Dear visitor,
Thanks for your interest in C programming. In this page, you will find a list of interesting C programming questions/puzzles, These programs listed are the ones which I have received as e-mail forwards from my friends, a few I read in some books, a few from the internet, and a few from my coding experiences in C.Most of the programs are meant to be compiled, run and to be explained for their behaviour. The puzzles/questions can be broadly put into the following categories:
- General typo errors, which C programmers do often and are very difficult to trace.
- Small programs which are extremely hard to understand at the first examination. These questions make a good excercise of reading and understanding effecient code written by others.
I have used Gnu/Linux/gcc for all of them.
The order in which the programs appear doesn't have
any relation with the level of difficulty.
Please feel free to contact me if you need any help in solving the problems.
My contact info. is available here
And you might be interested in a few references for C programming,
which I personally found very interesting.
If you are preparing for campus interviews, you might find the following
link interesting:
http://placementsindia.blogspot.com
http://www.interviewmantra.net/category/interview-questions/c
Regards,
Gowri Kumar
The expected output of the following C program is to print the elements in the array. But when actually run, it doesn't do so.
#include<stdio.h>#defineTOTAL_ELEMENTS (sizeof(array) /sizeof(array[0]))intarray[] = {23,34,12,17,204,99,16};Find out what's going wrong.
intmain()
{
intd;
for(d=-1;d<= (TOTAL_ELEMENTS-2);d++)
printf("%d\n",array[d+1]);
return0;
}
hint
I thought the following program was a perfect C program. But on compiling, I found a silly mistake. Can you find it out (without compiling the program :-) ?
#include<stdio.h>voidOS_Solaris_print() {printf("Solaris - Sun Microsystems\n"); }voidOS_Windows_print() {printf("Windows - Microsoft\n"); } voidOS_HP-UX_print() {printf("HP-UX - Hewlett Packard\n"); }intmain() {intnum;printf("Enter the number (1-3):\n");scanf("%d",&num);switch(num) {case1:OS_Solaris_print();break;case2:OS_Windows_print();break;case3:OS_HP-UX_print();break;default:printf("Hmm! only 1-3 :-)\n");break; }return0; }hint
What's the expected output for the following program and why?enum {false,true};intmain() {inti=1;do {printf("%d\n",i);i++;if(i<15)continue; }while(false);return0; }hint The following program doesn't "seem" to print "hello-out". (Try executing it) #include<stdio.h>#include<unistd.h>intmain()What could be the reason? |
#include<stdio.h>#definef(a,b) a##b#defineg(a) #a#defineh(a) g(a)intmain()Just by looking at the program one "might" expect the output to be, the same for both the printf statements. But on running the program you get it as:
{
printf("%s\n",h(f(1,2)));
printf("%s\n",g(f(1,2)));
return0;
}
bash$ ./a.out
12
f(1,2)
bash$
Why is it so?
hint
#include<stdio.h>intmain()If you expect the output of the above program to beNONE, I would request you to check it out!!
{
inta=10;
switch(a)
{
case'1':
printf("ONE\n");
break;
case'2':
printf("TWO\n");
break;
defa1ut:
printf("NONE\n");
}
return0;
}
The following C program segfaults of IA-64, but works fine on IA-32.
intmain()Why does it happen so?
{
int*p;
p= (int*)malloc(sizeof(int));
*p=10;
return0;
}
Here is a small piece of program(again just 14 lines of program) which counts the number of bits set in a number.
| Input | Output |
| 0 | 0(0000000) |
| 5 | 2(0000101) |
| 7 | 3(0000111) |
intCountBits (unsignedintx )Find out the logic used in the above program.
{
staticunsignedintmask[] = { 0x55555555,
0x33333333,
0x0F0F0F0F,
0x00FF00FF,
0x0000FFFF } ;
inti ;
intshift ; /* Number of positions to shift to right*/for ( i=0, shift=1; i<5; i++, shift*=2)
x= (x&mask[i ])+ ( ( x>>shift) &mask[i]);
returnx;
}
What do you think would be the output of the following program and why? (If you are about to say "f is 1.0", I would say check it out again)
#include<stdio.h>intmain() {floatf=0.0f;inti;for(i=0;i<10;i++)f=f+0.1f;if(f==1.0f)printf("f is 1.0 \n");elseprintf("f is NOT 1.0\n");return0; }
I thought the following C program is perfectly valid (after reading about the comma operator in C). But there is a mistake in the following program, can you identify it?
#include<stdio.h>intmain() {inta=1,2;printf("a : %d\n",a);return0; }
What would be the output of the following C program? (Is it a valid C program?)
