Programming Language  C Control Flow 主講人：虞台文

Content Overview If-Else Statement Else-If Statement Switch Statement Loops - While and For Do-While Statement Break and Continue Goto and Labels

Programming Language  C Control Flow Overview

Control Flow Control flow statements specify the order in which computations are performed. Sequencing Conditional Selection Looping (or iterative processing)

Statements and Blocks Simple Statements Null Statement lower = 0; upper = 300; step = 20; fahr = lower; Null Statement ; // a null statement Compound Statements (block statements) { celsius = (5.0/9.0) * (fahr-32.0); printf("%3.0f %6.1f\n", fahr, celsius); fahr = fahr + step; } 4 simple statements 1 compound statement

More on Block Statements compound-statement : { declaration-listopt statement-listopt }

Programming Language  C Types, Operators and Expressions If-Else Statement

If-Else Statement if (expression) statement1 else statement2

If-Else Statement if (expression) statement1 else statement2 If expression is evaluated to nonzero, statement1 is executed; otherwise, statement2 is executed. if (expression) statement1 else statement2 expression option

Shortcut if(expression != 0) ... if(expression) ...

Dangling Else Problem z = ? z = 20 z = 3 n=5; a=2; b=3; z=20; if (n > 0) if (a > b) z = a; else z = b; if (n > 0) if (a > b) z = a; else z = b; z = ? z = 20 z = 3

Ambiguity Removal if (n > 0) if (a > b) z = a; else z = b; } else z = b; if (n > 0) if (a > b) z = a; else z = b; if (n > 0){ if (a > b) z = a; else z = b; }

Programming Language  C Types, Operators and Expressions Else-If Statement

Else-If Statement option if (expression) statement else if (expression) else option

Example: Binary Search int binsearch(int x, int v[], int n); 1 5 9 25 80 125 137 140 180 201 400 2 3 4 6 7 8 10 v[] 6  binsearch(137, v, 11) 9  binsearch(201, v, 11) -1  binsearch(45, v, 11)

Example: Binary Search binsearch(25, v, 11) Example: Binary Search low <= high mid = (low + high) / 2 = 5 1 5 9 25 80 125 137 140 180 201 400 2 3 4 6 7 8 10 v[] low = v[mid] = 125 > 25 mid = high =

Example: Binary Search binsearch(25, v, 11) Example: Binary Search v[] low = 1 1 5 2 9 v[mid] = 125 > 25 3 25 high = 4 80 high = mid - 1 mid = 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(25, v, 11) Example: Binary Search low <= high v[] mid = (low + high) / 2 = 2 low = 1 1 5 mid = 2 9 v[mid] = 9 < 25 3 25 high = 4 80 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(25, v, 11) Example: Binary Search v[] 1 1 5 mid = 2 9 v[mid] = 9 < 25 low = 3 25 high = 4 80 low = mid + 1 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search 3  binsearch(25, v, 11) Example: Binary Search low <= high v[] mid = (low + high) / 2 = 3 1 1 5 2 9 v[mid] = 25 == 25 mid = low = 3 25 high = 4 80 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search low <= high mid = (low + high) / 2 = 5 1 5 9 25 80 125 137 140 180 201 400 2 3 4 6 7 8 10 v[] low = v[mid] = 125 > 25 mid = high =

Example: Binary Search binsearch(45, v, 11) Example: Binary Search v[] low = 1 1 5 2 9 v[mid] = 125 > 45 3 25 high = 4 80 high = mid - 1 mid = 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search low <= high v[] mid = (low + high) / 2 = 2 low = 1 1 5 mid = 2 9 v[mid] = 9 < 45 3 25 high = 4 80 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search v[] 1 1 5 mid = 2 9 v[mid] = 9 < 45 low = 3 25 high = 4 80 low = mid + 1 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search low <= high v[] mid = (low + high) / 2 = 3 1 1 5 2 9 v[mid] = 25 < 45 mid = low = 3 25 high = 4 80 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search v[] 1 1 5 2 9 v[mid] = 25 < 45 mid = 3 25 low= high = 4 80 low = mid + 1 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search low <= high v[] mid = (low + high) / 2 = 4 1 1 5 2 9 v[mid] = 80 > 45 3 25 low= high = 4 80 mid = 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search binsearch(45, v, 11) Example: Binary Search v[] 1 1 5 2 9 v[mid] = 80 > 45 high = 3 25 low= 4 80 high = mid - 1 mid = 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search -1  binsearch(45, v, 11) Example: Binary Search  low <= high v[] 1 1 5 2 9 high = 3 25 low= 4 80 mid = 5 125 6 137 7 140 8 180 9 201 10 400

