C for Computer Scientists

Lecture Outline

Aims and Objectives
More about if
switch
Conditional Expressions
More Loops
break and continue

• To cover more Control Flow commands

• if-else

• else-if

• switch

• Conditional Expressions

• do-while

• break

• continue

if-else

	if (expression)
		statement1
	else
		statement2
	
	/* where the else is optional */

• What happens if N is < 0 in the following example?

  if (n >= 0)
    for (i = 0; i < n; i++)
      if (s[i] > 0) {
         printf("...");
         return i;
      }
  else
    printf("error -- n is negative\n");
  

	if (expression)
		statement
	else if (expression)
		statement
	else if (expression)
		statement
	else if (expression)
		statement
	else
		statement

• Multi-way decision

  //Function name: binsearch 
  
  int binsearch(int x, int v[], int n)
  {
  	int low, high, mid;
  
  	low = 0;
  	high = n - 1;
  
  	while (low <= high) {
  		mid = (low + high) / 2;
  		if (x < v[mid]) {
  			high = mid - 1;
  		}
  		else if ( x > v[mid]) {
  			low = mid + 1;
  		}
  		else /* found match */
  			return mid;
  	}
  	return -1; /* no match */
  }
  

• A function to populate the int array... Group Exercise

• This is also used as a multi-way decision

  switch (expression) {
  	case const-expr: statements
  	case const-expr: statements
  	case const-expr: statements
  	default: statements
  }
  

• Expression must evaluate to an integer

• const-expr must be a constant integer value

• switch sees which case the evaluation of expression matches, and switches control to it

• If expression does not match a case, then default: will catch it. default: is optional

  #include <stdio.h>
  //Program name: count.c
  /* Program to count digits, white space, others */
  
  main()
  {
  	int c, i, nwhite, nother, ndigits[10];
  
  	nwhite = nother = 0;
  
  	for (i = 0; i < 10; ndigits[i++] = 0)
  	;
  	
  	while ((c = getchar()) != EOF) {
  		switch (c) {
  		case '0': case '1': case '2': case '3': 
  		case '4': case '5': case '6': case '7': 
  		case '8': case '9':
  			ndigits[c - '0']++;
  			break;
  		case ' ': case '\n': case '\t':
  			nwhite++;
  			break;
  		default:
  			nother++;
  			break;
  		}
  	}
  
  	printf("digits = ");
  	for (i = 0; i < 10; i++)
  		printf(" %d", ndigits[i]);
  	printf(", white space = %d, other = %d\n",
           
              nwhite, nother);
  } 
  

• Let's look at the following if-else condition

  /* find the larger of a and b */
  
  if (a < b) 
  	max = b;
  else
  	max = a;
  

• If a variable is being set depending on the truth or false hood of a conditional, then C provides a short-cut - the conditional expression

  var = expr1 ? expr2 : expr3
  
  z = (a < b) ? b : a;
  
  printf("You have %d item%s\n", i, (i==1) ? "" : "s");
  

• Converting a string to an integer

  #include <ctype.h>
  
  int atoi(char s[])
  {
  	int i, n, sign;
  	
  	/* skip leading white space */
  	for (i = 0; isspace(s[i]); i++)
  		;
  
  	sign = (s[i] == '-') ? -1 : 1;
  
  	/* skip sign */
  	if (s[i] == '-' || s[i] == '+')
  		i++;
  
  	for (n = 0; isdigit(s[i]); i++)
  		n = 10 * n + (s[i] - '0');
  	return sign * n;
  }
  

The atoi function is in the stdlib library.

• Already covered for and while in other lectures

do-while

• The for and while loops test the condition at the top of the loop

• The do-while loop tests the condition at the bottom of the loop

• The for and while loops will not execute if the condition is false the first time the loop is encountered

• The do-while loop is executed at least once

  do
  	statement
  while (condition) ;
  

• break has already been encountered in the switch command

• break can also be used in for, while, and do-while loops

• break causes the loop to be prematurely terminated

• Example

  #include <string.h>
  #include <ctype.h>
  .
  .
  .
  /* remove trailing blanks from a string */
  
  int trim(char s[])
  {
  
  	int i;
  
  	for (i = strlen(s) - 1; i >= 0; i--)
  		if (! isspace(s[i]))
  			break;
  
  	s[i+1] = '\0';
  	return i;
  }
  

• continue forces the next iteration of for, while, and do-while loops to begin. Cannot be used with switch

  for (i = 0; i < n; i++)
  	if (a[i] < 0)
  		continue;
  	/* if a[i] is positive, do this */
  

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