#include<stdio.h>intmain() {inti=43;printf("%d\n",printf("%d",printf("%d",i)));return0; }
voidduff(registerchar*to, registerchar*from, registerintcount)Is the above valid C code? If so, what is it trying to acheive and why would anyone do something like the above?
{
registerintn=(count+7)/8;
switch(count%8){
case0: do{ *to++=*from++;
case7: *to++=*from++;
case6: *to++=*from++;
case5: *to++=*from++;
case4: *to++=*from++;
case3: *to++=*from++;
case2: *to++=*from++;
case1: *to++=*from++;
}while( --n>0);
}
}
Here is yet another implementation of CountBits. Verify whether it is correct (how do you that???). If so, find out the logic used.
intCountBits(unsignedintx)
{
intcount=0;
while(x)
{
count++;
x=x&(x-1);
}
returncount;
}
Are the following two function prototypes same?
intfoobar(void);The following programs should be of some help in finding the answer: (Compile and run both the programs and see what happens)
intfoobar();
Program 1:
#include<stdio.h>voidfoobar1(void)Program 2:
{
printf("In foobar1\n");
}
voidfoobar2()
{
printf("In foobar2\n");
}
intmain()
{
charch='a';
foobar1();
foobar2(33, ch);
return0;
}
#include<stdio.h>voidfoobar1(void)
{
printf("In foobar1\n");
}
voidfoobar2()
{
printf("In foobar2\n");
}
intmain()
{
charch='a';
foobar1(33, ch);
foobar2();
return0;
}
What's the output of the following program and why?
#include<stdio.h>intmain()
{
floata=12.5;
printf("%d\n", a);
printf("%d\n", *(int*)&a);
return0;
}
The following is a small C program split across files. What do you expect the output to be, when both of them compiled together and run?
File1.c
intarr[80];File2.c
externint*arr;
intmain()
{
arr[1] =100;
return0;
}
Explain the output of the following C program (No, the output is not 20).
#include<stdio.h>intmain()
{
inta=1;
switch(a)
{ intb=20;
case1: printf("b is %d\n",b);
break;
default:printf("b is %d\n",b);
break;
}
return0;
}
What is the output of the following program? (Again, it is not 40, (if the size of integer is 4)).
#defineSIZE10voidsize(intarr[SIZE])
{
printf("size of array is:%d\n",sizeof(arr));
}
intmain()
{
intarr[SIZE];
size(arr);
return0;
}
The following is a simple c program, in which there is a function called Error to display errors. Can you see a potential problem with the way Error is defined?
#include<stdlib.h>#include<stdio.h>voidError(char*s)
{
printf(s);
return;
}
intmain()
{
int*p;
p=malloc(sizeof(int));
if(p==NULL)
{
Error("Could not allocate the memory\n");
Error("Quitting....\n");
exit(1);
}
else {
/*some stuff to use p*/ }
return0;
}
What is the differnce between the following function calls to scanf?(Please notice the space carefully in the second call. Try removing it and observe the behaviour of the program)
#include<stdio.h>intmain()
{
charc;
scanf("%c",&c);
printf("%c\n",c);
scanf(" %c",&c);
printf("%c\n",c);
return0;
}
What is the potential problem with the following C program?
#include<stdio.h>intmain()
{
charstr[80];
printf("Enter the string:");
scanf("%s",str);
printf("You entered:%s\n",str);
return0;
}
What is the output of the following program?
#include<stdio.h>intmain()
{
inti;
i=10;
printf("i : %d\n",i);
printf("sizeof(i++) is: %d\n",sizeof(i++));
printf("i : %d\n",i);
return0;
}
Why does the following program give a warning? (Please remember that sending a normal pointer to a function requiring const pointer does not give any warning)
#include<stdio.h>voidfoo(constchar**p) { }
intmain(intargc, char**argv)
{
foo(argv);
return0;
}
What is the output of the following program?
#include<stdio.h>intmain()
{
inti;
i=1,2,3;
printf("i:%d\n",i);
return0;
}
The following is a piece of code which implements the reverse Polish Calculator. There is a(are) serious(s) bug in the code. Find it(them) out!!! Assume that the function getop returns the appropriate return values for operands, opcodes, EOF etc..
#include<stdio.h>#include<stdlib.h>#defineMAX80#defineNUMBER'0'intgetop(char[]);
voidpush(double);
doublepop(void);
intmain()
{
inttype;
chars[MAX];
while((type=getop(s)) !=EOF)
{
switch(type)
{
caseNUMBER:
push(atof(s));
break;
case'+':
push(pop() +pop());
break;
case'*':
push(pop() *pop());
break;
case'-':
push(pop() -pop());
break;
case'/':
push(pop() /pop());
break;
/* ... * ... * ... */ }
}
}
The following is a simple program which implements a minimal version of banner command available on most *nix systems. Find out the logic used in the program.
