RPN Command-Line Calculator

Project roadmap

1. 01

   Build a stack

   ~1.5h

   Create stack.h and stack.c with a Stack struct holding an array of 64 doubles and a top index, and the functions stack_push, stack_pop, stack_peek and stack_size. Push and pop return false when the stack is full or empty, instead of crashing. Add a Makefile compiling with -Wall -Wextra -std=c11. Done when: a small test in main pushes and pops numbers correctly, including the full and empty cases.

2. 02

   Split an expression into tokens

   ~1h

   Read an expression into a buffer and split it into tokens separated by spaces and tabs with strtok. For each token, decide whether it's a number, checked with strtod and making sure the whole token was used, so 3x is rejected, or an operator (+, -, *, /, ^). Done when: the program prints each token of 3 4 + 2 * with its kind.

3. 03

   Evaluate the expression

   ~1.5h

   Push numbers onto the stack. For an operator, pop two values (the second pop is the left operand, which matters for - and /), apply the operator, and push the result, using pow from math.h for ^ (link with -lm). At the end, exactly one value must be on the stack; print it. Done when: 3 4 + 2 * gives 14, and 5 1 2 + 4 * + 3 - gives 14.

4. 04

   Report errors clearly

   ~1h

   Detect and report: not enough numbers for an operator (stack underflow), too many numbers left at the end, division by zero, an invalid token (showing which one), and a stack overflow. Each error prints a clear message to stderr and the expression's result is skipped. Done when: each kind of mistake gives its own message.

5. 05

   Support interactive and file input

   ~1h

   With arguments, evaluate them as one expression (rpn "3 4 +"). Without arguments, read lines from standard input in a loop with fgets, showing a > prompt when the input is a terminal (check with isatty), so rpn < file.txt evaluates a file line by line. Skip empty lines and lines starting with #. Exit with code 1 if any expression failed. Done when: all three ways of using it work.

6. 06

   Test it

   ~1.5h

   Write a tests.txt file of expressions with their expected results or errors, and a make test target (a shell script) that runs each one and compares the output. Cover every operator, negative numbers, decimals, and each error case. Done when: make test passes.

Resources

• ArticleReverse Polish notation (Wikipedia)
• Docsstrtok reference
• Docsstrtod reference
• DocsGNU Make manual