Data and processes in computing
Data and processes in computing

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

4.3 Character code functions

Many programming languages provide two functions associated with the character codes (see Table 2). We shall call these functions ASC and CHR. ASC takes a character as input, and returns the integer giving the ASCII code of the input character. CHR returns the character whose ASCII code is the input integer. These functions are described below. We have set a precondition on CHR to conform to our earlier restriction on the set of characters that we will consider in this course.

function ASC(c in Char) return in Int

pre   true.

post   The returned value is the ASCII code of c, as given in Table 2.

function CHR(n in Int) return in Char

pre 32 ≤ n ≤ 126.

post The returned value is the character with ASCII code n, as given in Table 2.

Activity 18

Find each of:

  • (a) ASC(‘j’);

  • (b) CHR(43);

  • (c) ASC(CHR(43));

  • (d) CHR(ASC(‘j’)).


  • (a) Referring to Table 2, ASC(‘j’) = 106.

  • (b) CHR(43) = ‘+’.

  • (c) ASC(CHR(43)) means the result of applying the function ASC to CHR(43). So ASC(CHR(43)) = ASC(‘+’) = 43.

  • (d) CHR(ASC(‘j’)) = CHR(106) = ‘j’.

Looking up the ASCII code of a character, and then looking to see the character corresponding to the number you just found, will always take you back to the character you started with. We can express this in a general equation, as CHR(ASC(c)) = c. (This holds for any character c.)

As illustrated in Activity 18, the two functions ASC and CHR are closely related. Each reverses the effect of the other. We can express this relationship by writing the two equations below (which hold for every c in Char and for every n in Int satisfying the condition 32 ≤ n ≤ 126).

ASC(CHR(n)) = n

CHR(ASC(c)) = c

We say that ASC and CHR are inverse functions.

Not every function can be paired up with an inverse function in this way. For example, the function FIRSTCHAR returns the first character in a string. Now different strings may have the same first character. (“This” and “The” both start with ‘T’, but are different strings.) With input ‘T’, a process attempting to reverse the effect of FIRSTCHAR would not know whether to return “This” or “The” (or “To”, or “Tom”, etc). The function FIRSTCHAR has no inverse function.

Activity 19

A simple cipher replaces each lower-case letter with the next one in alphabetical order. So ‘a’ is replaced by ‘b’, ‘b’ by ‘c’,. . . , ‘t’ by ‘u’, and so on. The letter z ‘ ’ is replaced by ‘a’. The function NEXT, specified below, corresponds to this coding.

function NEXT(c in Char) return in Char

pre c is a lower-case letter.

post If c lies between ‘a’ and ‘y’ then the returned value is the letter following c in alphabetical order. If c = ‘z’ then the returned value is ‘a’.

Does the function NEXT have an inverse function? If it does, describe this inverse function.


We can reverse the effect of NEXT by moving each character back one place in alphabetical order. This time, the special case is ‘a’, which must be moved to ‘z’. So NEXT does have an inverse function. If we call it BEFORE, it can be described as below.

function BEFORE(c in Char) return in Char

pre c is a lower-case letter.

post If c lies between ‘b’ and ‘z ’ then the returned value is the letter preceding c in alphabetical order. If c = ‘a’ then the returned value is ‘z’.

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371