I have a very simple program like this
int main()
{
int opt;
int n;
int flag = 1;
while(flag)
{
printf("m inside while.Press c to continue\n");
if((opt = getchar())== 'c')
{
printf("choose a number\n");
scanf(" %d",&n);
switch(n)
{
case 0:
printf("m zero\n");
break;
case 1:
printf("entered one\n");
break;
case 3:
printf("m exit\n");
flag = 0;
break;
}
printf("m broke\n");
}
}
printf("m out\n");
return 0;
}
I get output like this:
m inside while.Press c to continue
c
choose a number
1
entered one
m broke
m inside while.Press c to continue
m inside while.Press c to continue
c
choose a number
My doubt is why "m inside while.Press c to continue" gets printed twice after every loop??
Thanks in advance
This is because of \n character left behind by previous scanf. When you input a number and press Enter key, an additional \n character passed to the standard input buffer. scanf reads that nuber leaving behind \n in the buffer. On next iteration of loop getchar reads \n before pressing any character by you and hence m inside while.Press c to continue printed twice as \n is not c.
Place this snippet of code just after the scanf statement in your while loop to eat up the newline characters
while(getchar() != '\n');
This will eat up any number of \n.
For more detailed explanation on the behavior of getchar read this answer.
You final code should be
int main()
{
int opt;
int n;
int flag = 1;
while(flag)
{
printf("m inside while.Press c to continue\n");
if((opt = getchar())== 'c')
{
printf("choose a number\n");
scanf(" %d",&n);
while(getchar() != '\n');
switch(n)
{
case 0:
printf("m zero\n");
break;
case 1:
printf("entered one\n");
break;
case 3:
printf("m exit\n");
flag = 0;
break;
}
printf("m broke\n");
}
}
printf("m out\n");
return 0;
}
After scanf reads the input there is a '\n' still in the buffer you have to clear it otherwise it will be readed by getchar in the next time and as it's != 'c' it will prompt again:
Try this :
printf("choose a number\n");
scanf(" %d",&n);
char c;
while (c = getchar != '\n' && c != EOF); // clear the buffer
while(flag)
{
printf("m inside while.Press c to continue\n");
while((opt=getchar()) != '\n') {
if(opt == 'c')
{
printf("choose a number\n");
scanf(" %d",&n);
switch(n)
{
case 0:
printf("m zero\n");
break;
case 1:
printf("entered one\n");
break;
case 3:
printf("m exit\n");
flag = 0;
break;
}
printf("m broke\n");
}
}
}
Related
getchar() not waiting for input and jumping directly to next line. There is some mismatch between scanf and getchar I think but not able to make out what is it exactly.
#include<stdio.h>
#include<ctype.h>
int main()
{
char ch;
int n1,n2;
printf("Enter the operation of your choice\n");
printf("a. add\ts. subtact\nm. multiply\td. divide\nq. quit\n");
while((ch=getchar())!='q')
{
printf("\nEnter 1st number:\n");
if(scanf("%d",&n1)!=1)
{
printf("Please enter an integer value.\n");
continue;
}
printf("Enter 2nd number:\n");
if(scanf("%d",&n2)!=1)
{
printf("Please enter an integer value.\n");
continue;
}
switch(ch)
{
case 'a':
{ printf(" %d + %d = %d\n",n1,n2,n1+n2);
break;
}
case 's':
{
printf(" %d - %d = %d\n",n1,n2,n1-n2);
break;
}
case 'm':
{
printf(" %d * %d = %d\n",n1,n2,n1*n2);
break;
}
case 'd':
{
if(n2!=0)
{
printf(" %d / %d = %f\n",n1,n2,(float)n1/n2);
break;
}
else
{
printf("Enter a non-zero number for n2\n");
continue;
}
break;
}
}
printf("Enter the operation of your choice\n");
printf("a. add\ts. subtact\nm. multiply\td. divide\nq. quit\n");
}
printf("Bye.");
}
OUTPUT:
Enter the operation of your choice
a. add s. subtact
m. multiply d. divide
q. quit
a
Enter 1st number:
50
Enter 2nd number:
25
50 + 25 = 75
Enter the operation of your choice
a. add s. subtact
m. multiply d. divide
q. quit
Enter 1st number:
your getchar reads the newline and other spaces you enter when you give the value read by the scanf, you need to bypass these spaces
just replace
while((ch=getchar())!='q')
by
while ((scanf(" %c", &ch) == 1) && (ch != 'q'))
notice the space before '%', this is thank to it the spaces including newline are bypassed
I am creating a calculator file in C with a while loop and a switch statement. The first time through the while loop, everything works fine, but when it goes through the second time, the my printf gets called before I have the opportunity to enter the data into the preceding scanf.
