Selection Statements Selects statements to execute based on the value of an expression The expression is sometimes called the controlling expression Selection.

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 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 statement if (expression) statement 1 else statement 2 if expression is true, statement 1 is executed. if expression is false, statement 2 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) 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 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 example int 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 int main(void) { int grade; printf("Please enter your grade: "); scanf("%d", &grade); if (grade 100) printf("This is not a valid grade!\n"); else printf("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 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 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 step fib1 = 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 lim 0 fib1 1 fib2 --- fib_next

Fibonacci – step by step fib1 = 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 lim 0 fib1 1 fib2 --- fib_next

Fibonacci – step by step fib1 = 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 lim 0 fib1 1 fib2 1 fib_next

Fibonacci – step by step fib1 = 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 lim 1 fib1 1 fib2 1 fib_next

Fibonacci – step by step fib1 = 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 lim 1 fib1 1 fib2 1 fib_next

Fibonacci – step by step fib1 = 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 lim 1 fib1 2 fib2 2 fib_next

Fibonacci – step by step fib1 = 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 lim 2 fib1 3 fib2 3 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 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 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 step while (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 step while (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 step while (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 step while (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 step while (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 step while (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 step while (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 step while (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’ c upper_c YE Screen

Low2up – step by step while (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’ c upper_c YES Screen

Low2up – step by step while (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 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.

Selection Statements Selects statements to execute based on the value of an expression The expression is sometimes called the controlling expression Selection.

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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.

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Presentation on theme: "Selection Statements Selects statements to execute based on the value of an expression The expression is sometimes called the controlling expression Selection."— Presentation transcript:

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