Strings: working with text

What is a string?

A string is a piece of text: a name, a city, a message, even a phone number written in quotes. In Python the type of a string is str. You can write a string with single quotes or double quotes. Both mean the same thing.

city = "Mumbai"
food = 'vada pav'
print(city, food)
print("It's hot today")
note = """Line one
Line two"""
print(note)

Length, joining and repeating: len() tells you how many characters a string has. Spaces count as characters too. The + sign joins two strings into one; joining is also called concatenation. The * sign repeats a string a number of times.

name = "Priya"
print(len(name))
full = "Priya" + " " + "Sharma"
print(full)
print(len(full))
print("-" * 20)
print("ha" * 3)

Indexing: picking one character

An index is the position number of a character. Python starts counting at 0, so the first character is at index 0. Negative indexes count from the end: -1 is the last character, -2 the one before it.

word = "PYTHON"
print(word[0])   # first character: P
print(word[2])   # third character: T
print(word[-1])  # last character: N
print(word[-2])  # second from the end: O

Slicing: cutting out a piece

A slice is a piece of a string. You write [start:stop]. Python starts at index start and stops just BEFORE index stop. Leave out start to begin at the beginning; leave out stop to go to the end. An optional third number, the step, jumps over characters.

pnr = "4521367890"
print(pnr[0:3])   # 452
print(pnr[3:])    # 1367890
print(pnr[:4])    # 4521
print(pnr[-4:])   # 7890, the last four
print(pnr[::2])   # 42379, every second digit
print(pnr[::-1])  # 0987631254, reversed

Strings are immutable. Immutable means "cannot be changed after it is made". You cannot replace one character inside a string. Instead, you build a new string and store it in the variable.

name = "ravi"
name = "R" + name[1:]
print(name)

Turning numbers into text and text into numbers

str() turns a value into a string. int() turns a string of digits into a whole number, and float() turns it into a decimal number. You need these because Python never mixes text and numbers on its own.

age = 21
print("Age: " + str(age))
marks = "85"
print(int(marks) + 5)
print(float("99.5"))