CS 509, Advanced Programming II

Fall 2001 - Test 1

1. A colleague of yours believes that using declarations do not obey block scope. Write a small piece of code, which, when compiled, will convincingly demonstrate that your friend's belief is wrong.

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   #include <iostream>
   
   void t(void) {
     { using std::out;
       cout << "hello\n";   // Ok.
     }
     cout << "hello\n";     // Not ok.
     }
   

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2. Explain whether or not the following statement is correct:

   const int i;
   

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   The declaration statement doesn't assign i a value. the statement should be

   const int i = expr;

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3. Given the following code

   int main() {
     int a[] = { 0, 1, 2, 3 };         // Note that a[i] == i for 0 <= i < 4.
     int i, j, k;
   
     i = j                             // Assume 0 <= j < 4; 
     k = ?;	                    // The mystery statement.
     if ((a[j] == j) && (k == j + 1))
       cout << "Yes!\n";
     }
   

   which, if any, of the following expressions could be substituted for the ? in the mystery statement to have the program print Yes! when run:

   a) a[i++]++
   b) a[++i]++
   c) ++a[i++]
   d) ++a[++i]

   Justify your answer.

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   The expression i++ returns the value of j, which means that both a[i++]++ and ++a[i++] are equivalent to a[j]++ and ++a[j] respectively. Either of these two expressions increases the value of a[j] by one, which would make a[j] == j be false. Replacing ? with either a or c would not print Yes!.

   The expression ++i returns the value of j + 1, which means that both a[++i]++ and ++a[++i] are equivalent to a[j + 1]++ and ++a[j + 1] respectively. The value of a[j + 1] is j + 1, so a[j + 1]++ sets k to j + 1 and ++a[j + 1] sets k to j + 2. Substituting d for ? makes k == j + 1 false, while substituting b for ? makes k == j + 1 true.

   Substituting b for ? makes both a[j] == j and k == j + 1 true, printing Yes!.

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4. Explain whether or not the following statement is syntactically correct.
   if (b-expr) ;

   Assume b-expr is any syntactically correct boolean expression.

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   It's syntactically correct, although logically nonsensical. A semicolon with no statement is known as a empty statement. Because errant semicolons are a source of subtle programming errors, the preferred way to indicate empty statements is with a pair of braces:

   if (b-expr) { }

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This page last modified on 21 September 2001.