Example: Binary Search /* binsearch: find x in v[0] <= v[1] <= ... <= v[n-1] */ int binsearch(int x, int v[], int n) { int low, high, mid; low = 0; high = n - 1; while (low <= high) { mid = (low+high)/2; if (x < v[mid]) high = mid - 1; else if (x > v[mid]) low = mid + 1; else /* found match */ return mid; } return -1; /* no match */

Example: Binary Search

Exercises Consider the following grading table: Write a program that reads students’ grades and prints out the corresponding class for each grade. Grade Class 100-90 A 89-80 B 79-70 C 69-60 D <60 F

Exercises Write a program to compute by bisection the square root of a positive integer N given by the user. The algorithm proceeds as follows: Start with N and 1 as the upper and lower bounds, and then find the midpoint of the two bounds. If the square of the midpoint is equal or very close to N, then output the midpoint as the answer and the algorithm terminates. If the square of the midpoint is greater than N, then let the upper bound be the midpoint; otherwise let the lower bound be the midpoint. Repeat the process until the two most recent estimates of the square root of N is within 0.000005 of each other. Output the last estimate as the answer and the algorithm terminates.

Programming Language  C Types, Operators and Expressions Switch Statement

Switch Statement option Execution falls through if without break. switch (expression) { case item1: statement1; break; case item2: statement2; . . . . . . . . case itemn: statementn; default: statement; } Execution falls through if without break. option

Example  switch (letter) { case 'A': numberofvowels++; break; case 'E': case 'I': case 'O': case 'U': case ' ': numberofspaces++; default: numberofconsonants++; } switch (letter) { case 'A': case 'E': case 'I': case 'O': case 'U': numberofvowels++; break; case ' ': numberofspaces++; default: numberofconsonants++; } 

Exercise Write a function escape(s, t) that converts characters like newline and tab into visible escape sequences like \n and \t as it copies the string t to s. Use a switch. Write a function for the other direction as well, converting escape sequences into the real characters.

Programming Language  C Types, Operators and Expressions Loops – While and For

While Statement while (expression) statement The statement is cyclically executed as long as the expression is evaluated non-zero. This cycle continues until expression becomes zero, at which point execution resumes after statement. while (expression) statement

for Statement for (expr1; expr2; expr3) statement Continuing check. Loop variable(s) update Loop variable(s) initialization for (expr1; expr2; expr3) statement

While vs. For The choice between while and for is arbitrary, based on which seems clearer The for is usually used for a counter-like loop expr1; while (expr2) { statement expr3; }  while (expression) statement for (expr1; expr2; expr3) statement

While vs. For any of them can be omitted while (expression) statement for (expr1; expr2; expr3) statement

Infinite Loops while(1){ } for(; ;){ } while (expression) statement for (expr1; expr2; expr3) statement

Infinite Loops How to break the loop? while(1){ } for(; ; ;){ } if(…) break; or if(…) return; . . . . . . . . if(…) break; or if(…) return; . . . . . . . . while (expression) statement for (expr1; expr2; expr3) statement

Example: Bubble Sort

Example: Bubble Sort

Example: Bubble Sort (I) void BubbleSort(int data[], int n) { int tmp, i, j; for(i=0; i<n-1; i++) for(j=0; j<n-i-1; j++) if(data[j] > data[j+1]){ tmp = data[j]; data[j] = data[j+1]; data[j+1] = tmp; }

Example: Bubble Sort (II) void BubbleSort(int data[], int n) { int tmp, i, j, sorted; for(i=0, sorted=FALSE; !sorted && i<n-1; i++) for(j=0, sorted=TRUE; j<n-i-1; j++) if(data[j] > data[j+1]){ tmp = data[j]; data[j] = data[j+1]; data[j+1] = tmp; sorted = FALSE; } void BubbleSort(int data[], int n) { int tmp, i, j; for(i=0; i<n-1; i++) for(j=0; j<n-i-1; j++) if(data[j] > data[j+1]){ tmp = data[j]; data[j] = data[j+1]; data[j+1] = tmp; }

Exercises Modify the BubbleSort function such that its outer loop is a while statement. Verify your result. Modify the BubbleSort function such that its all loops are a while statement. Verify your result.

