Week 2 / Lecture 2 15 March 2017 NWEN 241 Control constructs, Functions Alvin Valera School of Engineering and Computer Science Victoria University of Wellington
Admin stuff Tutorial #2 Expectations on programming assignment Development environment issues Data input/output with standard C library functions getchar putchar gets puts printf scanf General debugging tips 2
Recap Built-in operators Arithmetic Assignment Increment/decrement Relational Equality and logical Arithmetic, assignment, increment/decrement operators evaluate to some numeric value Relational, equality and logical operators evaluate to either 1 (true) or 0 (false) 3
Recap When evaluating expressions Operator precedence determines the sequence in which operators in an expression are evaluated Associativity determines execution for operators of equal precedence Precedence can be overridden by explicit grouping using ( and ) which has the highest precedence When expression involves operands of diferent types C automatically converts intermediate values to the proper type so that the expression can be evaluated without losing any significance 4
Clarification on conversion hierarchy Conversion hierarchy short int char unsigned long int long int double float long double Operands that are of type char, short and enum are always converted up to int if it is suficient, otherwise to unsigned int This is known as integral promotion 5
Clarification on relational operator example Suppose: double x = 7e+33, y = 0.001; Expression Equivalent expression Value x < x + y x < (x + y) Expression is evaluated as (x - (x + y)) < 0.0 Because is x extremely large and y is small, there is loss in accuracy in evaluating x + y That is, x and x + y becomes equal The expression simplifies to 0.0 < 0.0 which is false (0) 0 6
This lecture Control blocks Ternary operator Functions Function-like macros 7
Recallf compound statement Compound statement Begins with { and ends with } Consists of 0 or more declarations and 0 or more statements Main use is to group statements into an executable unit Also known as block { int i, j=1; } i = 2*j + 1; printf("%d", i); 8
When to declare variables Prior to C99 Variables can only be declared at the beginning of a block With C99 and afer Variables can be declared anywhere in block, before usage { int i, j; { int i; scanf("%d", &i); } scanf("%d", &i); scanf("%d", &j); } int j; scanf("%d", &j); 9
Control constructs in C Control flow If-else Else-if Switch Loop While-loop For-loop Do-while-loop 10
Control flowf if-else statement if (expression) statement 1 else statement 2 The else part is optional If expression evaluates to true*, statement 1 is executed Else, statement 2 is executed if the else part is present *Recall: true non-zero; false zero statement 1 and statement 2 can be single or compound statements 11
Examplesf if-else statement if (y!= 0.0) x /= y; if (c == ' ') { ++blank_cnt; printf("found another blank\n"); } if (a > b) max = a; else max = b; if (n > 0) if (a > b) z = a; else z = b; To which if does this else belong to? 12
Conditional expression (ternary operator) expr 1? expr 2 : expr 3 expr 1 is evaluated first If expr 1 evaluates to true, then expression expr 2 is evaluated and that is used as the value of the expression Otherwise, expr 3 is evaluated and that is used as the value of the expression Example: z = (a > b)? a : b; /* z = max(a, b) */ 13
Control flowf else-if statement if (expression 1 ) statement 1 else if (expression 2 ) statement 2... else if (expression N ) else statement N statement The else part is optional expression 1 to expression N are evaluated in sequence If expression k (where k is 1,2,...,N) evaluates to true, statement k is executed The rest of else if and else are not evaluated any further If none of expression 1 to expression N is true, then the else statement is executed (if present) The statements can be single or compound statements 14
Examplef else-if statement if (temp <= 0) printf("it's freezing out there.\n"); else if (temp <= 10) { too_cold++; printf("it's too cold for me.\n"); } else if (temp <= 20) printf("it's still cold.\n"); else printf("awesome!\n"); 15
Control flowf switch statement switch (expression) { case const_expr 1 : } statements 1 case const_expr 2 : statements 2... case const_expr N : statements N default: statements The default part is optional const_expr 1 to const_expr N must be integer constants or constant expressions If expression matches const_expr k, execution starts at that case default is executed if none of the cases match The statements can consist of single or multiple statements statements, or compound statements 16
Examplef switch statement char c = getchar(); switch(c) { case 'Y': case 'y': printf("you answered yes.\n"); break; case 'N': case 'n': printf("you answered no.\n"); break; default: printf("what was that?\n"); break; Is this necessary? } 17
Loopf while-loop statement while (expression) statement If expression evaluates to true: statement is executed expression is re-evaluated again Cycle continues until expression evaluates to false statement can be single or compound statement 18
