May 22, 2015

In the previous exercise I needed a string-replace function, and was surprised not to find one in my code library. I quickly wrote a very simple function, noting that it was “awful” because it had quadratic performance.

Your task is to write a string-replace function that has linear time instead of quadratic. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.


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2 Responses to “String-Replace”

  1. Rutger said

    C++ implemenation for Python.

    [sourecode lang=”C++”]

    /* len(self)>=1, len(from)==len(to)>=2, maxcount>=1 */
    Py_LOCAL(PyStringObject *)
    replace_substring_in_place(PyStringObject *self,
    const char *from_s, Py_ssize_t from_len,
    const char *to_s, Py_ssize_t to_len,
    Py_ssize_t maxcount)
    char *result_s, *start, *end;
    char *self_s;
    Py_ssize_t self_len, offset;
    PyStringObject *result;

    /* The result string will be the same size */

    self_s = PyString_AS_STRING(self);
    self_len = PyString_GET_SIZE(self);

    offset = stringlib_find(self_s, self_len,
    from_s, from_len,
    if (offset == -1) {
    /* No matches; return the original string */
    return return_self(self);

    /* Need to make a new string */
    result = (PyStringObject *) PyString_FromStringAndSize(NULL, self_len);
    if (result == NULL)
    return NULL;
    result_s = PyString_AS_STRING(result);
    Py_MEMCPY(result_s, self_s, self_len);

    /* change everything in-place, starting with this one */
    start = result_s + offset;
    Py_MEMCPY(start, to_s, from_len);
    start += from_len;
    end = result_s + self_len;

    while ( –maxcount > 0) {
    offset = stringlib_find(start, end-start,
    from_s, from_len,
    if (offset==-1)
    Py_MEMCPY(start+offset, to_s, from_len);
    start += offset+from_len;

    return result;


  2. This looked so suitable for C that I couldn’t resist trying it. The result is at
    On a first run, the times were about ten times longer than those above, not what I expected. It turned out that my library strstr function execution was quadratic in string length. Replacing strstr with local code produced a times about 20 times faster than those above, and memory 200 times less.
    So even coding close to the machine, you can’t be sure of a simple library function.

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