Selection Statements
Selects statements to execute based on the value of an expression The expression is sometimes called
the controlling expression Selection statements:
if statement switch statement
Selection statements: if used to execute conditionally a
statement or block of code. if (expression) statement
If expression is true, statement is executed (what is true?).
statement can be replaced by a block of statements, enclosed in curly braces.
An example/* This program displays the absolute value of a number given by the
user */#include <stdio.h>
int main(void){
double num;
printf("Please enter a real number: ");scanf("%lf", &num);if (num<0)
num = -num;
printf("The absolute value is %g\n", num);
return 0;}
If-else statementif (expression)
statement1 else
statement2
if expression is true, statement1 is executed. if expression is false, statement2 is executed both statements can be (and very often are)
replaced by blocks of statements (“compound statements”)
An example (fragment)int first, second, min;/* … */if (first < second) { min = first; printf ("The first number is smaller than the second.\n");} else { min = second; printf ("The second number is smaller than the first\n");}
printf("The smaller number is equal to %d\n", min);
True or false In C, every expression has a numeric
value An expression is ‘true’ when its non-
zero If it is zero, it is false Therefore, in the following –
if (expression) statementstatement is executed if expression is non zero.
More about operators
In C, every expression has a numeric value
When using arithmetical operators (+, -, *, /) this is straight forward The value of A+B is the sum of A and
B And so on…
More about operators Expressions with relational operators
(<, <=, etc’) have values as well (intuitively, we are used to thinking about them as ‘true’ or ‘false’)
A < B equals zero if A is larger than or equal to B (false), and some non-zero value if A is smaller than B (true)
The exact non-zero value varies (and is not important for that matter)
Relational operators They are –
A == B (Note the difference from A = B) A != B A < B A > B A <= B A >= B
The value of the expression is non-zero if it’s true, zero if it’s false
An exampleint a, b;
printf("Enter two Numbers\n");scanf("%d%d", &a, &b);
if (a == b){
printf("The numbers equal %d\n", a);printf("The expression a == b is %d\n", a == b);
}else{
printf("The numbers are not equal\n");printf("The expression a == b is %d\n", a == b);
}
The assignment operator = The assignment operator is also an
operator. Hence, expressions involving it have a numeric value
This value equals to whatever appears on the right of the assignment operator
For example – (x = 4) equals 4 (y = 0) equals 0
A very common mistake
Very often a programmer might confuse between the equality operator and the assignment operator - if (x==4) … if (x=4) …
The second is usually a mistake, but legal in C so the compiler doesn’t call it!
Examples
val.c, eqn_sign.c
Logical operators
Allows to evaluate two or more expressions - !A – ‘not’ - True when A is not, and
vice versa. A && B – ‘and’ - True when both A
and B are true A || B – ‘or’ (inclusive or) - True when
either A or B (or both) are true
A silly example#include <stdio.h>
int main(void) {
int grade;
printf("Please enter your grade: ");scanf("%d", &grade);
if (grade<0 || grade>100)printf("This is not a valid grade!\n");
elseprintf("This is indeed a grade.\n");
return 0;}
Else-if
if statements distinguish between exactly 2 cases and execute different code in each case
The else-if construction allows for a multi-way decision
Else-if
if (expression)statement
else if (expression)statement
else if (expression)statement
elsestatement
An exampleif (grade >= 90)
printf ("A\n"); else if (grade >= 80)
printf ("B\n"); else if (grade >= 70)
printf ("C\n"); else if (grade >= 60)
printf ("D\n"); else
printf ("F\n");
Validating input When getting input from the user, it is
highly recommended to check whether it is valid.
If it’s not, you should display an appropriate message and return a non-zero value.
For example –if (grade<0 || grade>100){
printf(“Invalid input!\n”);return 1;
}
The return keyword For now, used to terminate the
program and return a value to the operating system
If the program is successful the return value should be zero, non-zero otherwise
The exact nature of this keyword will become clear in the future
Exercise Input –
An English letter Output –
If input is a lowercase letter – the corresponding uppercase letter
If input is an uppercase letter - corresponding lowercase letter
Note – Remember to check for input validity!
Solution
switch_case.c
Exercise (difficult!)
Write a program such that – Input – a 3-digit number Output – the same number with digits
sorted Example – if input is 132, output
should be 123 Note – if input is not a 3-digit number,
display an error message and exit!
Solution
Sort_digits.c
Loops
Used to repeat the same instruction(s) over and over again.
C provides some flexible ways of deciding how many times to loop, or when to exit a loop.
for, while, do-while loops.
While loops
while (condition) {
statement(s);}
The statements are executed as long as condition is true
When the condition is no longer true, the loop is exited.
Example - factorial#include <stdio.h>int main(void){
int i,n,fact = 1;printf("Enter a number\n");scanf("%d", &n);i=1; while (i<=n){
fact = fact*i;i++;
}printf("the factorial is %d\n", fact);return 0;
}
This is a counter
Every iteration i is incremented by 1. Equivalent to i=i+1.
