Introduction to C Programming CE Lecture 4 Further Control Structures in C

Program Control - Iteration Executing a series of one or more statements a number of times Also referred to as repetition or looping Non-deterministic No pre-set number of iterations Use While Do While Deterministic Known number of iterations Use For

Non-deterministic Loop: While statement Format while (condition) statement; where the statement can be a compound statement e.g. sum=0; scanf(“%d”,&x); while (x!=-9999) { sum=sum+x; scanf(“%d”,&x); }

Read Ahead Technique Input data: –9999 Unwinding the iteration: Read num 1 Process num Read num4 Process num Read num5 Process num Read num6 Process num Read num-9999

The iterated step is: Process num Read num Which leads to: Read num While num != Process num Read num

Example: While Loop 1. Define Problem Problem: Read a series of integers terminated by –9999. Count the number of integers that are greater than 150 and find the largest and smallest numbers greater than 150.Print a message if none of the integers are greater than 150. Example Input: Expected Output: Number of values>150=2 Max=400 Min=200

2. Design Initialise:- count=0:max=0:min=99999 Read first number Input values and process While number <> loop If number > 150 then Increment count Check if new largest If number>=max then max=number Check if new smallest If number <=min then min=number Read next number End loop Print results If count>0 then Print count, max, min Else Print “No values ”

3. Code #include int main(void) /*program description*/ { int num,max,min,count; count=0; min=99999; max=0; scanf(“%d”, &num); while (num !=-9999) { if (num>150) { ++count; if (num>max) max=num; if (num<min) min=num; } scanf(“%d”,&num); } if (count>0) { printf(“Number of values>150= %d\n”, count); printf(“Max = %d\n”,max); printf(“Min = %d\n”,min); } else printf(“No values over 150”); return 0; }

Non-deterministic loop – Do While loop General Form do statement; while (expression); Unlike the while statement, the expression in the ‘do while’ comes after ‘statement’ (the loop body) which is thus executed at least once. Note that it is very easy to make mistakes with the do while loop, although it may compile and appear to produce valid code.

Deterministic Loop: For statement For statement format for (initialising list; condition; altering list) statement; initialising list: Executed once altering list: Executed at the end of each iteration condition: Evaluated and checked at the start of each iteration i.e. a leading decision

Example:For loop for (count=1;count<=10;count=count+2) printf(“%3d”,count); Gives output: Equivalent While loop: count=1; while (count<=10) { printf(“%3d”,count); count=count+2; }

Increment and Decrement Operators count=count+1; -> ++count; count++; count=count-1; ->--count; count--; count=count+5: -> count+=5; Increment can occur before or after statement actioned count=3; x=++count; == count: 4, x: 4 count=3; x=count++; == count:4, x:3 Similarly with decrement operators.

Using increment and decrement operators in for loops for (count=0;count<=10;++count) printf(”%3d”,count); Prints : for (count=0;count<=10;count++) printf(”%3d”,count); Prints : Classic Error: for (i=0;i<10;++i); printf(“%d”, i);

Example 1 Deterministic Loop Problem: Print a multiplication table of size N. e.g where N= The structure of the data determines the structure of the program

Design Data: Repetition of rows Repetition of values (row*column) Design Read N Perform N times Print a row Perform N times Print a value

Code #include /* Reads an integer N and prints the N times multiplication table */ int main(void) { int row, col, n; printf(“\nPlease enter the value of N: “); scanf(“%d”, &n); printf(“\n\n”); for (row=1;row<=n;row++) { for (col=1;col<=n;++col) printf(“%6d”, row*col); printf(“\n”); } return 0; }

Deterministic Loops – Example 2 Problem: Write a program to encode lower case letters in a message by performing a cyclic shift one place to the right. Example Input: The cat sat on the mat Example Output: Uif dbu tbu po uif nbu

Code #include /* Character processing example */ int main(void) { char c; while (scanf(“%c”, &ch) != EOF) if (ch == ‘z’) printf(“%c”, ‘a’); else if (ch >= ‘a’ && ch <=’y’) printf(“%c”, ch+1); else printf(“%c”, ch); return 0; }

End of File Condition All files have an end of file marker When scanf() attempts to read this marker, it returns –1 The following code copies integers until the end of file is found EOF can also be generated by ctrl-D while (scanf(“%d”, &x) !=-1) printf(“%d”, x); stdio.h contains #define EOF –1 hence: while (scanf(“%d”, &x) !=EOF) printf(“%d”, x); End of file markers avoid having to select a data terminator result in neater code.

Example of buffered input #include int main(void) { int num, sum, count; float mean; sum=0; count=0; while (scanf(“%d”, &num) !=EOF) { sum=sum+num; ++count; } printf(“%f\n”, (float)sum/(float)count); return 0; }

Buffered Input Using scanf() Problem Consider the following segment of code: char ch1, ch2; printf(“\n Input first value: “); scanf(“%c”, &ch1); printf(“\n Input second value: “); scanf(“%c”, &ch2); At run time: Prompt: Input first value User response: Q [RETURN] Program Action: ch1=’Q’, ch2=10 (ASCII [RETURN] = 10) Why?

Solution char ch1, ch2; printf(“\n Input first value: “); scanf(“%c%c”, &ch1, &rtn); printf(“\n Input second value: “); scanf(“%c”, &ch2); NOTE: if ch2 is a non-character type e.g. an integer, then there isn’t a problem. Care must be taken when processing characters. Consider: #define EOLN 10 …… while (scanf(“%c”, &ch1)!=EOLN) printf(“%c”,ch1);