Structure chart
Graphical representation of the modules (user-defined, not built-ins) to be implemented in a program, demonstrating in what order they will be called and which functions call which other functions
 
IPO chart: Input/Process/Output chart
Tabular representation of the modules to explain the data communication between modules as well as the function of the module itself.
 
IPO Layout
4 columns: module name, I, P, and O
Variable number of rows: 1 header row followed by 1 row for each module in the design
VERY IMPORTANT: Input and Output describe DATA. They do NOT describe input from file/keyboard or output to file/screen. Those tasks are part of a module's PROCESS – what it does.
 
Input data to a module is data the module needs to accomplish its task. Sent from calling to called module.
Output data from a module is data the module calculates or creates and sends back to the calling module.
 
Examples:
Specification: create a program to read in the length and width of a rectangle from the keyboard, calculate the area and perimeter, and display the input and calculations to screen.
 
Structure chart 

		main
getInput		calcAreaPerim		displayReport

main() will call getInput() and wait for it to finish, then main() will call calcAreaPerim() and wait for it, then it will call displayReport() and wait, then main() will finish.
 
 
IPO chart

Module	I	P	O
main	none	controls calling other functions and data communication between them	completion status to OS
getInput	none	prompts the user for a length and width, reads values from keyboard	length, width
calcAreaPerim	length, width	calculates area as length * width and perim as 2 * (length + width)	area, perim
displayReport	length, width, area, perim	displays the original input as well as calculated values to screen	none

 
Notice that keyboard input is part of the PROCESS of getInput and screen output is part of the PROCESS of displayReport. No data is sent to getInput, then it prompts and reads, and it sends the length and width back out to main. displayReport accepts 4 pieces of input, displays them to screen, and does not send any data back out to main. It does not calculate anything therefore does not send out any data.
 
This document is for design purposes: to help figure out the problem and necessary structure and steps to solve each piece BEFORE implementation. Therefore there should not be any reference to actual C code in the document. No data types, no variable names, just abstract logical descriptions.