I have tried using '\n' before the text in the printf and I have also tried using fflush(stdout) before and after the scanf call.
Current output:
Welcome to the Calculator
Operation choices: Addition(A)
Subtraction(S)
Multiplication(M)
Division(D).
Enter choice: A
Enter both numbers in required sequence: 50 50
// the output of the calculator does <>= 100 // Equal to 100.
Welcome to the Calculator
Operation choices: Addition(A)
Subtraction(S)
Multiplication(M)
Division(D).
Enter choice: Enter both numbers in required sequence:
What I want:
Welcome to the Calculator
Operation choices: Addition(A)
Subtraction(S)
Multiplication(M)
Division(D).
Enter choice: A
Enter both numbers in required sequence: 50 50
// the output of the calculator does <>= 100 // Equal to 100.
Welcome to the Calculator
Operation choices: Addition(A)
Subtraction(S)
Multiplication(M)
Division(D).
Enter choice: // then I can enter a new choice for the switch //
The code I've tried:
while(input != 'q'){
printf("Welcome to the Calculator\nOperation choices:\tAddition(A)\n\t\t\tSubtraction(S)\n\t\t\tMultiplication(M)\n\t\t\tDivision(D)\nEnter choice: ");
fflush(stdout);
scanf("%c", &input);
fflush(stdout);
printf("\nEnter both numbers in required sequence: ");
scanf("%f %f", &num1, &num2);
switch(input){
case 'A':
result = num1 + num2;
break;
case 'S':
result = num1 - num2;
break;
case 'M':
result = num1 * num2;
break;
case 'D':
result = num1 / num2;
break;
default:
printf("Please choose a valid operation.");
break;
}
if(result > 100){
printf("Greater than 100.\n");
}
else if(result < 100) {
printf("Less than 100.\n");
}
else{
printf("Equal to 100.\n");
}
}
printf("Quit the menu.\n");
return(0);
}
The sequence of events in you program is correct, what happens is that scanf() reads a lingering '\n' new line character that is left in the stdin buffer from a previous input. The '\n' is consumed by scanf() and the program continues the execution.
You will need to clear the buffer before scanf() is executed.
Option 1 - Clear the buffer at the bottom of the while cycle:
//...
int c;
//...
else{
printf("Equal to 100.\n");
}
while((c = fgetc(stdin)) != '\n' && c != EOF){}
//...
Option 2 (simpler) - Use a space before %c specifier:
scanf(" %c", &input);
^
Try this : scanf(" %c", &input); (add space before %c)
scanf will likely take \n in buffer as input and placed in you character input.
The current problem is with the Evaluate another interval (Y/N)? prompt. Let's say I run the program 4 times; in order to end it, it requires me to type N 4 times.
int main() {
int trap, test;
double low, hi;
char repeat, c;
//Gather End Points
do {
printf("Enter endpoints of interval to be integrated (low hi): ");
test = scanf("%lf %lf", &low, &hi);
if (test != 2) {
printf("Error: Improperly formatted input\n");
while((c = getchar()) != '\n' && c != EOF); //Discard extra characters
} else
if (low > hi)
printf("Error: low must be < hi\n");
} while ((test != 2 || low > hi));
//Gather amount of triangles
do {
printf("Enter number of trapezoids to be used: ");
test = scanf("%d", &trap);
if (test != 1) {
printf("Error: Improperly formated input\n");
while((c = getchar()) != '\n' && c != EOF); //Discard extra characters
} else
if (trap < 1)
printf("Error: numT must be >= 1\n");
} while ((trap < 1 || test != 1));
//Output integrate
printf("Using %d trapezoids, integral between %lf and %lf is %lf",
trap, low, hi, integrate(low, hi, trap));
//Prompt user for another time
while (1) {
printf("\nEvaluate another interval (Y/N)? ");
scanf(" %c", &repeat);
switch (repeat) {
case 'Y':
main();
case 'y':
main();
case 'N':
return 0;
case 'n':
return 0;
default:
printf("Error: must enter Y or N");
}
}
return 0;
}
I expect it so that no matter what run of the program I'm on it will close when I type one N.
There are many ways to achieve what you want but calling main recursively is not a good idea.
