C Interview Programs in Function Pointers, string functions, fibonacci numbers

Here are five challenging questions in C language. These are subset of frequently asked C interview questions in Interview Mantra. Question.51 is an advanced question that tests the ability to declare and use function pointers and array of function pointers. Question.52(simulation of strcmp), Question.53(simulation of strcat) test the knowledge of C standard library ‘stdio.h’ and programming ability to simulate a function of known functionality. Question.54(generation of fibonacci numbers) tests the adroit ability of programming and math skills. Question.55 tests basic knowledge and ability to use control structures in C.

51. Declare an array of three function pointers where each function receives two integers and returns float.

Declaration:

float (*fn[3])(int, int);

Program: Illustrates the usage of above declaration

include<stdio.h>
float (*fn[3])(int, int);
float add(int, int);

int main() {
 int x, y, z, j;
  for (j = 0; j < 3; j++){
  fn[j] = &add;
 }
 x = fn[0](10, 20);
 y = fn[1](100, 200);
 z = fn[2](1000, 2000);
 printf("sum1 is: %d \n", x);
 printf("sum2 is: %d \n", y);
 printf("sum3 is: %d \n", z);
 return 0;
}

float add(int x, int y) {
 float f = x + y;
 return f;
}

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Output:

sum1 is: 30
sum2 is: 300
sum3 is: 3000

Explanation:
Here ‘fn[3]‘ is an array of function pointers. Each element of the array can store the address of function ‘float add(int, int)’.

fn[0]=fn[1]=fn[2]=&add
Wherever this address is encountered add(int, int) function is called. External link to this question
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52. Write a program to compare two strings without using strcmp() function.

strcmp() function compares two strings lexicographically. strcmp is declared in stdio.h
Case 1: when the strings are equal, it returns zero.
Case 2: when the strings are unequal, it returns the difference between ascii values of the characters that differ.
a) When string1 is greater than string2, it returns positive value.
b) When string1 is lesser than string2, it returns negative value.

Syntax:
int strcmp (const char *s1, const char *s2);
Read more about strcmp.

Program: to compare two strings.

#include<stdio.h>
#include<string.h>
int cmpstr(char s1[10], char s2[10]);

int main() {
 char arr1[10] = "Nodalo";
 char arr2[10] = "nodalo";
 printf(" %d", cmpstr(arr1, arr2));
 //cmpstr() is equivalent of strcmp()
 return 0;
}

//s1, s2 are strings to be compared
int cmpstr(char s1[10], char s2[10]) {
 //strlen function returns the length of argument string passed
 int i = strlen(s1);
 int k = strlen(s2);
 int bigger;

 if (i < k) {
  bigger = k;
 }
 else if (i > k) {
  bigger = i;
 }
 else {
  bigger = i;
 }

 //loops 'bigger' times
 for (i = 0; i < bigger; i++) {
  //if ascii values of characters s1[i], s2[i] are equal do nothing
  if (s1[i] == s2[i]) {
  }
  //else return the ascii difference
  else {
   return (s1[i] - s2[i]);
  }
 }
 //return 0 when both strings are same
 //This statement is executed only when both strings are equal
 return (0);
}

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Output:

-32

Explanation:
cmpstr() is a function that illustrates C standard function strcmp(). Strings to be compared are sent as arguments to cmpstr().

Each character in string1 is compared to its corresponding character in string2. Once the loop encounters a differing character in the strings, it would return the ascii difference of the differing characters and exit.
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53. Write a function to concatenate two strings without using strcat function.

strcat(string1,string2) is a C standard function declared in the header file string.h
The strcat() function concatenates string2, string1 and returns string1. Read more about strcat().

