Ruby how to increment a string number

user_14069773 0
Viewed 781

How can I confidently increment a string number in Ruby? I understand I can call "1".next and produce "2" but this does not work for negative numbers.

"-3".next
=> "-4"

How can I call to increment both positive and negative string digits? I want to return value as String.

ruby
3 Answers

You can convert that string to integer, call #next and then convert it back to string

'-3'.to_i.next.to_s
user_142451210

You could just do ("-3".to_i + 1).to_s but that does not account for float values (if you cared for that). You can call gsub with a block and match with regex to convert and increment. next does not work for negatives as it only evaluates the rightmost numeric and pays no attention to the minus character. The below matches for integers or floats and increments them based on the integer type:

INCREMENTAL = 1

def increment_str(str)
  str.gsub(/(-)?\d+(.\d+)?/) { |x| x.include?(".") ? x.to_f + INCREMENTAL : x.to_i + INCREMENTAL }
end

increment_str("-3") => "-2"
increment_str("3") => "4"
increment_str("-3.0") => "-2.0"
increment_str("3.0") => "4.0"
user_106103520

Your answer depends on whether the input is integer or float. In each case, cast it to the appropriate numeric type (to_i to to_f), increment by 1, and cast back to string. For example:

['-1.23', '-1', '0', '1', '1.23'].each do |str|
  int_str_plus_1 = (str.to_i + 1).to_s
  float_str_plus_1 = (str.to_f + 1).to_s
  puts [str, int_str_plus_1, float_str_plus_1].join("\t")
end

Prints:

-1.23  0  -0.22999999999999998
-1     0  0.0
0      1  1.0
1      2  2.0
1.23   2  2.23
user_9676210
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