Loopf for-loop statement for (expr 1 ; expr 2 ; expr 3 ) statement expr 1 ; while (expr 2 ) { } statement expr 3 ; The expressions are optional expr 1 and expr 3 are usually assignments or function calls expr 2 is usually a relational expression If expr 2 is missing, it is taken as permanently true 19
Loopf do-while-loop statement do statement while (expression); statement is executed, then expression is evaluated If expression evaluates to true, statement is executed again Loop terminates when expression evaluates to false 20
Examplef loop statements Infinite loops: while(1); for (;;); do {} while(1); int i = 10; while(i > 0) { printf("%d\n", i); i--; } for(int i = 10; i > 0; i--) { printf("%d\n", i); } Are these necessary? do { printf("do you agree with the contract?\n"); ans = getchar(); } while (ans!= 'Y' ans!= 'y'); 21
Statements that can alter control flow & loop break, return, continue and goto break: jumps out of the loop or switch return: jumps out of the loop or switch (the loop or switch must be inside a function) continue: stops current loop iteration and starts next iteration goto jumps to a labelled statement 22
Functions in C programs Every C program has at least one function: main() No C program needs to have more than one function in it Everything can be put in main(): not a good idea Any C program with only a main function is almost certainly for training purposes What are functions good for? structuring our thoughts (structured programming) allowing us to re-use code, reducing work and reducing errors A C program can be modularised by functions A big program can be broken down into a number of smaller ones 23
Creating a simple function Suppose we frequently wanted to compare two integers and then use the larger. We might have code like this repeatedly written in our program: int p, q, l;... /* p, q initialised */ if (p > q) l = p; else l = q;... /* l gets used */ 24
Creating a simple function How about making it a stand-alone function? What we need to do: l = larger(p, q); Pick a name for the function: larger() Specify what type of variables that larger() is going to compare: larger(int, int) Specify what type of value that larger (int, int) is going to return: int larger(int, int) int larger(int, int): this is called function prototype / declaration Code it: function definition/implementation 25
Creating a simple function Function definition: int larger(int x, int y) { if (x > y) return x; else return y; } x and y are called formal parameters, whose scope is the body of the function. 26
Invoking a function... int main(void) {... l = larger(p, q); /* p and q are called actual */... /* parameters. Their values are */ } /* going to be copied to x and y. */ int larger(int x, int y) { /* x and y (NOT p and q) are */ if (x > y) /* going to be compared here. */ return x; /* the larger value is going to be */ else return y; /* returned to larger(p, q). */ } 27
Role of function prototype Function prototype is used by compiler to check whether your program is passing the right parameters For functions that do not return a value, the compiler ensures that the function is not part of an expression Hence, a function prototype must be declared prior to its use For functions in standard library (and other provided libraries), include the appropriate header files For user-defined functions, you have to declare the prototypes before the main function 28
Role of function prototype int main(void) {... l = larger(p, q); /* larger() not declared yet */... } int larger(int x, int y) {... } Problem: function prototype not declared before being used int larger(int, int); int main(void) {...} int larger(int x, int y) {...} 29
Passing arguments to a function Function call: l = larger(p, q); Pass by value The values of actual parameters (p, q) are copied to formal parameters (x, y) actual parameters and formal parameters are separate entities What happens thereafer to formal parameters has nothing to do with actual parameters Any changes on x, y will not be transferred back to p, q There is another way of passing arguments to functions 30
Function-like macros Recall: C preprocess can used to define constants #define PI 3.14 Can also define macros that can be invoked like functions Append () in the macro name #define READ_CHAR() getchar() 31
Function-like macros Just like functions, function-like macros can take arguments Insert comma-separated parameter names between ( and ) Parameter names must be valid C identifiers #define max(x, Y) ((X) > (Y)? (X) : (Y)) Invoke just like normal functions z = max(1, 3); z = ((1)>(3)?(1):(3)); This expression evaluates to 3 32
Problems with function-like macros Suppose: #define SQ(X) X * X Then: (int)sq(r); (int)r * r; SQ(r1 + r2); r1 + r2 * r1 + r2; Solution: enclose individual variables with (), including the whole replacement text #define SQ(X) ((X) * (X)) 33
Problems with function-like macros Suppose: #define SQ(X) ((X) * (X)) Then: (int)sq(r); (int)((r) * (r)); SQ(r1 + r2); ((r1 + r2) * (r1 + r2)); 34
Problems with function-like macros Suppose: #define SQ(X) ((X) * (X)) How about these? SQ(++r); ((++r) * (++r)); r incremented twice SQ(f()); ((f()) * (f())); f() invoked twice Be careful when defining and calling function-like macros! 35
Next week Arrays 36