The document discusses the for loop statement in C language. It explains that a for loop contains three expressions: (1) an initialization expression (exp1) run before the loop, (2) a condition expression (exp2) that determines if the loop runs, and (3) an expression (exp3) run after each loop iteration. It provides an example of using a for loop to calculate the sum of the first n natural numbers. The document also discusses how while loops can be simulated with for loops and vice versa.

1. PDS: CS 11002 Computer Sc & Engg: IIT Kharagpur 1
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for Statement
Another iterative construct in C language is the
for-statement (loop). The structure or the
syntax of this statement is,
for (exp1; exp2; exp3) statement
Lect 9 Goutam Biswas

2. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 2
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for Loop
true false
exp1
exp2
statement
exp3
Lect 9 Goutam Biswas

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for Statement
• The expression1 is used for initialization
before entering the loop.
• The expression2 is to control the loop,
whether to enter the loop or to skip it.
• The expression3 executed after the execution
of the statement part of the loop, is used
essentially to update the loop control
condition.
Lect 9 Goutam Biswas

4. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 4
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for Statement
• All three expressions can be omitted. The
omitted control expression, expression2,
makes the condition true.
• The statement can also be null ‘;’.
Lect 9 Goutam Biswas

5. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 5
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Example III
We write program to solve the problem of
example III, the sum of first n natural numbers.
Lect 9 Goutam Biswas

6. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 6
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#include stdio.h
int main() // temp29.c
{
int n, i, sum = 0;
printf(Enter a +ve integer: );
scanf(%d, n);
for(i=1, sum=0; i=n; ++i)
sum += i;
printf(0+ ... + %d = %dn, n, sum);
return 0;
}
Lect 9 Goutam Biswas

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for Statement
for (i=1, sum=0; i=n; ++i)
sum + = i;
Lect 9 Goutam Biswas

8. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 8
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expression1
expression1 is ‘i=1, sum=0’. We have used the
comma operator (,). This is evaluated left to
right and is of lowest precedence among the
operators. The value of the expression is the
value of the rightmost operand. It is sum = 0
in this case.
Lect 9 Goutam Biswas

9. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 9
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while and for Loops
A while-statement can be simulated by a
for-statement.
while(exp) stmt ≡ for(; exp;) stmt
Lect 9 Goutam Biswas

10. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 10
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while andfor Loops
Similarly a for-statement can be simulated by
a while-statement and expression statements.
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for(exp1; exp2; exp3) stmt
≡
exp1; while(exp2) {stmt exp3;}
This equivalence is not true if there is a
continue statement in stmt.
Lect 9 Goutam Biswas

11. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 11
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Example IV
Read n int data and print the largest among
them.
The first input is the number of data, n.
Following inputs are a sequence of n int data.
It is essential to print the input.
Lect 9 Goutam Biswas

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Inductive Definition
lrgst(d1, d2, · · · , dn) =



d1 if n = 1,
max(d1, lrgst(d2, · · · , dn)) if n 1.
Lect 9 Goutam Biswas

13. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 13
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Sequence of Operations
• Read the number of data n(≥ 1).
• Read the first data in a variable named
largesta
.
• In a for-loop read the ith
data (i = 2 · · · n).
If it is larger than the data present in
largest, copy it to largest.
aUpto this point of execution, this is the largest data.
Lect 9 Goutam Biswas

14. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 14
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#include stdio.h
int main() // temp30.c
{
int n, largest, i;
printf(Enter a +ve integer: );
scanf(%d, n);
printf(Enter %d integers:n, n);
scanf(%d, largest);
printf(Input data: %d , largest) ;
for(i=2; i=n; ++i) {
int temp ; // local to block
scanf(%d, temp);
Lect 9 Goutam Biswas

15. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 15
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printf(%d , temp) ;
if (temp largest) largest = temp ;
}
printf(nLargest: %dn, largest);
return 0;
}
Lect 9 Goutam Biswas

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Note
• It is not necessary to know the number of
data a priori.
• We shall use EOF (end-of-file) from the
keyboard (Ctrl+D) to terminate the input.
• Every call to scanf() returns the number of
data read. If the Ctrl+D is pressed, scanf()
returns a special constant EOF (end-of-file,
defined in stdio.h).
Lect 9 Goutam Biswas

17. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 17
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#include stdio.h
int main() // temp31.c
{
int largest, count = 0, temp;
printf(Enter integer datan);
printf(Terminate by Ctrl+Dn);
scanf(%d, largest); // at least one data
printf(Input data: %d , largest) ;
++count ;
for(; scanf(%d, temp) != EOF;) {
printf(%d , temp);
if (temp largest) largest = temp ;
++count;
Lect 9 Goutam Biswas

18. PDS: CS 11002 Computer Sc Engg: IIT Kharagpur 18
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}
printf(nLargest among %d data: %dn,
count, largest);
return 0;
}
Lect 9 Goutam Biswas