Programming C for Engineers
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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(){
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 statement
if (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 value is
non-zero If it is zero, it is false Therefore, in the following –
if (expression) statement
statement is executed if expression is non zero.
More about operators
In C, every expression has a numeric value
When using arithmetical operators (+, -, *, /) this is straightforward 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 evaluates to zero if A is larger than or equal to B, and some non-zero value if A is smaller than B
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) evaluates to 4 (y = 0) evaluates to 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 warn us about 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
else
statement
An example
if (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
The ?: operator
expr1 ? expr2 : expr3 Nicer way to write:
(expr1)? expr2 : expr3 If expr1 is true (non-zero), expr2 is
evaluated. Otherwise, expr3 is evaluated
The ?: operator
#include <stdio.h>
int main(){
int i, j, min;
printf("Please enter two numbers: ");scanf("%d%d", &i, &j);
min = (i < j)? i : j;printf("The minimum between %d and %d is %d\n", i, j, min);
return 0;}
The switch statement a multiway conditional statement
similar to if-else if-else allows the selection of an arbitrary number of choices
based on an integer value
switch (expression) { case const-expr:
statements case const-expr:
statements … default:
statements}
The switch statement
expression must have an integer value when the switch statement is executed:
the expression is evaluated if a case matches the value of the expression,
the program jumps to the first statement after that case label
otherwise, the default case is selected the default is optional
That grade example again
switch (grade/10) { case 10: case 9:
printf ("A\n"); break;
case 8: printf ("B\n"); break;
case 7: printf ("C\n"); break;
case 6: printf ("D\n"); break;
default: printf ("F\n");
}
Give me a break
when the switch transfers to the chosen case, it starts executing statements at that point
it will “fall through” to the next case unless you “break out”
break causes the program to immediately jump to the next statement after the switch statement
A more interesting example
operation.c
Exercise
Write a program that accepts a number between 1 and 100 from the user. If there is a coin of that value in cents, it should display its name. Otherwise, it should report that there is no such coin
1 = cent, 5 = nickel, 10 = dime, 25 = quarter, 100 = dollar
Remember to check for the validity of the input!
Solution
coins.c
Debugging
It is virtually impossible to program without errors
Syntax errors are detected by the compiler
However, often a program has no syntax errors and compiles, but still doesn’t perform as desired
Debugging
Debuggers are software tools designed to help find software bugs
Both Visual C and the lcc compiler include a debugger
Debugging
The debugger allows us to – Execute the program one line at a time At each step see the values of all
variables and expressions Run the program up to a pre-specified
point And more…