1. The for Statement A most versatile loop
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2. The for The for is the most popular of the C family of language loops
This is because the initialization, test and advancement are all obvious as part of the header of the statement
Many other languages have a similar statement but this one has very few restrictions on the pieces
It is a leading decision loop
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3. The generalized for
The general form is: for(initialization;test;advancement) one statement
The for is a reserved word
The parentheses are required
The three items are quite flexible
Must be separated by a semicolon
The one statement is the body of the loop
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4. The Initialization
Always executed and always prior to first execution of the test
Usually is a simple assignment but may be more complicated
The same restrictions as on any assignment statement for (c = abs(3*i); c< …
The variable may also be declared there
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5. Control Variable A typical for has the same variable referenced in:
The initialization
The test
The advancement for(i=0; i<n; i++)…
Such a variable is known as the control variable
Its value controls the iteration of the for
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6. Variable Declaration in a For
The variable in the initialization may be declared at that time: for(int j = 0; j < k; j++)
It does not have to be a pre-existing variable
Thus the for and the compound statement allow declarations within them
This is the only location that allows a declaration within the for
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7. Scope So where is the variable known?
The variable declared in a for is known only in the for header and the one statement that follows it
The variable is created by the for
It is destroyed when the for is exited
Consider the next example
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8. Scope of a for control variable
int j = 5, k;
cin >> k; int sum = 0; for(int j = 2; j<k;j++){ sum = sum*j – k/2; k += 2; }
cout << k << “\n”; int val = 1; for(int k = 1;k<j;k++) val = val*k; cout << val << “\n”;
// larger scope
// scope of j
// scope of k
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9. Final words on initialization
The control variable does not have to be any particular type
It may also be left out
Suppose that the variable that you want as a control variable is already properly set
Leave out the initialization
Retain the semicolon
int a,b=2; cin >>a; for(;b<a;b++)…
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10. The test
The test is evaluated before every execution of the body of the loop
If it is true then the body is executed
Otherwise the loop is exited
The test is any boolean expression
May include comparisons
Of any type on any variables
May include boolean operators
&&, || or !
May also be a boolean variable
May be any numeric
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11. More on the test It is usually some test on the control variable
Does not have to be related to either the initialization or advancement k = 1; for(int i = 1;k>0;i++){ cin >> k; cout << i << “: “ << k; }
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12. Advancement Third piece of the for header
Only executed after the body of the loop
Always before the test on the second and subsequent times through the loop
May be any expression
Usually a ++ or – applied to control variable
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13. Advancement examples
The usual is count by constant one: for(i=0;i<j;i++) … for(i=0;i<j;i--) …
By any constant: for(double f=0;f<g;f+=0.25) …
The count by n: for(i=0;i<j;i+=n) …
Power of 2 for(i=1;i<j;i*=2) …
Something else: for(d=1.0; d<m; d+=ln(d))
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14. The Body of the Loop is one statement
Compound statement
Assignment
Function or Method call
Another for
Nested loops are also interesting
Any other statement
Empty statement
How can the body be empty?
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15. Empty statement How can the body be empty?
If we do all the work in the header
Suppose we want to compute the factorial: n! = 1*2*3*…*n
int fact = 1,n;
cin >> n;
for(int i=1;i<n;fact*=++i);
cout << fact; // not in for
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16. Common Error Sometimes we want the body of the for to be empty
Much more frequently we put a semicolon there by accident: for(i=1;i<=n;i++); fact *= i;
The assignment on fact is only executed once since the empty statement is the body of the loop
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17. Nested Loops
Nested loops are complicated from an execution viewpoint, but not from syntax
There is nothing like the else matching problem of the if statement
The only thing to remember is that the inner loop must not interfere with the outer loop’s test and advancement
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18. Nested Interference Consider the following two problems:
for(i=0;i<10;i++) for(i=0;i<12;i++)…
The outer loop is nullified
The inner loop leaves i larger than the test requirements of the outer loop
for(i=0;i<10;i++) for(i=0;i<5;i++)…
This is an infinite loop
The inner loop always leave i at 5
Outer loop gets it to 6 but is then reset to 5 by the inner loop
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19. Control error problem 1 We can get similar problems without nesting
Modification of the control variable in the loop: for(i=0;i<5;i++){ … i--; … }
This is an infinite loop
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20. Control error problem 2
Modification of the control variable in another way: for(i=0;i<5;i++){ … i++; … }
This will terminate but is deceptive
The double counting makes it hard to read
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21. Moral of story Do not change the control variable except in the header
If the control variable needs to be changed in multiple places or not at end of loop use a while
Programmers reading the code will not look for another change
The best you can get is hard to read code
The worst is an infinite loop
There are better ways to get that if desired
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22. For flow chart a Suppose the following loop: for(a;b;c) d; c b d true
false
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23. Order of execution Suppose the following loop: for(a;b;c) d;
The order of execution is: a, b, (d, c, b)*
Thus, these are possible: a, b a, b, d, c, b a, b, d, c, b , d, c, b a, b, d, c, b , d, c, b , d, c, b …
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24. For Equivalent to a While
The while is the most general loop of many languages, such as Pascal or VB
Not so in the C family of languages
Any while may be converted to an equivalent for: while(cond)… can be written as for(;cond;)… just leaving out the initialization and advancement
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25. String Processing
Fors are frequently used to process strings and arrays
s[i] is the ith character of the string s
Process an AnsiString:
for(int i=1;i<=s.Length(); i++){ char ch = s[i]; // process ch }
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26. Example: counting blanks
Count the blanks in string s
int blanks=0;
for(int i=1;i<=s.Length(); i++){ char ch = s[i]; if(ch==‘ ‘) blanks++;
} // end for
Count control characters by changing == to < or <=
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27. Equality tests
Beware
Not just real number equality comparisons but all
If the value is not hit, then infinite loop
Don’t do: for(i=0,i!=5;i+=2) instead for(i=0,i<=5;i+=2) especially when counting by things other than one
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28. The Comma Operator In C family comma is an operator
The unusual operator
It appears that is was first intended to be the way to bundle statements together
Perhaps before braces
There are some things that you can do with commas and others you should do with braces
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29. What does it do? The comma operator cuts off expression evaluation
It specifies that the thing on the left is executed first and the thing on the right second
Because of this there is no carryover from one side of the statement to the other
Does not define pieces as statements
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30. More Unlike use of the semicolon, it does not constitutes a statement
Usually used only in places where you need to do multiple thing but can only do one thing and cannot use braces:
The initialization and incrementation of for loops
Anywhere else a pair of braces is not acceptable
Very low precedence
Below =
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31. Example 1
In a for doing several things, with comma: for(i=5,s=0.8;i<10;i++,s+=k)
This presumes i and s are declared else where
A declaration forces all types to be the same for(int i=5,j=2;i<10;j+=k*++i)
Is legal but
for(int i=5,double j=2;i<10;i++)
Is not
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32. Example 2
Can also be used elsewhere: if(a>b) cout << “True”, b++, a--;
The comma separates the cout from b and a
No error but no output of b and a
Although this is better: if(a>b){ cout << “True”; b++; a--; }
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33. Last thoughts
The for control variable usually has no real world meaning
One of the few variables where a one character name is acceptable
Indentation of the for body is also a good idea
This gives better readability
We will also later consider the break and continue
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