Interview Mantra

This post covers some of the interview puzzles asked in C language interviews. Below are five of the C interview programs that are frequently asked in freshers interview and experienced people interview. Question36(print helloworld without using semicolon), Question37(print semicolon without using semicolon) are puzzles asked in interviews. These two questions test problem solving ability and test the awareness of return value of printf function. Question38(delete a line in file), Question39(replace a line in file) are C programs that test the file handling concepts of C and general thinking ability of the interview candidate. Question40 tests the candidate’s knowledge of file reading and new line character ‘\n’ in C. See all C interview questions in this blog in one page.

36. Write a program in C to print “Hello World” without using semicolon anywhere in the code.

Generally when we use printf(“”) statement, we have to use a semicolon at the end. If printf is used inside an if condition, semicolon can be avoided.

Program: Program to print some thing with out using semicolon(;)

#include <stdio.h>
int main() {
 //printf returns the length of string being printed
 if (printf("Hello World\n")) //prints Hello World and returns 11
 {
  //do nothing
 }
 return 0;
}

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Output:

Hello World

Explanation:
The if statement checks for condition whether the return value of printf(“Hello World”) is greater than 0. printf function returns the length of the string printed. Hence the statement if (printf(“Hello World”)) prints the string “Hello World”.
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37. Write a program in C to print a semicolon without using a semicolon anywhere in the code.

Generally when use printf(“”) statement we have to use semicolon at the end.
If we want to print a semicolon, we use the statement: printf(“;”);
In above statement, we are using two semicolons. The task of printing a semicolon without using semicolon anywhere in the code can be accomplished by using the ascii value of ‘ ; ‘ which is equal to 59.

Program: Program to print a semicolon without using semicolon in the code.

#include <stdio.h>
int main(void) {
 //prints the character with ascii value 59, i.e., semicolon
 if (printf("%c\n", 59)) {
  //prints semicolon
 }
 return 0;
}

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Output:

;

Explanation:
If statement checks whether return value of printf function is greater than zero or not. The return value of function call printf(“%c”,59) is 1. As printf returns the length of the string printed. printf(“%c”,59) prints ascii value that corresponds to 59, that is semicolon(;). See complete list of ascii values.
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38. Write a program in C to delete a specific line from a text file.

In this program, user is asked for a filename he needs to change. User is also asked for the line number that is to be deleted. The filename is stored in ‘filename’. The file is opened and all the data is transferred to another file except that one line the user specifies to delete.

Program: Program to delete a specific line.

#include <stdio.h>
int main() {
 FILE *fp1, *fp2;
 //consider 40 character string to store filename
 char filename[40];
 char c;
 int del_line, temp = 1;

 //asks user for file name
 printf("Enter file name: ");
 //receives file name from user and stores in 'filename'
 scanf("%s", filename);
 //open file in read mode
 fp1 = fopen(filename, "r");
 c = getc(fp1);
 //until the last character of file is obtained
 while (c != EOF)
 {
  printf("%c", c);
  //print current character and read next character
  c = getc(fp1);
 }

 //rewind
 rewind(fp1);
 printf(" \n Enter line number of the line to be deleted:");
 //accept number from user.
 scanf("%d", &del_line);
 //open new file in write mode
 fp2 = fopen("copy.c", "w");
 c = getc(fp1);
 while (c != EOF) {
  c = getc(fp1);
  if (c == '\n')
   temp++;
  //except the line to be deleted
  if (temp != del_line)
  {
   //copy all lines in file copy.c
   putc(c, fp2);
  }
 }
 //close both the files.
 fclose(fp1);
 fclose(fp2);

 //remove original file
 remove(filename);
 //rename the file copy.c to original name
 rename("copy.c", filename);

 printf("\n The contents of file after being modified are as follows:\n");
 fp1 = fopen(filename, "r");
 c = getc(fp1);
 while (c != EOF) {
  printf("%c", c);
  c = getc(fp1);
 }
 fclose(fp1);
 return 0;
}

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Output:

Enter file name:abc.txt
hi.
hello
how are you?
I am fine
hope the same
Enter line number of the line to be deleted:4
The contents of file after being modified are as follows:
hi.
hello
how are you?
hope the same

Explanation:
In this program, user is asked for a filename that needs to be modified. Entered file name is stored in a char array ‘filename’. This file is opened in read mode using file pointer ‘fp1′. Character ‘c’ is used to read characters from the file and print them to the output. User is asked for the line number in the file to be deleted. The file pointer is rewinded back and all the lines of the file except for the line to be deleted are copied into another file “copy.c”. Now “copy.c” is renamed to the original filename. The original file is opened in read mode and the modified contents of the file are displayed on the screen.
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39. Write a program in C to replace a specified line in a text file.

