Loop while double not equal character?

scanf() shown below returns 0 when you pass character so when you enter q while loop will terminate

double fahrenheit; while(scanf("%lf",&fahrenheit) == 1) { // Do your stuff } 

Else

1. Read the input into char array using fgets()
2. Use strcmp() to check whether the input is q
3. If the input is not q use strtod() to convert string to double and assign that value to your variable.

A better way to do it is to read a line of text, and parse it to see if it's the q letter or if it can be converted to a number, something like this

#include <stdio.h> #include <string.h> #include <stdlib.h> int main() { char line[100]; printf("Enter Temperatur (F) > "); if (fgets(line, sizeof(line), stdin) != NULL) { size_t length; double fahrenheit; char *endptr; /* get the length of the input string */ length = strlen(line); if (length == 0) return -1; if (line[length - 1] == '\n') /* remove the '\n' possibly read by fgets */ line[--length] = '\0'; /* check if the input string equals "q" */ if (strcmp(line, "q") == 0) return 0; /* convert the input string to a double */ fahrenheit = strtod(line, &endptr); /* check if the conversion was successful */ if (*endptr != '\0') { printf("invalid input\n"); return -1; } printf("value = %f\n", fahrenheit); } return 0; } 

you can retry on invalid input and process other commands too apart from "q" if you need to, this gives you more control on the input.

For a simple solution with minimal error checking.

Read user input as a string.
Test if q was entered.
Otherwise convert to double.

double fahrenheit; while (1) { // repeat forever char buffer[100]; fputs("Enter temperature in Fahrenheit:", stdout); if (fgets(buffer, sizeof buffer, stdin) == NULL) break; // exit on IO error or EOF if (buffer[0] == 'q') break; if (sscanf(buffer, "%lf", &fahrenheit) == 1) { // was scan successful? printf("Success: %f\n", fahrenheit); } else { printf("Failed: '%s'\n", buffer); } } 

What is missing for a robust solution:
1) Distinguish between leaving the loop due to q or EOF.
2) Will quit loop on "q", "quit", "qxyz", but not quit on "Q", " q".
3) Will succeed on input like "123 C".
4) No degree range check like failing for "-1000".
5) Does not complain about crazy input > 100 chars.