#include<stdio.h>#include<ctype.h>chart[]={
0,0,0,0,0,0,12,18,33,63,
33,33,62,32,62,33,33,62,30,33,
32,32,33,30,62,33,33,33,33,62,
63,32,62,32,32,63,63,32,62,32,
32,32,30,33,32,39,33,30,33,33,
63,33,33,33,4,4,4,4,4,4,
1,1,1,1,33,30,33,34,60,36,
34,33,32,32,32,32,32,63,33,51,
45,33,33,33,33,49,41,37,35,33,
30,33,33,33,33,30,62,33,33,62,
32,32,30,33,33,37,34,29,62,33,
33,62,34,33,30,32,30,1,33,30,
31,4,4,4,4,4,33,33,33,33,
33,30,33,33,33,33,18,12,33,33,
33,45,51,33,33,18,12,12,18,33,
17,10,4,4,4,4,63,2,4,8,
16,63 };
intmain(intargc,char**argv)
{
intr,pr;
for(r=0;r<6;++r)
{
char*p=argv[1];
while(pr&&*p)
{
into=(toupper(*p++)-'A')*6+6+r;
o=(o<0||o>=sizeof(t))?0:o;
for(pr=5;pr>=-1;--pr)
{
printf("%c",( ( (pr>=0) && (t[o]&(1<<pr)))?'#':' '));
}
}
printf("\n");
}
return0;
}
What is the output of the following program?
#include<stdio.h>#include<stdlib.h>#defineSIZEOF(arr) (sizeof(arr)/sizeof(arr[0]))#definePrintInt(expr) printf("%s:%d\n",#expr,(expr))intmain()
{
/* The powers of 10 */intpot[] = {
0001,
0010,
0100,
1000 };
inti;
for(i=0;i<SIZEOF(pot);i++)
PrintInt(pot[i]);
return0;
}
The following is the implementation of the Euclid's algorithm for finding the G.C.D(Greatest Common divisor)of two integers. Explain the logic for the below implementation and think of any possible improvements on the current implementation.
BTW, what does scanf function return?
#include<stdio.h>intgcd(intu,intv)Also implement a C function similar to the above to find the GCD of 4 integers.
{
intt;
while(v>0)
{
if(u>v)
{
t=u;
u=v;
v=t;
}
v=v-u;
}
returnu;
}
intmain()
{
intx,y;
printf("Enter x y to find their gcd:");
while(scanf("%d%d",&x, &y) !=EOF)
{
if(x>0&&y>0)
printf("%d%d%d\n",x,y,gcd(x,y));
printf("Enter x y to find their gcd:");
}
printf("\n");
return0;
}
What's the output of the following program. (No, it's not 10!!!)
#include<stdio.h>#definePrintInt(expr) printf("%s : %d\n",#expr,(expr))intmain()
{
inty=100;
int*p;
p=malloc(sizeof(int));
*p=10;
y=y/*p; /*dividing y by *p */;
PrintInt(y);
return0;
}
The following is a simple C program to read a date and print the date. Run it and explain the behaviour
#include<stdio.h>intmain()
{
intday,month,year;
printf("Enter the date (dd-mm-yyyy) format including -'s:");
scanf("%d-%d-%d",&day,&month,&year);
printf("The date you have entered is %d-%d-%d\n",day,month,year);
return0;
}
The following is a simple C program to read and print an integer. But it is not working properly. What is(are) the mistake(s)?
#include<stdio.h>intmain()
{
intn;
printf("Enter a number:\n");
scanf("%d\n",n);
printf("You entered %d\n",n);
return0;
}
The following is a simple C program which tries to multiply an integer by 5 using the bitwise operations. But it doesn't do so. Explain the reason for the wrong behaviour of the program.
#include<stdio.h>#definePrintInt(expr) printf("%s : %d\n",#expr,(expr))intFiveTimes(inta)
{
intt;
t=a<<2+a;
returnt;
}
intmain()
{
inta=1, b=2,c=3;
PrintInt(FiveTimes(a));
PrintInt(FiveTimes(b));
PrintInt(FiveTimes(c));
return0;
}
Is the following a valid C program?
#include<stdio.h>#definePrintInt(expr) printf("%s : %d\n",#expr,(expr))intmax(intx, inty)
{
(x>y) ? returnx :returny;
}
intmain()
{
inta=10, b=20;
PrintInt(a);
PrintInt(b);
PrintInt(max(a,b));
}
The following is a piece of C code, whose intention was to print a minus sign 20 times. But you can notice that, it doesn't work.