Program: Program to concatenate two strings

#include<stdio.h>
#include<string.h>
char *strct(char *c1, char *c2);

char *strct(char *c1, char *c2) {
 //strlen function returns length of argument string
 int i = strlen(c1);
 int k = 0;
 //loops until null is encountered and appends string c2 to c1
 while (c2[k] != '\0') {
  c1[i + k] = c2[k];
  k++;
 }
 return c1;
}

int main() {
 char string1[15] = "first";
 char string2[15] = "second";
 char *finalstr;
 printf("Before concatenation:"
 " \n string1 = %s \n string2 = %s", string1, string2);
 //addresses of string1, string2 are passed to strct()
 finalstr = strct(string1, string2);
 printf("\nAfter concatenation:");
 //prints the contents of string whose address is in finalstr
 printf("\n finalstr = %s", finalstr);
 //prints the contents of string1
 printf("\n string1 = %s", string1);
 //prints the contents of string2
 printf("\n string2 = %s", string2);
 return 0;
}

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Output:

Before concatenation:
string1 = first
string2 = second
After concatenation:
finalstr = firstsecond
string1 = firstsecond
string2 = second

Explanation:
string2 is appended at the end of string1 and contents of string2 are unchanged.

In strct() function, using a for loop, all the characters of string ‘c2′ are copied at the end of c1. return (c1) is equivalent to return &c1[0] and it returns the base address of ‘c1′. ‘finalstr’ stores that address returned by the function strct().
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54. Write a program to generate the fibonacci series

Fibonacci series: Any number in the series is obtained by adding the previous two numbers of the series.

Let f(n) be n’th term.
f(0)=0;
f(1)=1;
f(n)=f(n-1)+f(n-2); (for n>=2)
Series is as follows
0
1
1 (1+0)
2 (1+1)
3 (1+2)
5 (2+3)
8 (3+5)
13 (5+8)
21 (8+13)
34 (13+21)
…and so on

Program: to generate Fibonacci Series(10 terms)

#include<stdio.h>
int main() {
 //array fib stores numbers of fibonacci series
 int i, fib[25];
 //initialized first element to 0
 fib[0] = 0;
 //initialized second element to 1
 fib[1] = 1;
 //loop to generate ten elements
 for (i = 2; i < 10; i++) {
  //i'th element of series is equal to the sum of i-1'th element and i-2'th element.
  fib[i] = fib[i - 1] + fib[i - 2];
 }
 printf("The fibonacci series is as follows \n");
 //print all numbers in the series
 for (i = 0; i < 10; i++) {
  printf("%d \n", fib[i]);
 }
 return 0;
}

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Output:

The fibonacci series is as follows
0
1
1
2
3
5
8
13
21
34

Explanation:
The first two elements are initialized to 0, 1 respectively. Other elements in the series are generated by looping and adding previous two numbers. These numbers are stored in an array and ten elements of the series are printed as output.
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55. Write a c program which ask the user for a number between 1 to 9 and shows the number, if the user inputs anything out of the specified range the program should show an error and then again ask for a valid input. Compute this program without using a switch case.

Program: Program for accepting a number in a given range.

#include<stdio.h>
int getnumber();

int main() {
 int input = 0;
 //call a function to input number from key board
 input = getnumber();
 //when input is not in the range of 1 to 9,print error message
 while (!((input <= 9) && (input >= 1))) {
  printf("[ERROR] The number you entered is out of range");
  //input another number
  input = getnumber();
 }
 //this function is repeated until a valid input is given by user.
 printf("\nThe number you entered is %d", input);
 return 0;
}

//this function returns the number given by user
int getnumber() {
 int number;
 //asks user for a input in given range
 printf("\nEnter a number between 1 to 9 \n");
 scanf("%d", &number);
 return (number);
}

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Output:

Enter a number between 1 to 9

45

[ERROR] The number you entered is out of range

Enter a number between 1 to 9

4

The number you entered is 4

Explanation:
getfunction() function accepts input from user. ‘while’ loop checks whether the number falls within range or not and accordingly either prints the number(If the number falls in desired range) or shows error message(number is out of range).
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About the Author:  This post was written by Ratan Dhoka. He is an alumnus of Kakatiya Institute of Technology. He enjoys coding in C and Java. You can reach him at ratan_jain77@yahoo.com

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