Flowcharts for C programmers

1. Flowcharts for C programmers Ian McCrum

2. flowcharts Diagrams are used in many places; they can be used as a design aid or a documentation aid. As a design aid they can allow visualisation of relationships, show how a system is partitioned and the structure of how things go together. We can do this before coding (or circuit design). There is also a much more formal method of using a variety of diagrams; management and systems analysis for example. A Unified Modelling Language (UML) can tease out the entities, attributes and relationships of all aspects of data for example. Here we will present the bare minimum of simple flowcharts, enough so that anyone can follow them without prior training.

3. Simple Flowcharts • Time moves from top to bottom • A single entry point at the top • Try to avoid lines crossing (not always possible) • One box one job • Decision points should only have two exits • Deliberately use English and not abbreviated C keywords, don’t use “IF”, use “IS”…? Instead • Use common sense! The diagram should be clear

4. The bare minimum The start and end symbols The input (or output box) The statement box The process or sub-program box (C function) The decision box Continuation circles to link to the next page) 6 shapes are enough; some texts add dozens, symbols for storage of 4 or 5 types, printing etc. KISS! Keep it simple

5. Making flowcharts • WORD is adequate ( use insert->shape) • Microsoft Office has flowchart plugins • Microsoft Viso is pretty good but expensive • There are open source alternatives such as the libreoffice or OpenOffice products • There is opensource flowcharting software as well – yED is easy to learn and quick to use – stick to its “flowchart” shapes, use F2… I like the way it self aligns the boxes. Don’t use colour. Simple black and white works… (IMHO) • http://www.yworks.com/en/products_yed_about.html

6. One start box, usually OVAL, you can use the words “START program1.c” if you wish. Best to use a white background and black lines. If using Word, insert a textbox with no lines and use the GROUP option using rightclick. Use the same symbol as a finish box START The input or output box. Use English here, if outputting say “Light the LED” for example Read in the state of the pushbutton For simple statements I just use a text box in WORD but set its border to a 2 pixel black line. It is good to avoid using the equals sign ‘=‘ and to use English language to make it read better Set X to zero, Initialise variables A function in C can be represented by a box with double lines at the sides. Make the contents English! Sub program sqr Calculate square root of x A decision box: use English words and have a question mark at it the end, this should ask a question. It will have two labelled exits It helps if you use the centre justify option for the textbox Is the switch pressed ? Finally, on this page!, the continuation circle, label it A

7. A Continuation circle on the next page. There should be pairs of letters, one of each pair That’s all there is too it! There are weaknesses in this system but it does serve its purpose well. Major weakness is that there is no mechanism to give information about any variables or even functions you use. Adding “declarations” makes the charts unwieldly. I often add “margin notes” to my charts. In final year, those of you that take the OOP module will study Object Orientated Programming where you focus on the design of data, and attach relevant functions inside the data definitions. (cf Classes and Methods) C does have a “struct” system but it was given a major overhaul in C++, an OOPS language. Computing professionals spend a lot of time designing good data objects. C promotes focussing on the functions, you actually need to think about both. If interested read the seminal text Algorithms Plus Data Structures Equals Programs by Niklaus Wirth

8. Relationship of flowcharts to C If statements if else statements YES YES NO NO

9. Do loops – test at the bottom! do{ this and this }while(this true); e.g do{ i++; }while(RAO==0); YES NO

10. While loops – test at the top – body may never get executed While(test is true){ do this and this } e.G while(RA0==0){ RA1=1; } YES NO

11. For loops – complicated in flowcharts; three things present in a for loop header • The first item in the for statement is executed before entering the loop • The next thing in the for statement is a conditional test, if true the for loop is exited, skipping over the body of the loop • The last thing is only executed if the body of the loop gets entered, it gets executed at the bottom of the body and after it a jump up to the conditional test is done. • Although semicolons separate each of the three sections, you can have multiple statements in each section, separated by a comma.

12. for(i=0,j=0 ; i<10 ; i++,j++) { x=i*j; }

13. Real examples void putch(char ch){ unsigned char status; do{ status=TXIF; }while(status==0); TXD=ch; } In practice most programmers would… while(!TXIF){;} TXD=ch; Or do{;/}while(!TXIF); TXD=ch;

14. Further example unsigned intgetch(void){ unsigned char sta; do{ sta=RXIF; }while(sta==0); return RXREG; Or while(!RXIF){;} return (RXREG); // often C is not an exact // match for the flowchart

15. Tutorials Q1 Write a flowchart to average 10 numbers brought in from the keyboard Q2 Work through and array of 10 numbers and output the smallest value Q3 Work through an array replacing each element with a weighted average of 25% of each immediate neighbour added to 50% of itself. Leave the first and last elements untouched Q4 Flowchart the strstr function (look it up in the standard C library – c.f website file, page 96 or 120)