Bug #15001: Peek causes cursor to advance when enumerating the lines of a file

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]
• Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN

[fletchto99 (Matthew Langlois)]

When a call to peek is made on the enumerator for the lines of a file the cursor advances.

File:

1
2
3

Sample script:

e = File.open("f").each_line
e.peek
p e.each.to_a

The output is => ["2\n", "3\n"]
The expected output is => ["1\n", "2\n", "3\n"]

When performing the same operation on a string we get the expected output.

e = "1\n2\n3\n".each_line
e.peek
p e.each.to_a

The output is: => ["1\n", "2\n", "3\n"]

[fletchto99 (Matthew Langlois)]

• Subject changed from Peek causes cursor to advance when reading files to Peek causes cursor to advance when enumerating the lines of a file

[ysfk (Yosfik Alqadri)]

with slight modification of the above code,

e = File.open("f").each_line
p e.each.to_a
e.peek
p e.each.to_a

it will give following result

["1\n", "2\n", "3\n"]
Traceback (most recent call last):
	1: from f.rb:3:in `<main>'
f.rb:3:in `peek': iteration reached an end (StopIteration)

while the string version,

e = "1\n2\n3\n".each_line
p e.each.to_a
e.peek
p e.each.to_a

will give following result

["1\n", "2\n", "3\n"]
["1\n", "2\n", "3\n"]

[Hanmac (Hans Mackowiak)]

i think the problem is that String Enumerations are rewindable while on the IO/File it is not

[jeremyevans0 (Jeremy Evans)]

• Status changed from Open to Closed