Passing string into Cython functions

Question

The past few days I've been experimenting with Cython and I have a quick questions regarding the way c / cython handles strings as character arrays / pointers:

def function(char *string):
    for i in xrange(len(string)):
        print string[i]
        print &string[i]

Now for example, when I compile and run the code, with "abc" as an argument, I get the following answer:

97

abc

98

bc

99

c

Now my questions are:

Why does cython print out the ascii value for each character in string[i]?
Why does cython print out the suffix of the string starting at index i in &string[i]?

Thank you very much.

I believe the Cython docs recommend taking strings as str, not char *. — user2357112
– user2357112, Commented May 24, 2016 at 16:06

user2357112 · Accepted Answer · 2016-05-24 16:17:07Z

Like Python 3 bytes, when you index a char * in Cython, Cython treats the char at that index as a numeric value, not text. That's why print string[i] prints a number.

The behavior of print &string[i] is inherited from C. If the char * points to the following null-terminated string:

 |
 v
+-+-+-+-+
|a|b|c| |
+-+-+-+-+

then &string[1] is a char * that points here:

   |
   v
+-+-+-+-+
|a|b|c| |
+-+-+-+-+

which is also a null-terminated string, this one having characters bc in it. When you print it, Cython prints bc.

The Cython docs recommend not using char *s:

Generally speaking: unless you know what you are doing, avoid using C strings where possible and use Python string objects instead.

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