CSC300: Loop Mistakes

Contents [0/12]

Starter code [1/12]

Does this work? [2/12]

Does this work? [3/12]

Does this work? [4/12]

Does this work? [5/12]

Does this work? [6/12]

Does this work? [7/12]

Does this work? [8/12]

Does this work? [9/12]

Does this work? [10/12]

Does this work? [11/12]

Some common errors and advice [12/12]

(Click here for one slide per page)

Starter code [1/12]

package algs11;
import java.util.Arrays;
import stdlib.*;
public class Playground {
  /* Return number of times 5.0 occurs in the list */
  public static int numFives (double[] list) {
    return StdRandom.uniform (100); //TODO: fix this
  }
  /* This is a test function */
  public static void testNumFives (int expected, double[] list) {
    int actual = numFives (list);
    if (expected != actual) {
      StdOut.format ("Failed: Expecting [%d] Actual [%d] with argument %s\n", expected, actual, Arrays.toString (list));
    }
  }
  /* A main function for testing */
  public static void main (String[] args) {
    testNumFives (3, new double[] { 11, 21, 5, 31, 5, 41, 5, 51});
    testNumFives (4, new double[] { 11, 21, 5, 31, 5, 5, 41, 5, 51});
    testNumFives (4, new double[] { 11, 21, 5, 5, 5, 31, 41, 5, 51});
    testNumFives (0, new double[] { 11, 21, 31, 41 });
    testNumFives (1, new double[] { 11, 21, 5, 31, 41 });
    testNumFives (1, new double[] { 11, 21, 31, 41, 5 });
    testNumFives (1, new double[] { 5, 11, 21, 31, 41 });
    testNumFives (0, new double[] { 11 });
    testNumFives (1, new double[] { 5 });
    testNumFives (3, new double[] { 5, 5, 5 });
    testNumFives (0, new double[] { });
    StdOut.println ("Finished tests");
  }
}

Does this work? [2/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; i<a.length; i++)
      if (a[i] == 5.0)
        result++;   
    return result;
  }

Does this work? [3/12]

  public static int numFives (double[] a) {
    double[] list = new double[] { 4, 5, 6, 5, 3 };
    int result = 0;
    for (int i=0; i<list.length; i++)
      if (list[i] == 5.0)
        result++;   
    return result;
  }

Does this work? [4/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; i<a.length; i++) {
      if (a[i] == 5.0)
        result++; 
      else
        return result;
      return result;
    }
    StdOut.print (result); 
  }

Does this work? [5/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; i<a.length; i++)
      if (a[i] == 5.0)
        result++;   
      else
        i++;
    return result;
  }

Does this work? [6/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; i<a.length; i++)
      if (i == 5.0)
        result++;   
    return result;
  }

Does this work? [7/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; i<a.length; i++)
      if (a == 5.0)
        result++;   
    return result;
  }

Does this work? [8/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; 0<a.length; i++)
      if (a[i] == 5.0)
        result++;   
    return result;
  }

Does this work? [9/12]

  public static int numFives (double[] a) {
    int result = 0;
    for (int i=0; i>0; i++)
      if (a[i] == 5.0)
        result++;   
    return result;
  }

Does this work? [10/12]

  public static int numFives (double[] a) {
    int result = 0;
    int i = 0;
    while (i < a.length) {
      if (a[i] == 5.0) {
        result++; 
        i++;
      } else {
        i++;
      }
    }
    return result;
  }

Does this work? [11/12]

  public static int numFives (double[] a) {
    int result = 0;
    int i = 0;
    while (i < a.length) {
      if (a[i] == 5.0) {
        result++; 
        i++;
      }
      if (a[i] != 5.0)  
        i++;
    }
    return result;
  }

Some common errors and advice [12/12]

A common problem is using < when you mean >.

In the bound of a loop this can cause big trouble!

For example, consider:

LOOP1:
int i = 1;
while (i > a.length) {
  i += 1;
}

and

LOOP2:
int i = 1;
while (i < a.length) {
  i += 1;
}

If a.length is 0 or 1, then the LOOP1 runs forever!

Another common issue that comes up is incrementing in the wrong place.

LOOP3:
int i = 1;
while (i < a.length) {
  if (a[i] == 5.0) {
    result += 1;
    i += 1;
  }
}

If a is length 2 or more, and a[1] != 5.0, then this is an infinite loop!

Sometimes your indentation can get messed up if you are typing for a while... To fix that, use Source > Format. That is, use the Format option on the Source menubar item.

A final common issue I will mention for now. Sometimes you may forget to put in the curly braces. This can be confusing!

LOOP4:
int i = 1;
while (i < a.length) 
  if (a[i] == 5.0) {
    result += 1;
  }
  i += 1;

LOOP4 looks okay, right?

But if you format, it will turn into this:

LOOP5:
int i = 1;
while (i < a.length) 
  if (a[i] == 5.0) {
    result += 1;
  }
i += 1;

LOOP5 is the same as LOOP4, but the indentation has changed. It makes it clear that i is being incremented outside the loop. To fix this, you need to add curly braces:

LOOP6:
int i = 1;
while (i < a.length) {
  if (a[i] == 5.0) {
    result += 1;
  }
  i += 1;
}

Hurrah! finally a correct loop!

Revised: 2023-09-12 18:09