Programming - for loops & break(3)

Administrivia

Practice Web Page - http://www.cs.tau.ac.il/~efif/courses/Programming_Fall_04

for loops

for ( expr₁; expr₂; expr₃ ) statement
this is (almost) equivalent to writing:
expr₁;
while (expr₂) {
statement
expr₃;
}
for (i = 0; i < 10; i = i + 1) printf ("i is %d\n", i);
in this case:
- expr₁ is i = 0
- expr₂ is i < 10
- expr₃ is i = i + 1
- statement is printf ("i is %d\n", i);

Factorial with for loop

#include <stdio.h>

int main()
{
  int i, n, fact = 1;

  printf("Enter a number: ");
  if (scanf("%d", &n) != 1) {
    printf("ERROR: failed to rean a number!\n");
    return -1;
  }
  for (i = 1; i <= n; i++)
    fact *= i;
  printf("Factorial of is %d\n", fact);
  return 0;
}

fact2.c

The define directive

A technique to define constants in C
- There are other ways

Constants are defined using the preprocessor directive #define

The preprocessor is a program that modifies your source file prior to compilation

#include <stdio.h>

#define MONDAY 1
#define TUESDAY 2
#define WEDNESDAY 3
#define THURSDAY 4
#define FRIDAY 5
#define SATURDAY 6
#define SUNDAY 7

int main()
{
  int today = MONDAY;
  if ((today == SATURDAY) || (today == SUNDAY)) {
    printf("Weekend\n");
  } else {
    printf("Go to work or school\n");
  }

  return 0;
}

define.c

Nesting of for loops

A program that prints a square of asterisks ('*')

#include

#define SIZE 5

int main()
{
  int i, j;
  for (i = 1; i <= SIZE; i++) {
    for(j = 1; j <= SIZE; j++)
      printf("*");
    printf("\n");
  }
  return 0;
}

ast_square.c

More nesting

A program that prints an isosceles triangle of asterisks ('*')

#include <stdio.h>

int main()
{
  int i, j;
  int num;
  if (scanf("%d", &num) != 1) {
    printf("ERROR: failed to read a number!\n");
    return -1;
  }
  for (i = 0; i < num; i++) {
    for (j = 0; j < num - i - 1; j++)
      printf(" ");
    for (j = 0; j < 2 * i + 1; j++)
      printf("*");
    printf("\n");
  }

  return 0;
}

ast_tri.c

Characters

A program that prints the number of appearances of a character in a line

#include 

int main()
{
  char c, ch = 0;
  int counter = 0;
  printf("Enter a character: ");
  if (scanf("%c", &c) != 1) {
    printf("ERROR: failed to read a character!\n");
    return -1;
  }
  printf("Now enter the text: ");
  flushall();
  while (ch != '\n') {
    scanf("%c", &ch);
    if (ch == c)
      counter++;
  }
  if (counter != 0)
    printf("%c has appeared %d times\n", c, counter);
  else
    printf("%c was not found\n", c);
  return 0;
}

flushall() - flushes all input/output buffers
appears.c

The math library

A program that figures out whther a number is prime or not

#include <stdio.h>
#include <math.h>

int main()
{
  int i, prime, sq;
  printf("Enetr a number: ");
  if (scanf("%d", &prime) != 1) {
    printf("ERROR: failed to read a number!\n");
    return -1;
  }
  sq = (int) sqrt(prime);
  for (i = 2 ; i <= sq; i++) {
    if (prime % i == 0) {
      printf("%d is not prime (divided by %d)\n", prime, i);
      break;
    }
  }
  if (i > sq)
    printf("%d is prime\n", prime);
  return 0;
}

is_prime.c
sqrt()
- Declared in math.h
- double sqrt ( double x );
- returns the square root of x

Breaking from a loop

A program that prints all primes smaller than a number

#include <stdio.h>
#include <math.h>

int main()
{
  int i, j, sq, last;
  printf("Enetr a number: ");
  if (scanf("%d", &last) != 1) {
    printf("ERROR: failed to read a number!\n");
    return -1;
  }
  for (i = 2; i <= last; i++) {
    sq = (int) sqrt(i);
    for (j = 2 ; j <= sq; j++)
      if (i % j == 0)
        break;
    if (j > sq) 	
      printf("the number %d is prime\n", i);  
  }
  return 0;
}

primes.c

Maintained by Efi Fogel. Last modified: November 16 2003.