Example – fibonacci series
fibonacci.c
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0
Screen
5
lim0
fib11
fib2---
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0
Screen
5
lim0
fib11
fib2---
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1
Screen
5
lim0
fib11
fib2---
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1
Screen
5
lim0
fib11
fib21
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1
Screen
5
lim1
fib11
fib21
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1
Screen
5
lim1
fib11
fib21
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1
Screen
5
lim1
fib11
fib21
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1
Screen
5
lim1
fib11
fib21
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1
Screen
5
lim1
fib11
fib22
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1
Screen
5
lim1
fib11
fib22
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1
Screen
5
lim1
fib12
fib22
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1
Screen
5
lim1
fib12
fib22
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2
Screen
5
lim1
fib12
fib22
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2
Screen
5
lim1
fib12
fib23
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2
Screen
5
lim2
fib12
fib23
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2
Screen
5
lim2
fib13
fib23
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2
Screen
5
lim2
fib13
fib23
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2 3
Screen
5
lim2
fib13
fib23
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2 3
Screen
5
lim2
fib13
fib25
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2 3
Screen
5
lim3
fib13
fib25
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2 3
Screen
5
lim3
fib15
fib25
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2 3
Screen
5
lim3
fib15
fib25
fib_next
Fibonacci – step by stepfib1 = 0;fib2 = 1;
printf("%d ", fib1);
while(fib2 < lim){
printf("%d ", fib2);fib_next = fib1 + fib2;fib1 = fib2;fib2 = fib_next;
}
printf("\n");
0 1 1 2 3
Screen
5
lim3
fib15
fib25
fib_next
getchar getchar() gets a single character from
the user. Requires including stdio.h Returns a non-positive number on
failure. Similar to scanf.
char c;
c = getchar();
char c;
scanf(“%c”, &c);
====
putchar
putchar(‘char’) prints out the character inside the brackets.
Requires including stdio.h Similar to printf.
char c;
putchar(c);
char c;
printf(“%c”, c);====
Example – lower-case to upper case.
low2up.c
Low2up – step by step#include <stdio.h>
int main(void){ char c; char upper_c;
printf ("Please enter a string: ");
c = getchar();
Buffer
‘#’
‘@’
c upper_c
Screen
Low2up – step by step#include <stdio.h>
int main(void){ char c; char upper_c;
printf ("Please enter a string: ");
c = getchar();
eS\n
Buffer
‘y’ ‘@’
c upper_c
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
eS\n
Buffer
‘y’ ‘@’
c upper_c
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
eS\n
Buffer
‘y’ ‘@’
c upper_c
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
eS\n
Buffer
‘y’ ‘Y’
c upper_c
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
eS\n
Buffer
‘y’ ‘Y’
c upper_c
Y
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
S\n
Buffer
‘e’ ‘Y’
c upper_c
Y
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
S\n
Buffer
‘e’ ‘Y’
c upper_c
Y
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
S\n
Buffer
‘e’ ‘Y’
c upper_c
Y
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
S\n
Buffer
‘e’ ‘E’
c upper_c
Y
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
S\n
Buffer
‘e’ ‘E’
c upper_c
YE
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
\n
Buffer
‘S’ ‘E’
c upper_c
YE
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
\n
Buffer
‘S’ ‘E’
c upper_c
YE
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
\n
Buffer
‘S’ ‘E’
c upper_c
YE
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
\n
Buffer
‘S’ ‘S’
c upper_c
YE
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
\n
Buffer
‘S’ ‘S’
c upper_c
YES
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
Buffer
‘\n’ ‘S’
c upper_c
YES
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
Buffer
‘\n’ ‘S’
c upper_c
YES
Screen
Low2up – step by stepwhile (c != '\n' && c >= 0){ if (c >= 'a' && c <= 'z') upper_c = c - 'a' + 'A'; else upper_c = c;
/* Print the converted character. */ putchar(upper_c);
/* Get the next character */c = getchar();
} putchar('\n');
Buffer
‘\n’ ‘S’
c upper_c
YES
Screen
Exercise
Input – Two integers – A and B
Output – How many times A contains B This is the result of the integer
division A/B Note –
Do not use the division operator!
Solution#include <stdio.h>
int main(void){
int a, b, res;
printf("Please enter two numbers.\n");scanf("%d%d", &a, &b);
res = 0;while ( (res+1) * b <= a)
res = res + 1;
printf("%d / %d = %d", a, b, res);return 0;
}
Break in a loop
When break is encountered, the loop is exited regardless of whether the condition is still true.
The program then continues to run from the first line after the while loop.
If called within a nested loop, break breaks out of the inner loop only.
Example – counting letters
break.c
Continue
When continue is encountered, the rest of the loop is ignored.
The program then continues to run from the beginning of the loop.
Rarely used. Can usually be replaced by an
appropriate if-else statement.