Hey I want to show all of you a supposed "shortcut" evaluating those nasty prefix /posfix expression I found over at the Java boards
Example:
Evaluate for b:
a = 10;
b = a++; //Answer is 10
Now I would tell my students to just rewrite to equvalent:
a = 10;
b = a; // b = 10
a++;
Easy right?
How about these:
Evaluate for x, given a = 2, b = 3, and c = 2
1. x= a++ + b++; //x = 5
2. x = ++a - --c; // x = 2
3. x = ++a - b++ + ++c; //x = 3
and given a = 10
4. x = a++ + ++a; //undefined in C, but x = 22 in Java
Ugh my student panic with these-- here are the shortcut rules:
1. for any i, subsitute the latest value of i
2. for ++symbol, increment the latest value
3. for -- symbol, decrement the latest value
4. finally do the calculation for all non brackted values
5. be careful when the assigned var is the same as the var which undergone all these ++/-- operations in the statement. In this case the assigned var takes its lastest value. (Which was corrected to say: Since the left hand side var happened to be the var i inteself it takes the value of calculated value.
So using the rules
1. x = 2(3) + 3(4) = 5
2 x = (3)3 - (1)1 = 2
3 x= (3)3 - 3(4) + (3)3 = 3
4. for Java, 10(11) + (12)12 = 22
Now, this shortcut seem to always work for all kinds of ++/-- expressions- we have tried this with a variety of student sand it seems to work.
The written rules seem not to be the clearest.
So I would like all of your comments about this shortcut and how can we clean up the wording?
Thanks to Anna over at Java. We are going to write these rules in notes for our C, c++, C# and Java classes. if you can help us I will give you credit as well for your help.
Thanks Mr. C.
