The escape character in the string and the length of the string

Tips:

(1) When calculating the length of the string, the key is to recognize the escape character;

(2) An escape character is always preceded by a backslash, plus one other character. Therefore, you should pay special attention when you encounter the backslash!!!

Example:

1. String without escape character

such as: "abc!x=/", strlen the length of 7, but because the string at the end there is a Terminator ' \ ', so the string occupies 8 bytes, the length is 8.

2. String with escape character

(1) string "abc\n": where ' \ n ' is an escape character (line break), the length of a string can only be counted as one character, so the length of the string is 5 (not 6)

(2) The string "abc\n\\\" ": There are 4 escape characters: ' \ n ' (newline character), ' \ \ ' (backslash), ' \ ' (single quote), ' \ ' (double quotation mark), so the length of the string is 8 (instead of 12).

(3) string "ABC\0XYZ": There is an escape character ' \ s ', which is a string terminator, so when the length of the string is tested with the function strlen, the result should be 4 (not 8).

(4) The string "Abc\\0xy": where there is an escape character ' \ \ ' (backslash), so that the string "0xy" after the same calculation, so that the length of the string is 7 (instead of the second backslash and the subsequent 0 is combined into an escape character ' "", if so, The first backslash cannot be processed because an escape character is always made up of backslashes plus other characters, and a single backslash cannot be any valid character.

(5) If the string "Abc\\0xy" is changed to "Abc\\\0xy": There are two escape characters ' \ \ ' (backslash) and ' \ S ' (string terminator), then when the function strlen to test the length of the string, the result should be 5 (instead of 8).

(6) If the string "Abc\\\0xy" is changed to "Abc\\\061xy": Then there are two escape characters ' \ \ ' (backslash) and ' \061 ' (ASCII value equals 061 characters, that is, the numeric character ' 1 '), When a function strlen is used to test the length of the string, the result should be 7 (not 4 or 9). Therefore, when encountering the escape character ' + ', it is also necessary to see if there is a number behind it, if so, the following number (one to two bits) should be combined with the preceding ' + ' to count the length of the whole string as one character.

If you use printf ("Abc\\\061xy") and output, the output is: Abc\1xy

Note that using strlen to find the length of a string is not the end of the Terminator, but the number of bytes that the string occupies is a terminator.

Example one: [1995 level two C written selection question (26)]

Please select the output of the following statement ()

printf ("%d\n", strlen ("\t\" \065\xff\n "));

(A) 5 (B) (C) 8 (D) output not valid, no normal output

The correct answer is (A): The 5 characters contained in the string are: ' \ t ' (Jump Geff), ' \ ' (double quotes), ' \065 ' (ASCII value is octal 065, which is the decimal 51 character), ' \xff ' (ASCII value is hexadecimal ff, or decimal 255 character) , ' \ n ' (line break).

Example two: [1998 level two C written selection question (44)]

If you have the following program sections:

Char str[]= "ab\n\012\\\" ";

printf ("%d", strlen (str));

The output of the above program segment is

A) 3 B) 4 C) 6 D) 12

The correct answer is (C): The 6 characters contained in the string are: ' A ', ' B ', ' \ n ', ' \012 ', ' \ \ ' and ' \ '. If the question is to ask the length of the string, it is 7.

The escape character in the string and the length of the string