Why does Ruby's String#to_i sometimes return 0 when the string contains a number?

Question

I was just trying out Ruby and I came across String#to_i. Suppose I have this code:

var1 = '6 sldasdhkjas'
var2 = 'aljdfldjlfjldsfjl 6'

Why does puts var1.to_i output 6 when puts var2.to_i gives 0?

Solution 1

The to_i method returns the number that is formed by all parseable digits at the start of a string. Your first string starts with a with digit so to_i returns that, the second string doesn't start with a digit so 0 is returned. BTW, whitespace is ignored, so " 123abc".to_i returns 123.

Solution 2

From the documentation for String#to_i:

Returns the result of interpreting leading characters in str as an integer

Solution 3

More exhaustive examples of to_i:

irb(main):013:0* "a".to_i
=> 0
irb(main):014:0> "".to_i
=> 0
irb(main):015:0> nil.to_i
=> 0
irb(main):016:0> "2014".to_i
=> 2014
irb(main):017:0> "abc2014".to_i
=> 0
irb(main):018:0> "2014abc".to_i
=> 2014
irb(main):019:0> " 2014abc".to_i
=> 2014

Comments

[hsinxh]

So that means if there is non-integer character in the beginning of the string, to_i will ignore the rest of the string ?

[DarkDust]

Exactly, if the string starts with a non-integer character it immediately stops parsing and returns 0, ignoring any numbers that might come later in the string.

[Marc Talbot]

That isn't completely true - it will parse through whitespace characters. " 123".to_i will evaluate to 123, as would a string beginning with a tab.