Strings and the slice operator - PowerPoint Presentation

Slide1

Strings and the slice operatorSlide2

The slice operator

This is the “substring” method which most languages have

Given a string, you can slice pieces of the string

the syntax of the operator uses the square brackets

s[5:7] means the part of the string s which starts at position 5 and includes everything

up to

but not including

position 7, in other words, the characters at position 5 and at position 6

Omitting an argument before the colon means to start at the beginning of the string s[:3] means s[0], s[1] and s[2]

Omitting an argument after the colon means to include the rest of the string s[4:] means s[4], s[5]… up to and including the end of the stringSlide3

slice returns a new

string

Suppose s were “

abcdef

” and you wrote an expression like s[1:4

]

Its value is “

bcd

”, a new string (the original string is not changed at all)

If the expression were on a line by itself, it does nothing!

You need to use the expression IN some statement, like an assignment statement, an if statement, a while statement, a print statement

print(s[1:4])

makes sense

t = s[1:4]

makes a copy of the 3 characters as a string and puts it into t

s = s[1:4]

this does the same as the statement above, but the previous value of s is also discarded

if s[1:4] == “

bcd

”:

would result in TrueSlide4

Be careful of your range

str

[i:i+1] is the same as saying

str

[

i

]

It is possible to go

out of range with slice – don’t go past the end of the

string!

You can leave off both the starting and ending points, as in

s[:].

This produces a copy of the whole string.