Debugging and Ajax

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Debugging and Ajax

• Session 7
• INFM 718N
• Web-Enabled Databases

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Agenda

• Debugging
• Teams
• Ajax

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• Relational normalization
• Structured programming
• Software patterns
• Object-oriented design
• Functional decomposition

Client Hardware
Web Browser
Database
Server Hardware
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Types of Errors

• Syntax errors
• Detected at compile time
• Run time exceptions
• Cause system-detected failures at run time
• Logic errors
• Cause unanticipated behavior (detected by you!)
• Design errors
• Fail to meet the need (detected by stakeholders)

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Debugging Syntax Errors

• Focus on the first error message
• Fix one thing at a time
• The line number is where it was detected
• It may have been caused much earlier
• Understand the cause of warnings
• They may give a clue about later errors
• If all else fails, comment out large code regions
• If it compiles, the error is in the commented part

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Run Time Exceptions

• Occur when you try to do the impossible
• Use a null variable, divide by zero,
• The cause is almost never where the error is
• Why is the variable null?
• Exceptions often indicate a logic error
• Find why it happened, not just a quick fix!

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Debugging Run-Time Exceptions

• Run the program to get a stack trace
• Where was this function called from?
• Print variable values before the failure
• Reason backwards to find the cause
• Why do they have these values?
• If necessary, print some values further back

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Logic Errors

• Evidenced by inappropriate behavior
• Cant be automatically detected
• Inappropriate is subjective
• Sometimes very hard to detect
• Sometimes dependent on user behavior
• Sometimes (apparently) random
• Cause can be hard to pin down

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Debugging Logic Errors

• First, look where the bad data was created
• If that fails, print variables at key locations
• if (DEBUG) echo \foobar foobar
• Examine output for unexpected patterns
• Once found, proceed as for run time errors
• define (DEBUG, FALSE) to clean the output

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Ajax Applications

• Google Maps
• http//maps.google.com
• Google Suggest
• http//www.google.com/webhp?complete1hlen
• Sajax Tables
• http//labs.revision10.com/?p5
• Sajax
• http//www.modernmethod.com/sajax/

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Sajax Example
lt? require("Sajax.php") function multiply(x,
y) return x y sajax_init() //
sajax_debug_mode 1 sajax_export("multiply")
sajax_handle_client_request() ?gt
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lthtmlgtltheadgt lttitlegtMultiplierlt/titlegt ltscriptgt
lt? sajax_show_javascript() ?gt function
do_multiply_cb(z) document.getElementById("z")
.value z function do_multiply() // get
the folder name var x, y x
document.getElementById("x").value y
document.getElementById("y").value x_multiply(x
, y, do_multiply_cb) lt/scriptgt lt/headgt ltbodygt
ltinput type"text" name"x" id"x" value"2"
size"3"gt ltinput type"text" name"y" id"y"
value"3" size"3"gt ltinput type"text"
name"z" id"z" value"" size"3"gt ltinput
type"button" name"check" value"Calculate" onc
lick"do_multiply() return false"gt lt/bodygtlt/html
gt
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Code Walkthrough

• Syntax
• How layout helps reading
• How variables are named
• How strings are used
• How forms are used
• How output is created
• MySQL Integration
• Opening a database
• Handling slashes
• Posing queries
• Using result sets

• Structured Programming
• How things are nested
• How arrays are used
• Modular Programming
• Functional decomposition
• How functions are invoked
• How arguments work
• How scope is managed
• How errors are handled
• How results are passed