Snow_Mac Snow_Mac - 3 months ago 35
C Question

C: Multiple scanf's, when I enter in a value for one scanf it skips the second scanf

I have this block of code (functions omitted as the logic is part of a homework assignment):

#include <stdio.h>

int main()
{
char c = 'q';
int size;

printf("\nShape (l/s/t):");
scanf("%c",&c);

printf("Length:");
scanf("%d",&size);

while(c!='q')
{
switch(c)
{
case 'l': line(size); break;
case 's': square(size); break;
case 't': triangle(size); break;
}


printf("\nShape (l/s/t):");
scanf("%c",&c);

printf("\nLength:");
scanf("%d",&size);
}

return 0;
}


The first two Scanf's work great, no problem once we get into the while loop, I have a problem where, when you are supposed to be prompted to enter a new shape char, it instead jumps down to the
printf
of Length and waits to take input from there for a char, then later a decimal on the next iteration of the loop.

Preloop iteration:

Scanf: Shape. Works Great

Scanf: Length. No Problem

Loop 1.

Scanf: Shape. Skips over this

Scanf: length. Problem, this scanf maps to the shape char.

Loop 2

Scanf: Shape. Skips over this

Scanf: length. Problem, this scanf maps to the size int now.

Why is it doing this?

Answer

scanf("%c") reads the newline character from the ENTER key.

When you type let's say 15, you type a 1, a 5 and then press the ENTER key. So there are now three characters in the input buffer. scanf("%d") reads the 1 and the 5, interpreting them as the number 15, but the newline character is still in the input buffer. The scanf("%c") will immediately read this newline character, and the program will then go on to the next scanf("%d"), and wait for you to enter a number.

The usual advice is to read entire lines of input with fgets, and interpret the content of each line in a separate step. A simpler solution to your immediate problem is to add a getchar() after each scanf("%d").

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