#17. Concepts in C -- only through Examples & Explanations

This is a continuation of last post. 

Before seeing the answer try to predict it & then see the explanation.

All the Best!!

46) main( )
{
void *vp;
char ch = ‘g’, *cp = “goofy”;
int j = 20;
vp = &ch;
printf(“%c”, *(char *)vp);
vp = &j;
printf(“%d”,*(int *)vp);
vp = cp;
printf(“%s”,(char *)vp + 3);
}

47) main ( )
{
static char *s[ ] = {“black”, “white”, “yellow”, “violet”};
char **ptr[ ] = {s+3, s+2, s+1, s}, ***p;
p = ptr;
**++p;
printf(“%s”,*--*++p + 3);
}

48) main()
{
int i, n;
char *x = “girl”;
n = strlen(x);
*x = x[n];
for(i=0; i<n; ++i)
{
printf(“%s\n”,x);
x++;
}
}

49) int i,j;
for(i=0;i<=10;i++)
{
j+=5;
assert(i<5);
}

50) main()
{
int i=-1;
+i;
printf("i = %d, +i = %d \n",i,+i);
}

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46.
Answer:
g20fy
Explanation:
Since a void pointer is used it can be type casted to any other type pointer.
vp = &ch stores address of char ch and the next statement prints the value
stored in vp after type casting it to the proper data type pointer. the output is
‘g’. Similarly the output from second printf is ‘20’. The third printf statement
type casts it to print the string from the 4th value hence the output is ‘fy’.

47.
Answer:
ck
Explanation:
In this problem we have an array of char pointers pointing to start of 4 strings.
Then we have ptr which is a pointer to a pointer of type char and a variable p
which is a pointer to a pointer to a pointer of type char. p hold the initial value
of ptr, i.e. p = s+3. The next statement increment value in p by 1 , thus now
value of p = s+2. In the printf statement the expression is evaluated *++p
causes gets value s+1 then the pre decrement is executed and we get s+1 –
1 = s . the indirection operator now gets the value from the array of s and
adds 3 to the starting address. The string is printed starting from this position.
Thus, the output is ‘ck’.

48.
Answer:
(blank space)
irl
rl
l
Explanation:
Here a string (a pointer to char) is initialized with a value “girl”. The strlen
function returns the length of the string, thus n has a value 4. The next
statement assigns value at the nth location (‘\0’) to the first location. Now the
string becomes “\0irl” . Now the printf statement prints the string after each

iteration it increments it starting position. Loop starts from 0 to 4. The first
time x[0] = ‘\0’ hence it prints nothing and pointer value is incremented. The
second time it prints from x[1] i.e “irl” and the third time it prints “rl” and the
last time it prints “l” and the loop terminates.

49.
Answer:
Runtime error: Abnormal program termination.
assert failed (i<5), <file name>,<line number>
Explanation:
asserts are used during debugging to make sure that certain conditions are
satisfied. If assertion fails, the program will terminate reporting the same.
After debugging use,
#undef NDEBUG
and this will disable all the assertions from the source code. Assertion
is a good debugging tool to make use of.

50.
Answer:
i = -1, +i = -1
Explanation:
Unary + is the only dummy operator in C. Where-ever it comes you can just
ignore it just because it has no effect in the expressions (hence the name dummy operator).

To be continued...

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