Programming Language  C Types, Operators and Expressions Do-While Statement

Do-While Statement do statement while (expression);

Example: itoa /* itoa: convert n to characters in s */ void itoa(int n, char s[]) { int i, negative; n = (negative = (n < 0)) ? –n : n; /* record sign */ /* make n positive */ i = 0; do { /* generate digits in reverse order */ s[i++] = n % 10 + '0'; /* get next digit */ } while ((n /= 10) > 0); /* delete it */ if (negative) s[i++] = '-'; s[i] = '\0'; reverse(s); }

Exercises Write the function itob(n,s,b) that converts the integer n into a base b character representation in the string s. In particular, itob(n,s,16) formats s as a hexadecimal integer in s. Write a version of itoa that accepts three arguments instead of two. The third argument is a minimum field width; the converted number must be padded with blanks on the left if necessary to make it wide enough.

Programming Language  C Types, Operators and Expressions Break and Continue

break and continue A break causes the innermost enclosing loop (for, while, do) or switch to be exited immediately. A continue causes the next iteration of the enclosing for, while, or do loop to begin.

Example: (break) Remove Trailing Blanks /* trim: remove trailing blanks, tabs, newlines */ int trim(char s[]) { int n; for (n = strlen(s)-1; n >= 0; n--) if (s[n] != ' ' && s[n] != '\t' && s[n] != '\n') break; s[n+1] = '\0'; return n; }

Example: (continue) Remove Factor 2 /* trim: remove all of factor 2 in int array */ int remove2(int s[], int n) { int i, nchange; for (i = 0, nchange = 0; i < n; i++){ if(s[i] & 1 || s[i] == 0) continue; while(!(s[i] & 1)) s[i] /= 2; nchange++ } return nchange;

Programming Language  C Types, Operators and Expressions Goto and Labels

The goto and Labeled Statements labeled-statement jump-statement .......................... jump-statement : goto identifier ; labeled-statement : identifier : statement

The goto Statement The goto statement performs an unconditional transfer of control to the named label. The label must be in the current function. Don’t use goto frequently appropriate case  error handling

Example: goto void main() { int i, j; for ( i = 0; i < 10; i++ ){ printf( "Outer loop executing. i = %d\n", i ); for ( j = 0; j < 3; j++ ){ printf( "Inner loop executing. i = %d j = %d\n", i, j ); if ( j == 1 && i==5 ) goto stop; } /* This message does not print: */ printf( "Loop exited. i = %d j = %d\n", i, j ); stop: printf( "Jumped to stop. i = %d j = %d\n", i, j );

Example: goto void main() { int i, j; for ( i = 0; i < 10; i++ ){ printf( "Outer loop executing. i = %d\n", i ); for ( j = 0; j < 3; j++ ){ printf( "Inner loop executing. i = %d j = %d\n", i, j ); if ( j == 1 && i==5 ) goto stop; } /* This message does not print: */ printf( "Loop exited. i = %d j = %d\n", i, j ); stop: printf( "Jumped to stop. i = %d j = %d\n", i, j );

Exercise Modify the above program as a gotoless program.

Example: Using Label for Error Handling for( p = 0; p < NUM_PATHS; ++p ){ NumFiles = FillArray( pFileArray, pszFNames ) for( i = 0; i < NumFiles; ++i ){ if( (pFileArray[i] = fopen( pszFNames[i], "r" )) == NULL ) goto FileOpenError; // Process the files that were opened. } FileOpenError: printf( "Fatal file open error. Processing interrupted.\n" );