Yacc provides a general tool for describing the input to a computer program. The Yacc user specifies the structures of his input, together with code to be invoked as each such structure is recognized. Flex is a tool for generating scanners: programs which recognized lexical patterns in text.
The above tools have been used to build a simple interpreter to perform mathematical operations. The whole program is based on a user defined grammar, which can be changed as needed. It supports basic mathematical operations such as +, -, * and / including the operator precedence. Integer and floating point numbers as well as positive and negative numbers can be used to perform calculations. It is possible to run the exponentiation (** operator) or logarithms (log operator).
This interpreter gives you the option to run the functions. An example is the max function that returns the largest number of arguments passed as parameters. max(5,13,0,55,27) returns 55, max(2 ** 4, 2 ** 6, 4 ** 3) returns 64 and so on.
10 + 2 ** 5 = In this case, the exponentiation operation will be performed first. The next operation will be adding and the final result will be 42.
4 ** ((max(4.25,12,6.9) * 4) / 8) = This example shows the possibility of nesting operations. The result will be 4096.
You need yacc and flex to generate the required files. To download packages, follow the instructions below:
apt-get update
apt-get install flex
apt-get install bison
apt-get install libc-dev
After downloading the packages, perform the following operations which will generate the following files: y.tab.c, y.tab.h and lex.yy.c
yacc -d parser.y
flex scanner.l
Then build an interpreter using gcc compiler:
gcc y.tab.c lex.yy.c -o interpreter -lm
If everything went well you can run the built program by typing in the terminal
./interpreter
This will start the program, which will start analyzing the content of the data.txt file and will perform actions. Each operation saved in the file must end with the operator =. This is information for the interpreter thanks to whom he knows where the instruction entry ends. Inside the data.txt file it is possible to use multi-line comments using /* */ notation. The content of the comments is not interpreted. The result of each operation will be displayed on separate lines. At the very end of the file there is an exit instruction which is used for terminating the program.
The tools that were used to create this calculator are also used in the open source PostgreSQL database. If you would like to deepen your knowledge and if you are interested in implementation please visit
https://github.com/postgres/postgres/tree/master/src/backend/parser