Learn Fortran by writing a "guess the number" game

The first compiled programming language I learned was Fortran 77. While growing up, I taught myself how to write programs in BASIC on the Apple II and later in QBasic on DOS. But when I went to university to study physics, I learned Fortran.

Fortran used to be quite common in scientific computing. And once upon a time, all computer systems had a Fortran compiler. Fortran used to be as ubiquitous as Python is today. So if you were a physics student like me, working in the 1990s, you learned Fortran.

I always thought Fortran was somewhat similar to BASIC, so I quickly took to Fortran whenever I needed to write a quick program to analyze lab data or perform some other numerical analysis. And when I got bored, I wrote a "Guess the number" game in Fortran, where the computer picks a number between one and 100 and asks me to guess the number. The program loops until I guess correctly.

The "Guess the number" program exercises several concepts in programming languages: how to assign values to variables, how to write statements, and how to perform conditional evaluation and loops. It's a great practical experiment for learning a new programming language.

The basics of Fortran programming

While Fortran has been updated over the years, I am most familiar with Fortran 77, the implementation I learned years ago. Fortran was created when programmers wrote programs on punched cards, so "classic" Fortran is limited to the data you could put on a punched card. That means you could only write classic Fortran programs with these limitations:

• Only one line of source code per card is allowed.
• Only columns 1–72 are recognized (the last eight columns, 73-80, are reserved for the card sorter).
• Line numbers ("labels") are in columns 1–5.
• Program statements go in columns 7–72.
• To continue a program line, enter a continuation character (usually +) in column 6.
• To create a comment line, enter C or * in column 1.
• Only the characters A to Z (uppercase), 0 to 9 (numbers), and the special characters = + - * / ( ) , . $ ' : and space are used.

With these limitations, you can still write very useful and interesting programs.

Guess the number in Fortran

Explore Fortran by writing the "Guess the number" game. This is my implementation:

CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
C     PROGRAM TO GUESS A NUMBER 1-100
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC
      PROGRAM GUESSNUM
      INTEGER SEED, NUMBER, GUESS

      PRINT *, 'ENTER A RANDOM NUMBER SEED'
      READ *, SEED
      CALL SRAND(SEED)

      NUMBER = INT( RAND(0) * 100 + 1 )

      PRINT *, 'GUESS A NUMBER BETWEEN 1 AND 100'
 10   READ *, GUESS

      IF (GUESS.LT.NUMBER) THEN
         PRINT *, 'TOO LOW'
      ELSE IF (GUESS.GT.NUMBER) THEN
         PRINT *, 'TOO HIGH'
      ENDIF

      IF (GUESS.NE.NUMBER) GOTO 10

      PRINT *, 'THATS RIGHT!'
      END

If you are familiar with other programming languages, you can probably figure out what this program is doing by reading the source code. The first three lines are a comment block to indicate the program's function. The fourth line, PROGRAM GUESSNUM, identifies this as a program, and it is closed by the END statement on the last line.

After defining a few variables, the program prompts the user to enter a random number seed. A Fortran program cannot initialize the random number generator from the operating system, so you must always start the random number generator with a "seed" value and the SRAND subroutine.

Fortran generates random numbers between 0 and 0.999... with the RAND(0) function. The 0 value tells the RAND function to generate another random number. Multiply this random number by 100 to generate a number between 0 and 99.999…, and then add 1 to get a value between 1 and 100.999… The INT function truncates this to an integer; thus, the variable NUMBER is a random number between 1 and 100.

The program prompts the user, then enters a loop. Fortran doesn't support the while or do-while loops available in more modern programming languages. Instead, you must construct your own using labels (line numbers) and GOTO statements. That's why the READ statement has a line number; you can jump to this label with the GOTO at the end of the loop.

Punched cards did not have the < (less than) or > (greater than) symbols, so Fortran adopted a different syntax to compare values. To test if one value is less than another, use the .LT. (less than) comparison. To test if a value is greater than another, use .GT. (greater than). Equal and not equal are .EQ. and .NE., respectively.

In each iteration of the loop, the program tests the user's guess. If the user's guess is less than the random number, the program prints TOO LOW, and if the guess is greater than the random number, the program prints TOO HIGH. The loop continues until the user's guess is the same as the random number.

When the loop exits, the program prints THATS RIGHT! and ends immediately.

$ gfortran -Wall -o guess guess.f

$ ./guess
 ENTER A RANDOM NUMBER SEED
93759
 GUESS A NUMBER BETWEEN 1 AND 100
50
 TOO LOW
80
 TOO HIGH
60
 TOO LOW
70
 TOO LOW
75
 TOO HIGH
73
 TOO LOW
74
 THATS RIGHT!

Every time you run the program, the user needs to enter a different random number seed. If you always enter the same seed, the program will always pick the same random number.

Try it in other languages

This "guess the number" game is a great introductory program when learning a new programming language because it exercises several common programming concepts in a pretty straightforward way. By implementing this simple game in different programming languages, you can demonstrate some core concepts and compare each language's details.

Do you have a favorite programming language? How would you write the "guess the number" game in it? Follow this article series to see examples of other programming languages that might interest you.