A pretty simple way to change your program is to add an additional while(1) level. Something like:
int main(void)
{
char repeat;
while(1){ // Outer while to keep the program running
printf("running program\n");
// Put your program here
printf("program done\n");
repeat = '?';
while(repeat != 'y' && repeat != 'Y'){ // Repeat until input is 'Y' or 'y'
printf("\nEvaluate another interval (Y/N)? ");
scanf(" %c", &repeat);
switch (repeat){
case 'Y':
case 'y':
break;
case 'N':
case 'n':
return 0; // Stop if input is 'n' or 'N'
default:
printf("Error: must enter Y or N");
}
}
}
return 0; // This will never be reached
}
Another way (a simpler way, IMO) is to put the code where you ask the user into a function that you call from main. Like:
int continueProg()
{
char repeat = '?';
while(1){
printf("\nEvaluate another interval (Y/N)? ");
scanf(" %c", &repeat);
switch (repeat){
case 'Y':
case 'y':
return 1;;
case 'N':
case 'n':
return 0;
default:
printf("Error: must enter Y or N");
}
}
}
int main(void)
{
do {
printf("running program\n");
// Put your program here
printf("program done\n");
} while(continueProg());
return 0;
}
BTW: Take a look at getchar instead of using scanf
There are multiple problems in your program:
You test the return value of scanf() when reading the user's answer to the prompts, and you clear the pending input correctly, but you do not handle the potential end of file, leading to endless loops.
c must be defined as int to accommodate for all values returned by getchar(): 256 values of type unsigned char and the special value EOF.
You can main() recursively to repeat the program's action requiring multiple N answers. You should instead add an outer loop and exit from it upon a N answer or an end of file condition.
Here is a modified version:
#include <stdio.h>
double integrate(double low, double hi, int trap) {
...
}
int flush_line(void) {
// Consume the pending input and return `'\n`` or `EOF`
int c;
while ((c = getchar()) != EOF && c != '\n')
continue;
return c;
}
int main() {
// Main program loop
for (;;) {
int trap, test;
double low, hi;
char repeat;
//Gather End Points
for (;;) {
printf("Enter endpoints of interval to be integrated (low hi): ");
test = scanf("%lf %lf", &low, &hi);
if (test == EOF)
return 1;
if (test != 2) {
printf("Error: Improperly formatted input\n");
if (flush_line() == EOF)
return 1;
continue; // ask again
}
if (low > hi) {
printf("Error: low must be < hi\n");
continue;
}
break; // input is valid
}
//Gather amount of triangles
for (;;) {
printf("Enter number of trapezoids to be used: ");
test = scanf("%d", &trap);
if (test == EOF)
return 1;
if (test != 1) {
printf("Error: Improperly formated input\n");
if (flush_line() == EOF)
return 1;
continue;
}
if (trap < 1) {
printf("Error: numT must be >= 1\n");
continue;
}
break;
}
//Output integrate
printf("Using %d trapezoids, integral between %lf and %lf is %lf\n",
trap, low, hi, integrate(low, hi, trap));
//Prompt user for another time
for (;;) {
printf("\nEvaluate another interval (Y/N)? ");
if (scanf(" %c", &repeat) != 1)
return 1; // unexpected end of file
switch (repeat) {
case 'Y':
case 'y':
break;
case 'N':
case 'n':
return 0;
default:
printf("Error: must enter Y or N\n");
if (flush_line() == EOF)
return 1;
continue;
}
break;
}
}
}
I need to write a script for the user to enter a score between 0 and 10, flush the bad input out, if user inputs it and then using a switch statement, tell a user what grade did he/she got.
Here is my script:
...
int main()
{
int input; // input from user
printf("Enter the number between 0 and 10 and I will tell you your grade!");
while ((input=scanf("Your input:", &input) != EOF))
{
if (input < 0 || input > 10) //input is invalid
{
printf("Sorry, invalid character data.");
while (getchar() !='\n')
{
printf("Your input must be from 0 to 10.", input);
scanf("%d", &input); //This part looks very bad for me
}
}
else
switch (input)
{
case 0:
case 1:
case 2:
case 3:
case 4:
case 5:
printf("Your grade is F. \n");
break;
case 6:
printf("Your grade is D. \n");
break;
...
I got this far with my homework, and here are some "leftover" problems I can't fight with.
1) Whenever user submits anything after enter, it goes into infinite loop and prints Your grade is F., even when case = 6 for example.
2) I used break; at the end of each case. It looks like they don't work(?)
3) It looks like the problem in the second line in the second loop
scanf("%d", &input); //This part looks very bad for me
but then I guess the scripts accepts it as true since the else statements that includes switch begins to work, because otherwise it wouldn't print Your grade is F.
Try the following code. Have a look at what flush_stream is doing when we have invalid data...