Program: Program to replace a specified line in a text file.

#include <stdio.h>
int main(void) {
 FILE *fp1, *fp2;
 //'filename' is a 40 character string to store filename
 char filename[40];
 char c;
 int del_line, temp = 1;
 //asks user for file name

 printf("Enter file name: ");
 //receives file name from user and stores in 'filename'
 scanf("%s", filename);
 fp1 = fopen(filename, "r");
 //open file in read mode
 c = getc(fp1);
 //print the contents of file .
 while (c != EOF) {
  printf("%c", c);
  c = getc(fp1);
 }

 //ask user for line number to be deleted.
 printf(" \n Enter line number to be deleted and replaced");
 scanf("%d", &del_line);
 //take fp1 to start point.
 rewind(fp1);
 //open copy.c in write mode
 fp2 = fopen("copy.c", "w");
 c = getc(fp1);
 while (c != EOF) {
  if (c == '\n') {
  temp++;
 }

 //till the line to be deleted comes,copy the content from one file to other
 if (temp != del_line){
  putc(c, fp2);
 }
 else //when the line to be deleted comes
 {
  while ((c = getc(fp1)) != '\n') {
  }
  //read and skip the line ask for new text
  printf("Enter new text");
  //flush the input stream
  fflush(stdin);
  putc('\n', fp2);
  //put '\n' in new file
  while ((c = getchar()) != '\n')
   putc(c, fp2);
  //take the data from user and place it in new file
  fputs("\n", fp2);
  temp++;
 }
 //continue this till EOF is encountered
 c = getc(fp1);
 }
 //close both files
 fclose(fp1);
 fclose(fp2);

 //remove original file
 remove(filename);
 //rename new file with old name opens the file in read mode
 rename("copy.c", filename);

 fp1 = fopen(filename, "r");
 //reads the character from file
 c = getc(fp1);
 //until last character of file is encountered
 while (c != EOF){
  printf("%c", c);
  //all characters are printed
  c = getc(fp1);
 }
 //close the file pointer
 fclose(fp1);
 return 0;
}

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Output:

Enter file name:abc.txt
hi.
hello
how are you?
hope the same
Enter line number of the line to be deleted and replaced:4
Enter new text: sayonara see you soon
hi.
hello
how are you?
sayonara see you soon

Explanation:
In this program, the user is asked to type the name of the file. The File by name entered by user is opened in read mode. The line number of the line to be replaced is asked as input. Next the data to be replaced is asked. A new file is opened in write mode named “copy.c”. Now the contents of original file are transferred into new file and the line to be modified is deleted. New data is stored in its place and remaining lines of the original file are also transferred. The copied file with modified contents is replaced with the original file’s name. Both the file pointers are closed and the original file is again opened in read mode and the contents of the original file is printed as output.
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40. Write a program in C to find the number of lines in a text file.

Number of lines in a file can be determined by counting the number of new line characters present.

Program: Program to count number of lines in a file.

#include <stdio.h>
int main()
/* Ask for a filename and count number of lines in the file*/
{
 //a pointer to a FILE structure
 FILE *fp;
 int no_lines = 0;
 //consider 40 character string to store filename
 char filename[40], sample_chr;
 //asks user for file name
 printf("Enter file name: ");
 //receives file name from user and stores in a string named 'filename'
 scanf("%s", filename);
 //open file in read mode
 fp = fopen(filename, "r");
 //get character from file and store in sample_chr

 sample_chr = getc(fp);
 while (sample_chr != EOF) {
  //Count whenever sample_chr is '\n'(new line) is encountered
  if (sample_chr == '\n')
  {
   //increment variable 'no_lines' by 1
   no_lines=no_lines+1;
  }
  //take next character from file.
  sample_chr = getc(fp);
 }
 fclose(fp); //close file.
 printf("There are %d lines in %s \n", no_lines, filename);
 return 0;
}

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Output:

Enter file name:abc.txt
There are 4 lines in abc.txt

Explanation:
In this program, name of the file to be read is taken as input. A file by the given name is opened in read-mode using a File pointer ‘fp’. Characters from the file are read into a char variable ‘sample_chr’ with the help of getc function. If a new line character(‘\n’) is encountered, the integer variable ‘no_lines’ is incremented. If the character read into ‘sample_char’ is not a new line character, next character is read from the file. This process is continued until the last character of the file(EOF) is encountered. The file pointer is then closed and the total number of lines is shown as output.
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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