3.2.6 String Handling Operations in a Programming Language

Strings – A string is any sequence of characters from the keyboard, including digits,
letters, punctuation and any other characters you can see when you look at your keyboard.
It is a very important data structure in programming because it is used to achieve many things.

Casting – Converting a piece of data to a specific data type. For example, a user might enter
a string and the program might convert it to an integer in order to perform certain calculations.

Typical operations on strings

• Because strings are so important, programmers need to know how to carry out a number of frequently used operations on them. These include:
  • Finding the length of a string.
  • Finding the position of a character.
  • Getting a substring (also called a slice) from a string.
  • Concatenation of strings.
  • Converting a character into its ASCII character code.
  • Converting an ASCII character code into a character.
  • Converting digits between string digits and numerical digits.

• Strings are a useful way of storing any data that you do not need to perform calculations on.
• However, there are times when you need to be able to select a part of a string, manipulate a string or convert it to another data type.

Length:

• Using the Python code, we can ask the user for a name and then using the LEN function find out the length of the name as follows:

• Entering the name “George” then the LEN function would return “6”.

Position

• Position will find out where in the string a character appears. Don’t forget it will start counting from 0.
• In this example, it will find the letter “o” in the name and display the number of the position.

• With the name “George” the find function will return “2”.
• In Python, if the letter cannot be found it will return the value of -1.

Substring

• A substring is a group of two or more characters that appears inside another string.
• For instance, “the best of” is a substring of “It was the best of times”.
• The substring can be found as long as you know the start position and the end position of the string you are looking for.

• Don’t forget Python needs an extra position counted at the end, otherwise it will stop when it reaches the last letter and not show it.
• This substring will return “ngo”.
• It is assumed that the start position is 0 and not 1, so message[ 0 ] contains the first character of the string “M”.

Concatenation

• Concatenation means joining two strings together and this is done using the + symbol in both pseudocode and Python.

• Please note, the “ “ has been added in the middle to join the names with a space between them.

ASCII Character codes

• ASCII stands for “American Standard Code for Information Interchange”. Using this code, every character can be represented as a 7-bit binary pattern, allowing 128 different characters to be represented.
• The table below shows how each upper and lower-case letter is represented along with numbers and some symbols:

• If you want to convert a character into an ASCII code, you can convert that character to the corresponding ASCII code as shown below:

• If you want to convert an ASCII code into a character, you can convert this as shown below:

String Conversions

• Sometimes we need to change the data type of a variable from a string to an integer or a real (float) number or vice versa.

• Python is quite simple and does not really care about the current data type, it only needs to know what you are converting it to (int, float or str).
• Whereas the pseudocode is much more specific and needs you to know the current data type and the required data type you are converting it to.