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
void flush_stream();
void flush_stream() {
char c;
do {
c = getchar();
}
while (!isdigit(c) && c != '\n');
ungetc(c, stdin);
}
int main(void) {
const char *prompt = "Input please: ";
int input; // input from user
printf("Enter the number between 0 and 10 and I will tell you your grade!\n");
while(1) {
printf("%s", prompt);
int ret = scanf("%d", &input);
if(ret == 0) {
printf("Sorry, invalid character data, your input must be from 0 to 10.\n");
flush_stream();
continue;
}
if(ret > 0) {
if (input < 0 || input > 10) {
printf("Sorry, invalid character data, your input must be from 0 to 10.\n");
flush_stream();
continue;
}
switch (input) {
case 0:
case 1:
case 2:
case 3:
case 4:
case 5:
printf("Your grade is F. \n");
break;
case 6:
printf("Your grade is D. \n");
break;
}
}
}
}
It seems you are using scanf and printf wrong.
To input a number:
scanf("%d", &input);
To output a number:
printf("%d\n", input);
And in you while loop, when the input is illegal, why not just continue to the next loop?
while (true) {
printf("input your grade here: ");
if (scanf("%d", &input) == EOF) {
break;
}
if (input < 0 || input > 10) {
printf("your input is illegal.\n");
continue;
}
switch (input) {
...
}
}
I am implementing a polynomial using array. This is the Problem Statement:
Write a menu-driven program to represent Polynomials as a data structure using arrays. and write functions to add, subtract and multiply two polynomials; multiply a polynomial with a constant, find whether a polynomial is a "zero- polynomial, return the degree of the polynomial. Assume that a new polynomial is created after each operation. How would you input and output polynomials?
I have created the input and output functions. But my do while loop is running twice.. Help me finding out why.
The do-while loop
do{
print_menu();
scanf("%c",&ch);
printf("\nch = %c\n",ch);
switch(ch){
case '1':
create_poly(poly,termpool,&next_poly);
break;
case '2':
print_poly(poly,termpool,&next_poly);
break;
case 'q':
break;
default:
printf("Invalid choice.");
}
}while(ch != 'q');
return 0;
}
The print_menu() function
void print_menu()
{
printf("\n1. Create a new polynomial.");
printf("\n2. Print polynomial.");
printf("\nq. Exit");
printf("\nEnter Choice:");
}
The create_poly() function
void create_poly(int poly[][2], int termpool[][2], int *next_poly)
{
int beg = poly[*next_poly][0];
int end, size, i, j;
printf("Enter size of the polynomial:");
scanf("%d",&size);
poly[*next_poly][1] = beg + size - 1;
end = poly[*next_poly][1];
printf("Enter terms of the polynomial(coeff then exponent):\n");
for(i=beg; i<=end; i++){
for(j=0; j<2; j++){
scanf("%d ",&termpool[i][j]);
}
}
poly[++(*next_poly)][0] = end + 1;
}
The print_poly() function
void print_poly(int poly[][2],int termpool[][2],int *next_poly)
{
int pos,beg,end;
int i;
printf("Enter position of the polynomial:");
scanf("%d",&pos);
if(pos-1 > *next_poly){
printf("Invalid position.");
return;
}
beg = poly[pos-1][0];
end = poly[pos-1][1];
for(i=beg; i<=end; i++){
printf(" %dx^%d +",termpool[i][0],termpool[i][1]);
}
printf("\b = 0");
}
Here is a sample output:
1. Create a new polynomial.
2. Print polynomial.
q. Exit
Enter Choice:1
ch = 1
Enter size of the polynomial:2
Enter terms of the polynomial(coeff then exponent):
2 4
6 7
1. Create a new polynomial.
2. Print polynomial.
q. Exit
Enter Choice:
ch =
Invalid choice.
1. Create a new polynomial.
2. Print polynomial.
q. Exit
Enter Choice:q
ch = q
Tried flushing the stdin… The problem stays. Printing the value of ch in each step, I think it is a whitespace. Where does the white space comes?
The answer to abnormal behavior of scanf answers this question also.
If you test the next code you will note the same problem
int main() {
char c;
do {
scanf_s("%c", &c);
if (c != 'q')
printf("test scanf() function\n");
} while (c);
}
the scanf() function works when the enter key is pressed, but this insert another char in the buffer input, the char of new line '\n', it is taken again by scanf() because the loop block. Try to change the previous code by this code:`
do {
scanf_s("%c", &c); // or c = getchar();
switch (c){
case '\n':
break;
default:
printf("test scanf() function\n");
}
} while (c);`
and will work fine. In your code only add a new case in the switch block:
switch(ch) {
case '1':
create_poly(poly,termpool,&next_poly);
break;
case '2':
print_poly(poly,termpool,&next_poly);
break;
case '\n':
break;
case 'q':
break;
default:
printf("Invalid choice.");
}
sorry, English is not my native language
There's an extra character waiting to be consumed after you make your initial choice, that's why the loop is executing twice.
See this question on the comp.lang.c FAQ