CSCE 121:509-512Introduction to Program Design and ConceptsDr. J. Michael MooreSpring 2015Set 7: Errors

1Based on slides created by Bjarne Stroustrup and Jennifer Welch

CSCE 121:509-512 -- Set 7: Errors

Outline

• Kinds of errors• Function argument checking

– Error reporting– Error detection– Exceptions

• Debugging• Testing

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CSCE 121:509-512 -- Set 7: Errors

Errors

• Errors are natural and unavoidable when writing programs

• Organize software to minimize errors• Eliminate most of the errors we made through

testing and debugging• Ensure remaining errors are not serious• Avoiding, finding, and correcting errors is 95%

of effort for serious software development

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CSCE 121:509-512 -- Set 7: Errors

Your Programs

• Should produce desired result for all legal inputs

• Should give reasonable error messages for illegal inputs

• Need not worry about misbehaving hardware• Need not worry about misbehaving system

software• Is allowed to terminate after finding an error

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OK for beginner’s code,

not for p

rofessional’s

CSCE 121:509-512 -- Set 7: Errors

Sources of Error

• Poor specification• Incomplete programs• Unexpected arguments• Unexpected input• …

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CSCE 121:509-512 -- Set 7: Errors

Kinds of Errors• Compile-time errors

– Syntax errors– Type errors

• Link-time errors (can’t find other parts)• Run-time errors

– Detected by computer => crash– Detected by library => exception– Detected by user code

• Logic error– Detected by programmer (or user) => code runs but produces

wrong output

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CSCE 121:509-512 -- Set 7: Errors

Check Your Inputs

• Before trying to use an input value, make sure it meets your expectations/requirements– Function arguments– Data from input stream

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CSCE 121:509-512 -- Set 7: Errors

Bad Function Arguments

• The compiler helps– Number and types of arguments must match

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int area(int length, int width) { return length*width;}…int x1 = area(7); //error; wrong number of argsint x2 = area(“seven”,2); // error; first arg has wrong typeint x3 = area(7,10); // okint x4 = area(7.5,10); // ok but dangerous (7.5 => 7)int x5 = area(10,-7); // types are correct but nonsensical

CSCE 121:509-512 -- Set 7: Errors

Bad Function Arguments

• What to do about area(10,-7)?• Caller should check

– Hard to do systematically• Function should check

1.Return an “error value”, or2.Set an error status indicator, or3.Throw an exception

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CSCE 121:509-512 -- Set 7: Errors

Returning an Error Value

• Caller must check:if (area(x,y) < 0) cout << “error”;

• Problems:–What if caller forgets to check?–For some values, there is no bad value to return (e.g., max)

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int area(int length, int width) { if (length <= 0 || width <= 0) return -1; return length*width;}

Bad idea

CSCE 121:509-512 -- Set 7: Errors

Setting an Error Status Indicator

• Caller must check:int z = area(x,y);

if (errno == 7) cout << “bad area computation”;

• Problems:– What if caller forgets to check?– How to choose a unique value for errno?– How does caller deal with the error?

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int errno = 0; // global variableint area(int length, int width) { if (length <= 0 || width <= 0) errno = 7; return length*width}

Bad idea

CSCE 121:509-512 -- Set 7: Errors

Throwing an Exception

• Special syntax for dealing with errors• When an error situation is encountered, code

invokes a throw statement• Associated with the throw statement is an

object containing more information (can be of user-defined type) – the exception

• When throw is executed, control is returned to the calling function together with the thrown object

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CSCE 121:509-512 -- Set 7: Errors

Catching an Exception• Calling function should have the call to the throwing

function enclosed in a try statement• At the end of the try statement is one (or more)

catch clauses• Each catch clause has an argument and some code

that will be executed if an exception is thrown inside the try and the thrown object has the matching type– This is the error-handling code!

