Szathmáry László honlapja @ DEIK

string slicing

Warning! C#'s string.Substring() differs from Python's string slicing!

csharp> var s = "Fallout: New Vegas"
csharp> s.Substring(0, 4)
"Fall" // Seems good, right?

csharp> s.Substring(4, 7)
"out: Ne" // Ooops! What da hell?

string.Substring() has 2 parameters: start position and count. Thus, s.Substring(4, 7) means to start at position 4 and take 7 characters altogether.

Note that string.Substring() is overloaded and it also has a version with just 1 parameter. There, s.Substring(4) means to start at position 4 and take everything until the end (in Python it would be s[4:]).

To simulate Python's string slicing, I have this solution:

/// <summary>
/// Get the string slice between the two indexes.
/// Inclusive for start index, exclusive for end index.
/// </summary>
public static string Slice(this string s, int start, int end)
{
// based on https://www.dotnetperls.com/string-slice
if (start < 0) // support negative indexing
{
start = s.Length + start;
}
if (end < 0) // support negative indexing
{
end = s.Length + end;
}
if (start > s.Length) // if the start value is too high
{
start = s.Length;
}
if (end > s.Length) // if the end value is too high
{
end = s.Length;
}
var len = end - start; // Calculate length
if (len < 0)
{
len = 0;
}
return s.Substring(start, len); // Return Substring of length
}

For usage examples, here are some tests:

[Fact]
public void Slice_string()
{
const string s1 = "Fallout: New Vegas";
Assert.Equal("Fall", s1.Slice(0, 4));
Assert.Equal("out", s1.Slice(4, 7));
Assert.Equal("out: New Vega", s1.Slice(4, 17));
Assert.Equal("out: New Vegas", s1.Slice(4, 18));
Assert.Equal("out: New Vegas", s1.Slice(4, 19));
Assert.Equal("out: New Vegas", s1.Slice(4, 30));
Assert.Equal("gas", s1.Slice(-3, s1.Length));
Assert.Equal("Vegas", s1.Slice(-5, s1.Length));
Assert.Equal("Vega", s1.Slice(-5, -1));
Assert.Equal("", s1.Slice(100, 200));

const string s2 = "batman";
Assert.Equal("bat", s2.Slice(0, 3));
Assert.Equal("man", s2.Slice(3, s2.Length));
Assert.Equal("atm", s2.Slice(1, 4));
Assert.Equal("man", s2.Slice(-3, s2.Length));
}

See the current version here.

20180404a

string slicing