Beware of evaluating arbitrary strings using ToExpression. The extractInteger function I suggested earlier is the safe way to do it because ToExpression is only evaluated on a string of DigitCharacter.

It seems "pretty convoluted" because built-in functions are designed to handle the general case, not just a very specific case. The language would be pretty limiting otherwise. What if the string had more than one integer?

button = "There are 314 buttons, 10 are green and 20 are blue";
button // StringCases[d : DigitCharacter .. :> ToExpression[d]]
(* {314, 10, 20} *)

That is why it returns a List of the matches. You can then extract whichever number you need using Part.

If you just need to handle a specific case of extracting one integer then write a function to do that.

(* Returns the first one by default *)
extractInteger[s_String, position_Integer : 1] := 
 s // StringCases[d : DigitCharacter .. :> ToExpression[d]] // 
   Flatten // Part[#, position] &

extractInteger[button]
(* 314 *)

extractInteger[button, 3]
(* 20 *)

Hi Lewis,

There are many ways to do this. One way

button = "There are 314 buttons";
button // StringCases[d : DigitCharacter .. :> ToExpression[d]] // Flatten // First
(* 314 *)