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Characters

In programming, a sequence of characters is called a string(s). Also, a part of the sequence is substring(s). This chapter introduces some features to manipulate strings.

Some operators for characters

Concatinating characters

The // symbol concatenates two character-literals.

program ch
   print *,"Hello, " // "world!"
end program ch

You can use it for character variables.

program ch
   implicit none
   character(len=10) :: x,y
   x = "Hello, "
   y = "world!"
   print *,x // y
end program ch

 Hello,    world!

It seems wrong. Each variable has 10-character length, and print keeps the margin enough to show the full width of each variable. In the above case, print statement shows Hello, followed by padded spaces (as 4 characters). To avoid it, you cut the trailed spaces with the intrinsic function, trim.

! for exercise 1
   print *,trim(x) // y

 Hello,world!

But it removes all the trailing spaces in x while keeping the other spaces. The output has no space between the words. Exercise 1 asks you how you show Hello, world! in the right way.

Character functions

There are many built-in functions useful for manipulating characters.

See the following example.

! for excercise 1
program ch
   implicit none
   character(len=10) :: x
   x = '  123 45  '
   print *,len(x)
   print *,len_trim(x)
   print *,adjustl(x)
   print *,adjustr(x)
   print *,trim(x)   
end program ch

          10
           8
 123 45     
     123 45 
 123 45     

To remove the leading spaces, you apply adjust() to the variable.

x = adjustl(x)

Some other functions may be useful to manipulate the characters precisely.

Here are some examples of these functions.

program ch
   print *,index("1231234512","123")     ! 1
   print *,index("1231234512","234")     ! 5
   print *,scan("1231234512","1234")     ! 1
   print *,verify("1231234512","1234")   ! 8

   print *,index("1231234512","123",back=.true.)   ! 4
   print *,scan("1231234512","1234",back=.true.)   ! 10
   print *,verify("1231234512","1234",back=.true.) ! 8
end program ch

Alternative literal symbols

The character literals are defined by ' or ". If the literal contains one of such characters, you should use the alternative one.

program ch
   ! shows: It's my dog
   print *,"It's my dog"

   ! shows: You said "I was not there".
   print *,'You said "I was not there".'
end program ch

When you have both ' and " in the literal, it looks more complicated.

program ch
   ! shows: You said "it's my dog".
   print *,"You said ",'"',"it's my dog.",'"'

   ! the same result
   print *,"You said " // '"' // "it's my dog." // '"'
end program ch

Reading characters from keyboard

By default, the input characters are split by white spaces into pieces. It is useful to read some numbers, but you can not read the entire input with spaces from the keyboard.

program ch
   implicit none
   character(len=10) :: x
   read *,x
   print *,x
end program ch

When you type abc def in the terminal, you should find the following output.

 abc

You can read the entire characters with spaces using the format identifier. We do not see it here, and the later chapter deals with it.

Summary

• Characters can be concatenated with //.
• There are several functions to manipulate characters.
• With a naive approach, you can not read characters with spaces from the keyboard.

Exercises

See the above code.

1. Show Hello, world! in the right way (with a space between the words).
2. Show the length of x, excluding all leading and training spaces. In this case, make the program show 6 for x=" 123 45 ".

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