Hello Friends ,
Here we'll discuss about some common errors we face during programming
Missing semicolons
Every
statement must end with a semicolon. A missing semicolon may cause considerable
confusion to the compiler and result in misleading error messages. Consider the
following statements.
a = x+y
b = m/n;
The
compiler will treat the second line as a part of the first one and treat b as
a variable name.
Misuse of Semicolon
Another
common mistake is to put a semicolon in a wrong place. Consider the following
code:
for(i =1;
i<=10; i++);
sum = sum + i;
The code
is supposed to sum all the integers from 1 to 10. But what actually happens is
that only the last value of i is added to the sum.
Missing Braces
It is
common to forget a closing brace when coding a deeply nested loop. The number
of opening braces should match with the closing ones. However, if we put a
matching brace in a wrong place, the compiler won’t notice the mistake and the
program will produce unexpected result.
Consider
following example:
for(i=1;1<=10;i++)
sum1=sum1+i;
sum2=sum2+i*i;
printf("%d%d\n",sum1,sum2);
In this
code the for loop treat only the first statement sum1=sum1+i; as
its body and therefore the statement sum2=sum2+i*i; is
evaluated only once when the loop is exited. The correct code is:
for(i=1;1<=10;i++)
{
sum1=sum1+i;
sum2=sum2+i*i;
}
printf("%d%d\n",sum1,sum2);
Missing Quotes
Every
string must be enclosed in double quotes, while a single character constant in
single quotes. If we miss them out, the string or the character will be
interpreted as a variable name. For Example:
if(response==YES)
//correct code is if(response=="YES")
Grade=A;
//correct code is Grade='A'
Improper Comment Characters
Every
comment should start with /* and end with */.
Anything between them is ignored by the compiler. If we miss out the closing */,
then the compiler searches for a closing */ further down in
the program, treating all the lines as comments. In case, it fails to find to
find a closing */, we may get an error message. Consider following
lines:
. . . . .
. . .
/* comment
line 1
Statement1;
Statement2;
/* comment
line 2 */
Statement
3;
. . . . .
. . .
Undeclared Variables
Every
variable must be declared for its type, before it is used. During the
development of a large program, it is quite possible to use a variable to hold
intermediate results and to forget to declare it.
Forgetting the Precedence of Operators
Expressions
are evaluated according to the precedence of operators. It is common among
beginners to forget this. Consider the statement:
if(value =
product() >= 100)
tax=0.05*value;
The call product() returns
the product of two numbers, which is compared to 100. If it is equal to or
greater than 100, the relational test is true and 1 is assigned to value,
otherwise 0 is assigned. In either case, the only values value can
take is 1 or 0. This is not we actually want.
The
statement was actually expected to assign the value returned by product() to value and
then compare value with 100. If value was
equal to or greater than 100, tax is computed.
The error
is due to the higher precedence of the relational operator compared to the
assignment operator. We can force the assignment to occur first by using
parentheses as follows:
if((value
= product()) >= 100)
tax=0.05*value;
Mismatching of Actual and Formal Parameter Type in
Function Calls
When a
function with parameters is called, we should ensure that the type of values
passed, match with the type expected by the called function. Otherwise,
erroneous results may occur. If necessary, we may use the type cast to change
the type locally.
Example: y
= cos((double) x);
Missing & operator in scanf Parameters
All
non-pointer variables in a scanf call should be preceded by an & operator.
If the variable code an integer, then the statement scanf("%d",
code); is wrong . The correct one is scanf("%d",
&code); Remember that the compiler will not detect the error and
you may get a crazy output.
Crossing the Bounds of an Array
Array
indices start from 0. A common mistake is to start the index from 1. For
example:
int
x[10],sum,i;
for(i=1;1<=10;i++)
sum=sum+x[i];
The above
code will not find the correct sum of the elements of array x. The
for loop expression should be corrected as follows:
for(i=0;1<10 i="" span="">
Forgetting a Space for Null character in as String
All
character arrays are terminated with a null character and therefore their size
should be declared to hold one character more than the actual string size.
Using Uninitialized Pointers
An
uninitialized pointer points to garbage. The following program is wrong:
main()
{
int a,*ptr;
a=25;
*ptr=a+5;
}
The
pointer ptr has not been initialized.
Missing Indirection and Address Operators
Another
common error is to forget to use the operators * and & in
certain places. Consider the following program:
main()
{
int m,*p1;
m=25;
p1=m;
printf("%d\n",*p1);
}
This will
print some unknown value because the pointer assignment p1=m; is
wrong. It should be p1=&m;
Consider
the following expression: y=p1+10;
Perhaps, y was
expected to be assigned the value at location p1 plus 10. But
it does not happen. y will contain some unknown address value.
The above expression should be rewritten as: y=*p1+10;
Missing Parentheses in Pointer Expressions
The
following two statements are not the same:
X=*p1+1;
X=*(p1+1);
The first
statement would assign the value at location p1 plus 1 to x,
while the second would assign the value at location p1+1.
Omitting Parentheses around Arguments in Macro
Definitions
This would
cause incorrect evaluation of expression when the macro definition is
substituted.
Example:
#define f(x) x*x+1
The
call:
y=f(a+b);
Will be
evaluated as: y=a+b * a+b+1;
Some other mistakes that we commonly make are:
·
Wrong indexing and
termination of loops.
·
Unending loops (infinite
loops).
·
Use of incorrect
relational test.
·
Failure to consider all
possible conditions of a variable.
·
Trying to divide by zero.
·
Mismatching of data
specification and variables in scanf and printf statements.