• Execution resumes after the catch clause

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Exception Example

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class Bad_area {}; // user-defined type to be used // for making exception objects

int area(int length, int width) { if (length <= 0 || width <= 0) throw Bad_area(); // make an unnamed Bad_area object return length*width;}

…try { int z = area(x,y);}catch(Bad_area) { // parameter is an unnamed Bad_area object cerr << “Oops! Bad area calculation – fix program\n”;}…

CSCE 121:509-512 -- Set 7: Errors

Exceptions

• Exception handling is general– You cannot forget about an exception: program

will terminate if the exception is not handled– Just about every kind of error can be reported

using exceptions• You still have to figure out what to do about

an exception

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CSCE 121:509-512 -- Set 7: Errors

Out-of-Range Exceptions

• When using std_lib_facilities_4.h, vector will throw a Range_error exception if you use an invalid index

• The default behavior does not ensure this though!

• Mechanism is in std_lib_facilities_4.h for pedagogical purposes

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Exceptions – For Now

• For now, just use exceptions to terminate programs gracefully, like this:

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int main()try{ // your program here}catch(out_of_range) { // out of range exceptions cerr << “oops, some vector index is out of range\n”;}catch(…) { // all other exceptions cerr << “oops, some other exception\n”;}

CSCE 121:509-512 -- Set 7: Errors

Utility error Function

• Provided in std_lib_facilities_4.h• Combines two tasks:

– Allows you to print an error message of your choosing

– Throws a runtime error so that program halts

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void error(string s){ // make an unnamed object of type runtime_error with // parameter s (string that can be accessed in catch clause) throw runtime_error(s); // throw the object made}

CSCE 121:509-512 -- Set 7: Errors

Using error

• See using-error.cpp

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CSCE 121:509-512 -- Set 7: Errors

How to Find Errors• Make the program easy to read!• Comments – explain design ideas• Use meaningful names• Indent consistently• Break code into small functions (at most a

page)• Avoid complicated code sequences if possible

(such as nested loops, nested if-statements)• Use library facilities

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CSCE 121:509-512 -- Set 7: Errors

Common Compile-Time Errors• String literal not terminated

cout << “Hello, << name << ‘\n’;

• Character literal not terminatedcout << “Hello, “ << name << ‘\n;

• Block not terminatedif (a > 0) { /* do something */else { /* do something else */}

• Mismatched parenthesesif (a x = f(y);

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CSCE 121:509-512 -- Set 7: Errors

More Common Compile-Time Errors

• Name not declared before it’s used• Needed headers not included• Name spelled incorrectly (case matters)• Expression statement not terminated with a

semicolon

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CSCE 121:509-512 -- Set 7: Errors

Debugging

• Carefully follow the program through its sequence of steps

• Add debug output statements to check intermediate results and see where you are

• Check what the program actually says, not what you think it saysfor (int i=0; 0 < month.size(); ++i) { … }for (int i=0; i <= max; ++j) { … }

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CSCE 121:509-512 -- Set 7: Errors

Debugging: Corner Cases

• Is every variable initialized?• Did the function get the right values?• Did you handle the first / last element

correctly?• Did you handle the empty case correctly?• Did you open the files correctly?• Did you actually read that input?

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CSCE 121:509-512 -- Set 7: Errors

Preconditions

• What does a function require of its arguments?

• Such a requirement is called a precondition• Often a good idea to check if the requirement

holds• Example: arguments to area function should

be positive.

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CSCE 121:509-512 -- Set 7: Errors

Postconditions

• What must be true when a function returns?• Such a condition is called a postcondition• Example: area function must return a positive

integer that is the product of its arguments

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CSCE 121:509-512 -- Set 7: Errors

Pre- and Postconditions

• Always think about what they should be for each function you write

• At a minimum, put them in comments for the function

• Check them where “reasonable”• Check a lot when debugging

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Testing

• Be systematic and extensive• Test parts of program in isolation

– When you’ve written a complicated function, write a little program that just calls it to see how it behaves in isolation before putting it into the real program

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CSCE 121:509-512 -- Set 7: Errors

Acknowledgments

• Photo on slide 1: “Games Bank Error” by Chris Potter, licensed under CC BY 2.0

• Slides are based on those for the textbook:http://www.stroustrup.com/Programming/5_errors.ppt

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