String Compare, Whitespace Insensitive

In the process of writing some unit tests, I needed to compare the output string with the expected output (. Except the strings are XML snippets, and I didn't want the tests to break if I changed the formatting. So I need to write up a simple "string compare, whitespace insensitive" function (e.g " f o o " == "foo"). Very handy, but not built into any programming language that I can recall.

Writing this function is a good simple programming, weed-out question for interviewing. It works for any programming language, too. The higher level languages will have a few ways to do it. If the interviewee can't come up with something, well, it will be a short interview.

php string "startswith" let me count the ways

I bounce around between half a dozen different programming languages, so I forget all the little variations. Recently I needed to test in php if a string starts with another (a prefix). So I typed in php string startswith, expecting to get the php manual with some built-in function.

oh no. It's not built in, that's fine. It's the online advice that frightens me. In order from google:

Take 1: use strstr

if (strstr($source, $prefix) == $source)) {
    echo "'$source starts with '$prefix'\n";
} else {
    echo "'$source' does not start with '$prefix'\n";
}

Uhhh if the source does not contain the prefix, then the entire string is searched. Worse then you have to do a full string compare.

Take 2: use strpos

if (strpos($source, $prefix) === 0)) {
    echo "'$source starts with '$prefix'\n";
} else {
    echo "'$source' does not start with '$prefix'\n";
}

Marginally better. Very fast if the source does start with the prefix, very slow if it does not. Lots of confusion on how to test for false (!!!) online.

Take 4: Use substr

The first example I saw just had hardwired examples, but it was something like this:

if (substr($source, 0, strlen($prefix)) == $prefix)) {
    echo "'$source starts with '$prefix'\n";
} else {
    echo "'$source' does not start with '$prefix'\n";
}

Requires creating a new string and doing a full string comparison, however it does not scan the whole string.

Take 5: Use preg_match

good grief. My favorite was this "tip" to "check if a string starts with a specific character."

   if (preg_match('/^a/', $str)) {
                echo "String starts with an a";
   }

No comment on this one.

PLEASE READ

ok team, here it is.

function str_startswith($source, $prefix)
{
   return strncmp($source, $prefix, strlen($prefix)) == 0;
}

It fails fast, it doesn't create new strings that get thrown away, it works in all cases. Granted strncmp is a bit cryptic to people who have never used "C", but it's a of shocker that I didn't see this online.

Here's my "unit test" for it.

function mytest($source, $prefix) {
    if (str_startswith($source, $prefix)) {
       echo "'$source' starts with '$prefix'\n";
    } else {
       echo "'$source' does not start with '$prefix'\n";
    }
}

mytest("foobar", "foo");
mytest("foobar", "food");
mytest("foobar", "bar");
mytest("foobar", "FOO");
mytest("foobar", "foobar1");
mytest('', '');
mytest('', 'foo');
mytest('foobar', '');

It better output:

'foobar' starts with 'foo'
'foobar' does not start with 'food'
'foobar' does not start with 'bar'
'foobar' does not start with 'FOO'
'foobar' does not start with 'foobar1'
'' starts with ''
'' does not start with 'foo'
'foobar' starts with ''

Interview Question

Since it seems that people have trouble with this, it makes a great interview question.

Here's a list of the php string functions. Write down as many functions as you can that check to see if one string starts with another. What are the pros and cons for each method? What test cases do you need to check?