#include<stdio.h>intmain()Well fixing the above code is straight-forward. To make the problem interesting, you have to fix the above code, by changing exactly one character. There are three known solutions. See if you can get all those three.
{
inti;
intn=20;
for( i=0; i<n; i-- )
printf("-");
return0;
}
What's the mistake in the following code?
#include<stdio.h>intmain()
{
int*ptr1,ptr2;
ptr1=malloc(sizeof(int));
ptr2=ptr1;
*ptr2=10;
return0;
}
What is the output of the following program?
#include<stdio.h>intmain()
{
intcnt=5, a;
do {
a/=cnt;
} while (cnt--);
printf ("%d\n", a);
return0;
}
What is the output of the following program?
#include<stdio.h>intmain()
{
inti=6;
if( ((++i<7) && ( i++/6)) || (++i<=9))
;
printf("%d\n",i);
return0;
}
What is the bug in the following program?
#include<stdlib.h>#include<stdio.h>#defineSIZE15intmain()
{
int*a, i;
a=malloc(SIZE*sizeof(int));
for (i=0; i<SIZE; i++)
*(a+i) =i*i;
for (i=0; i<SIZE; i++)
printf("%d\n", *a++);
free(a);
return0;
}
Is the following a valid C program? If so, what is the output of it?
#include<stdio.h>intmain()
{
inta=3, b=5;
printf(&a["Ya!Hello! how is this? %s\n"], &b["junk/super"]);
printf(&a["WHAT%c%c%c%c%c%c !\n"], 1["this"],
2["beauty"],0["tool"],0["is"],3["sensitive"],4["CCCCCC"]);
return0;
}
What is the output of the following, if the input provided is:
Life is beautiful
#include<stdio.h>intmain()
{
chardummy[80];
printf("Enter a string:\n");
scanf("%[^a]",dummy);
printf("%s\n",dummy);
return0;
}
Note : This question has more to do with Linker than C language
We have three files a.c, b.c and main.c respectively as follows:
a.c
---
inta;b.c
---
inta=10;main.c
------
externinta;intmain() {printf("a = %d\n",a);return0; }Let's see what happens, when the files are compiled together:
bash$ gcc a.c b.c main.c bash$ ./a.out a = 10Hmm!! no compilation/linker error!!! Why is it so??
The following is the offset macros which is used many a times. Figure out what is it trying to do and what is the advantage of using it.
#defineoffsetof(a,b) ((int)(&(((a*)(0))->b)))
The following is the macro implementation of the famous, Triple xor swap.
#defineSWAP(a,b) ((a) ^= (b) ^= (a) ^= (b))What are the potential problems with the above macro?
What is the use of the following macro?
#defineDPRINTF(x) printf("%s:%d\n",#x,x)
Let's say you were asked to code a function IAddOverFlow which takes three parameters, pointer to an integer where the result is to be stored, and the two integers which needs to be added. It returns 0 if there is an overflow and 1 otherwise:
intIAddOverFlow(int*result,inta,intb)So, how do you code the above function? (To put in a nutshell, what is the logic you use for overflow detection?)
{
/* ... */ }
What does the following macro do?
#defineROUNDUP(x,n) ((x+n-1)&(~(n-1)))
Most of the C programming books, give the following example for the definition of macros.
#defineisupper(c) (((c) >='A') && ((c) <='Z'))But there would be a serious problem with the above definition of macro, if it is used as follows (what is the problem??)
charc;But most of the libraries implement the isupper (declared in ctypes.h) as a macro (without any side effects). Find out how isupper() is implemented on your system.
/* ... */if(isupper(c++))
{
/* ... */ }
I hope you know that ellipsis (...) is used to specify variable number of arguments to a function. (What is the function prototype declaration for printf?) What is wrong with the following delcaration?
intVarArguments(...)
{
/*....*/return0;
}
Write a C program to find the smallest of three integers, without using any of the comparision operators.
What does the format specifier %n of printf function do?
Write a C function which does the addition of two integers without using the '+' operator. You can use only the bitwise operators.(Remember the good old method of implementing the full-adder circuit using the or, and, xor gates....)
How do you print I can print % using the printf function? (Remember % is used as a format specifier!!!)
What's the difference between the following two C statements?
constchar*p;
char*constp;
What is the difference between memcpy and memmove?
What is the format specifiers for printf to print double and float values?
Write a small C program to determine whether a machine's type is little-endian or big-endian.
Write a C program which prints Hello World! without using a semicolon!!!
This work
by Gowri Kumar Chandramouli is licensed under a
Creative Commons Attribution-ShareAlike 3.0 Unported License.